Azooxanthellate cold-water corals (CWCs) have a global
distribution and have commonly been found in areas of active fluid seepage.
The relationship between the CWCs and these fluids, however, is not well
understood. This study aims to unravel the relationship between CWC
development and hydrocarbon-rich seepage in Pompeia Province (Gulf of
Cádiz, Atlantic Ocean). This region is comprised of mud volcanoes (MVs), coral
ridges and fields of coral mounds, which are all affected by the
tectonically driven seepage of hydrocarbon-rich fluids. These types of seepage, for example, focused, scattered, diffused or eruptive, is tightly controlled by a
complex system of faults and diapirs. Early diagenetic carbonates from the
currently active Al Gacel MV exhibit δ13C signatures down to
-28.77 ‰ Vienna Pee Dee Belemnite (VPDB), which indicate biologically derived methane
as the main carbon source. The same samples contain 13C-depleted lipid
biomarkers diagnostic for archaea such as crocetane (δ13C down
to -101.2 ‰ VPDB) and pentamethylicosane (PMI) (δ13C down to
-102.9 ‰ VPDB), which is evidence of microbially mediated
anaerobic oxidation of methane (AOM). This is further supported by next
generation DNA sequencing data, demonstrating the presence of AOM-related
microorganisms (ANMEs, archaea, sulfate-reducing bacteria) in the carbonate.
Embedded corals in some of the carbonates and CWC fragments exhibit less
negative δ13C values (-8.08 ‰ to -1.39 ‰ VPDB), pointing against the use of methane as the carbon source. Likewise,
the absence of DNA from methane- and sulfide-oxidizing microbes in sampled
coral does not support the idea of these organisms having a chemosynthetic lifestyle.
In light of these findings, it appears that the CWCs benefit rather indirectly
from hydrocarbon-rich seepage by using methane-derived authigenic carbonates
as a substratum for colonization. At the same time, chemosynthetic organisms
at active sites prevent coral dissolution and necrosis by feeding on the
seeping fluids (i.e., methane, sulfate, hydrogen sulfide), allowing
cold-water corals to colonize carbonates currently affected by
hydrocarbon-rich seepage.
Introduction
Cold-water corals (CWCs) are a widespread non-phylogenetic group of
cnidarians that include hard skeleton scleractinian corals, soft-tissue
octocorals, gold corals, black corals and hydrocorals (Roberts et al., 2006, 2009; Cordes et al., 2016). Typically, they thrive at low
temperatures (4–12 ∘C) and occur in water depths of ca.
50–4000 m. CWCs are azooxanthellate and solely rely on their nutrition as
an energy and carbon source (Roberts et al., 2009). Some scleractinian corals
(e.g., Lophelia pertusa, Madrepora oculata, Dendrophyllia cornigera, Dendrophyllia alternata,
Eguchipsammia cornucopia) are able to form colonies or even large carbonate mounds (Rogers et
al., 1999; Wienberg et al., 2009; Watling et al., 2011; Somoza et al.,
2014). Large vertical mounds and elongated ridges formed by episodic growth
of scleractinian corals (mainly Lophelia pertusa) are, for instance, widely distributed along
the continental margins of the Atlantic Ocean (Roberts et al., 2009). These
systems are of great ecological value since they offer sites for resting,
breeding and feeding for various invertebrates and fishes (Cordes et al.,
2016, and references therein).
Several environmental forces influence the initial settling, growth and
decline of CWCs. These include, among others, an availability of suitable
substrates for coral larvae settlement, low sedimentation rates,
oceanographic boundary conditions (e.g., salinity, temperature and density of
the ocean water) and a sufficient supply of nutrients through
topographically controlled current systems (Mortensen et al., 2001; Roberts
et al., 2003; Thiem et al., 2006; Dorschel et al., 2007; Dullo et al., 2008;
Van Rooij et al., 2011; Hebbeln et al., 2016). Alternatively, the
“hydraulic theory” suggests that CWC ecosystems may be directly fueled by
fluid seepage, providing a source of, e.g., sulfur compounds, nitrogen
compounds, P, CO2 and/or hydrocarbons (Hovland, 1990; Hovland and
Thomsen, 1997; Hovland et al., 1998, 2012). This relationship is supported
by the common co-occurrence of CWC mounds and hydrocarbon-rich seeps around
the world, for example at the Hikurangi Margin in New Zealand (Liebetrau et
al., 2010), the Brazilian margin (e.g., Gomes-Sumida et al., 2004), the Darwin
Mounds in the northern Rockall Trough (Huvenne et al., 2009), the Kristin
field on the Norwegian shelf (Hovland et al., 2012), the western Alborán
Sea (Margreth et al., 2011) and the Gulf of Cádiz (e.g.,
Díaz-del-Río et al., 2003; Foubert et al., 2008). However, CWCs
may also benefit rather indirectly from seepage. For instance,
methane-derived authigenic carbonates (MDACs) formed through the microbially
mediated anaerobic oxidation of methane (AOM, Suess and Whiticar, 1989;
Hinrichs et al., 1999; Thiel et al., 1999; Boetius et al., 2000; Hinrichs
and Boetius, 2002) potentially provide hard substrata for larval settlement
(e.g., Díaz-del-Rio et al., 2003; Van Rooij et al., 2011; Magalhães
et al., 2012; Le Bris et al., 2016; Rueda et al., 2016). In addition, larger
hydrocarbon-rich seepage-related structures such as mud volcanoes and
carbonate mud mounds act as morphological barriers favoring turbulent water
currents that deliver nutrients to the corals (Roberts et al., 2009;
Wienberg et al., 2009; Margreth et al., 2011; Vandorpe et al., 2016).
In the Gulf of Cádiz, most CWC occurrences are “coral graveyards” with
only a few living corals that are situated along the Iberian and Moroccan
margins. These CWC systems are typically associated with diapiric ridges,
steep fault-controlled escarpments and mud volcanoes (MVs) such as the Faro
MV, Hesperides MV, Meknes MV and mud volcanoes in the Pen Duick Mound Province (Foubert et al., 2008; Wienberg et al., 2009). Mud
volcanoes (and other conspicuous morphological structures in this region
such as pockmarks) are formed through tectonically induced fluid flow
(Pinheiro et al., 2003; Somoza et al., 2003; Medialdea et al., 2009;
León et al., 2010, 2012). The fluid flow is promoted through the of the
high regional tectonic activity and high fluid contents of sediments in this
area (mainly CH4 and, to a lesser extent, H2S, CO2, and
N2, Pinheiro et al., 2003; Hensen et al., 2007; Scholz et al., 2009;
Smith et al., 2010; González et al., 2012a). However, the exact influence
of fluid flow on CWC growth in this region remains elusive.
Bathymetric map of the study area. (a) Location of the Gulf of
Cádiz between Spain, Portugal and Morocco. The study area is marked with
a red star. (b) Pompeia Province including its different morphological
features. Red lines indicate remotely operated vehicle (ROV) paths, yellow stars mark sampling sites. (c) Detailed map of the Al Gacel MV including pathways of Dive 10 and 11 (black
and blue lines, respectively). Further details of the area are provided in
Figs. 2 and 3.
This study aims to elucidate the linkage between the present-day formation
of MDACs and CWCs development by testing whether CWCs are indeed
non-chemosynthetic fauna or, in fact, harbor chemosynthetic symbionts, which
allow them to consume some of the reduced compounds in sites of active
emission of under seafloor fluids. We address our hypothesis by the combined
analyses of high-resolution ROV underwater images, geophysical data (e.g.,
seabed topography, deep high-resolution multichannel seismic reflection
data) and sample materials (water analysis, petrographic features, δ13C and δ18O signatures of carbonates, lipid biomarkers
and environmental 16s rDNA sequences of the prokaryotic microbial
community). We focus our study in Pompeia Province (Fig. 1), which
encompasses mud volcanoes as the currently active Al Gacel MV (León et
al., 2012), diapiric coral ridges and mounds. Based on our findings, we
propose an integrated model to explain the spatiotemporal and genetic
relations between CWCs, chemosynthetic fauna and hydrocarbon-rich seepage in
the study area.
Materials and methods
This study is based on data and samples from Pompeia Province that were
collected during the SUBVENT-2 cruise in 2014 aboard the R/V Sarmiento de Gamboa (Fig. 1). In order to elucidate the spatiotemporal and genetic
relations between CWCs, chemosynthetic fauna and hydrocarbon-rich seepage in
this area, we explored geological features (mud volcanoes and coral ridges)
by means of underwater imaging and geophysical data. ROV dives were carried
out at the Al Gacel MV (D10 and D11) and the northern Pompeia Coral
Ridge (D03). Subsequently, we conducted detailed analyses on selected samples from
sites that were characterized by different types of seepage during sampling
(Table 1). Samples from the Al Gacel MV include authigenic carbonates
(D10-R3, D10-R7, D11-R8), porewater from the sediment (via micro-cores:
D10-C5, D10-C8, D11-C10) and water from above the seafloor (via Niskin
bottles: D10-N12, D11-N9). Furthermore, a scleractinian coral fragment was
recovered from the northern Pompeia Coral Ridge (D03-B1). All samples were
immediately stored at room temperature (petrographic analysis), 4 ∘C
(water, sediments and porewater analysis), -20∘C (stable isotopic analysis) or -80∘C (environmental DNA
analysis).
General description and characterization of recovered samples for
this study on the Al Gacel MV and northern Pompeia Coral Ridge. Please note
that samples D10-R3 and D11-R8 were carbonates with embedded corals (see Fig. 7 for more details).
Site descriptionCoordinatesDepth (m)TypeSampleBase of volcano characterized35∘26.51′ N850–890CarbonateD10-R3by non-chemosynthetic fauna-6∘58.22′ WActive pockmark35∘26.47′ N790CarbonateD10-R7-6∘58.27′ WWaterD10-N4Al Gacel MVD10-C5D10-C8Summit with metric35∘26.48′ N763CarbonateD11-R8carbonate blocks-6∘58.35′ W35∘26.48′ N760WaterD11-N9-6∘58.37′ WD11-C10Northern PompeiaSulfide-oxidizing bacterial mats35∘26.77′ N829Necrotic fragment of a livingD03-B1Coral Ridgeand shells of chemosynthetic bivalves-6∘59.94′ WMadrepora oculata coralGeophysical survey
Seabed topography of the studied sites was mapped by using an Atlas
Hydrosweep DS (15 kHz and 320 beams) multibeam echosounder (MBES).
Simultaneously, ultra-high-resolution sub-bottom profiles were acquired with
an Atlas Parasound P35 parametric chirp profiler (0.5–6 kHz). Deep
high-resolution multichannel seismic reflection data were obtained using an
array of seven Sercel GI guns (system composed of 4.1 + 2.46 + 1.8 + 0.74 L) with a total of 9.1 L. The obtained data were
recorded with an active streamer (SIG®16.3×40.175 with a 150 m
length of three sections of 40 hydrophones each). The shot interval was 6 s and the recording length 5 s two-way travel time (TWT). Data
processing (filtering and stacking) was performed onboard with Hot Shots
software.
Video survey and analysis
A remotely operated vehicle (ROV-6000 Luso, operated by EMEPC) was used for
photographic documentation (high-definition digital camera, 1024×1024 pixels)
and sampling. The ROV was further equipped with a STD/CTD-SD204 sensor
(in situ measurements of salinity, temperature, oxygen, conductivity, sound
velocity and depth), HydroC™ sensors (in situ measurements of CO2 and
CH4), Niskin bottles (CH4 concentrations, pH and redox potential
measurements) and a ROV core sampler (up to 16 cm).
Seawater and porewater analysis
Niskin water samples and micro-cores covering the water–sediment interface
were recovered from an active pockmark close to the summit of the Al Gacel
MV (D10-N4, D10-C5, D10-C8; the same site as carbonate sample D10-R7) as well as
directly from its summit (D11-N9, D11-C10). Redox potentials (ORPs) and
pH values of the water contained in the Niskin bottles were measured on-site
with Hanna portable instruments (HI 9025). Porewater from the micro-cores
was immediately extracted by centrifuging 10 cm thick slices of the
sediments. Upon extraction, the porewater was filtered with syringe filters
of cellulose acetate (0.2 µm pore), acidified with distilled nitric
acid (HNO3) and stored under 4 ∘C before further analysis.
Major and trace elements were subsequently measured with an Agilent 7500c
inductively coupled plasma mass spectrometer (ICP-MS). Method accuracy and
precision was checked by external standards (MIV, EPA, NASC, CASS). The
precision was better than 5 % RSD (residual standard deviation) and the
accuracy better than 4 %. Concentrations of S2- were measured with a
Hach–Lange DR 2800 spectrophotometer (cuvette test kit LCK 653).
Petrographic analysis
General petrographic analysis was performed on thin sections (ca. 60 µm
thickness) with a Zeiss SteREO Discovery.V8 stereomicroscope (transmitted
and reflected light) linked to an AxioCam MRc 5 Mp camera. Additional
detailed petrographic analysis of textural and mineralogical features was
conducted on polished thin sections (ca. 30 µm thickness) using a
DM2700P Leica Microscope coupled to a DFC550 digital camera. Carbonate
textures have been classified following Dunham (1962) and Embry III and Klovan (1971).
Stable isotope signatures (δ13C, δ18O) of
carbonates
Stable carbon and oxygen isotope measurements were conducted on ca. 0.7 mg
carbonate powder obtained with a high-precision drill (ø 0.8 mm). The
analyses were performed with a Thermo Scientific Kiel IV carbonate device
coupled to a Finnigan Delta Plus gas isotope mass spectrometer. Accuracy and
reproducibility were checked through the replicate analysis of a standard
(NBS19) and reproducibility was better than 0.1 ‰.
Stable carbon and oxygen isotope values are expressed in the standard
δ notation as per mill (‰) deviations relative
to Vienna Pee Dee Belemnite (VPDB).
Lipid biomarker analysisSample preparation
All materials used were pre-combusted (500 ∘C for > 3 h) and/or extensively rinsed with acetone prior to sample contact. A
laboratory blank (pre-combusted sea sand) was prepared and analyzed in
parallel in order to monitor laboratory contaminations.
The preparation and extraction of lipid biomarkers was conducted in
orientation with descriptions in Birgel et al. (2006). Briefly, the samples
were first carefully crushed with a hammer and internal parts were powdered
with a pebble mill (Retsch MM 301, Haan, Germany). Hydrochloric acid (HCl,
10 %) was slowly poured on the powdered samples that were covered with
dichloromethane (DCM)-cleaned water. After 24 h of reaction, the residues
(pH 3–5) were repeatedly washed with water and then lyophilized.
A total of 3 g of each residue was saponified with potassium hydroxide (KOH, 6 %) in
methanol (MeOH). The residues were then extracted with methanol (40 mL, 2×)
and, upon treatment with HCl (10 %) to pH 1, in DCM (40 mL, 2×) by using
ultra-sonification. The combined supernatants were partitioned in DCM vs.
water (3×). The total organic extracts (TOEs) were dried with sodium sulfate
(NaSO4) and evaporated with a gentle stream of dinitrogen (N2) to reduce loss
of low-boiling compounds (see Ahmed and George, 2004).
A total of 50 % of each TOE was separated over a silica gel column (0.7 g
Merck silica gel 60 conditioned with n-hexane: 1.5 cm i.d., 8 cm length) into
(a) hydrocarbon (6 mL n-hexane), (b) alcohol (7 mL DCM / acetone, 9:1, v:v) and
(c) carboxylic acid fractions (DCM / MeOH, 3:1, v:v). Only the hydrocarbons
were subjected to gas chromatography–mass spectrometry (GC–MS).
Gas chromatography–mass spectrometry (GC–MS)
Lipid biomarker analyses of the hydrocarbon fraction were performed with a
Thermo Scientific Trace 1310 GC coupled to a Thermo Scientific Quantum XLS
Ultra MS. The GC was equipped with a capillary column (Phenomenex Zebron
ZB-5MS, 30 m length, 250 µm inner diameter, 0.25 µm film
thickness). Fractions were injected into a splitless injector and
transferred to the column at 300 ∘C. The carrier gas was He at a
flow rate of 1.5 mL min-1. The GC oven temperature was ramped from
80 ∘C (1 min) to 310 ∘C at 5 ∘C min-1
(held for 20 min). Electron ionization mass spectra were recorded in full
scan mode at an electron energy of 70 eV with a mass range of m/z 50–600
and scan time of 0.42 s. Identification of individual compounds was based on
comparison of mass spectra and GC retention times with published data and
reference compounds.
Gas chromatography–combustion–isotope ratio mass spectrometer
(GC–C–IRMS)
Compound-specific δ13C analyses were conducted with a Trace GC
coupled to a Delta Plus IRMS via a combustion-interface (all Thermo
Scientific). The combustion reactor contained CuO, Ni and Pt and was
operated at 940 ∘C. The GC was equipped with two serially linked
capillary columns (Agilent DB-5 and DB-1, individually 30 m in length, 250 µm inner diameter, 0.25 µm film thickness). Fractions were injected into
a splitless injector and transferred to the GC column at 290 ∘C.
The carrier gas was He at a flow rate of 1.2 mL min-1. The temperature
program was identical to the one used for GC–MS (see above). CO2 with
known δ13C value and a standard (IAEA600) were used for
internal calibration. Instrument precision was checked using a mixture of
n-alkanes with known isotopic composition. Standard deviations of duplicate
sample measurements were generally better than 1.0 ‰.
Carbon isotope ratios are expressed as δ13C
(‰ ) relative to VPDB.
Amplicon sequencing of 16S rRNA genesDNA extraction and 16S rRNA gene amplification
Environmental DNA analyses of microbial communities were performed on a
carbonate sample with embedded corals from the base of the Al Gacel MV
(D10-R3), a carbonate sample from an active pockmark close to the summit of
the Al Gacel MV (D10-R7) and a necrotic fragment of a living Madrepora oculata recovered
from the northern Pompeia Coral Ridge (D03-B1). About 1–4 g of solid
samples were first mashed with mortar and liquid nitrogen into fine powder.
Three biological replicates were used per sample. Total DNA was isolated
with a Power Soil DNA Extraction Kit (MO BIO Laboratories, Carlsbad, CA).
All steps were performed according to the manufacturer's instructions.
Bacterial amplicons of the V3–V4 region were generated with the primer
set MiSeq_Bacteria_V3_forward
primer (5′-TCGTCGGCAGCGTCAGATGTGTATAAGAGACAGCCTACGGGNGGCWGCAG-3′) and
MiSeq_Bacteria_V4_reverse
primer (5′-GTCTCGTGGGCTCGGAGATGTGTATAAGAGACAGGACTACHVGGGTATCTAATCC-3′).
Likewise, archaeal amplicons of the V3–V4 region were generated with the
primer set MiSeq_Archaea_V3_forward primer
(5′-TCGTCGGCAGCGTCAGATGTGTATAAGAGACAG-GGTGBCAGCCGCCGCGGTAA-3′) and
MiSeq_Archaea_V4_reverse primer
(5′-GTCTCGTGGGCTCGGAGATGTGTATAAGAGACAG-CCCGCCAATTYCTTTAAG-3′). A total of 50 µL
of the polymerase chain reaction (PCR) mixture for bacterial DNA amplification, contained 1?U
Phusion high-fidelity DNA polymerase (Biozym Scientific, Oldendorf,
Germany), 5 % dimethyl sulfoxide (DMSO), 0.2 mM of each primer, 200 µM
deoxynucleotides (dNTPs) solution mix, 0.15 µL
of 25 mMMgCl2 and 25 ng of isolated DNA. The PCR protocol for
bacterial DNA amplification included (i) initial denaturation for 1 min at
98 ∘C; (ii) 25 cycles of 45 s at 98 ∘C, 45 s at
60 ∘C, and 30 s at 72 ∘C; and (iii) a final extension
at 72 ∘C for 5 min. The PCR reaction mixture for archaeal DNA
amplification was similarly prepared but contained instead 1 µL of
25 mMMgCl2 and 50 ng of isolated DNA. The PCR protocol for archaeal
DNA amplification included (i) initial denaturation for 1 min at
98 ∘C; (ii) 10 cycles of 45 s at 98 ∘C, 45 s at
63 ∘C, and 30 s at 72 ∘C; (iii) 15 cycles of 45 s at
98 ∘C, 45 s at 53 ∘C, and 30 s at 72 ∘C;
and (iv) a final extension at 72 ∘C for 5 min.
PCR products were checked by agarose gel electrophoresis and purified using
the GeneRead Size Selection Kit (QIAGEN GmbH, Hilden, Germany).
Data analysis and pipeline
Illumina PE sequencing of the amplicons and further processing of the sequence
data was performed in the Göttingen Genomics Laboratory (Göttingen,
Germany). After Illumina MiSeq processing, sequences were analyzed as
described in Egelkamp et al. (2017) with minor modifications. In brief,
paired-end sequences were merged using PEAR v0.9.10 (Zhang et al., 2014);
sequences with an average quality score below 20 and containing unresolved
bases were removed with QIIME 1.9.1 (Caporaso et al., 2010). Non-clipped
reverse- and forward-primer sequences were removed by employing cutadapt 1.15
(Martin, 2011). USEARCH version 9.2.64 was used following the UNOISE
pipeline (Edgar, 2010). In detail, reads shorter than 380 bp were removed,
de-replicated and denoised with the UNOISE2 algorithm of USEARCH resulting
in amplicon sequence variants (ASVs) (Callahan et al., 2017). Additionally,
chimeric sequences were removed using UCHIME2 in reference mode against the
SILVA SSU database release 132 (Yilmaz et al., 2014). Merged paired-end
reads were mapped to chimera-free ASVs and an abundance table was created
using USEARCH. Taxonomic classification of ASVs was performed with BLAST
against the SILVA database 132. Extrinsic domain ASVs, chloroplasts and
unclassified ASVs were removed from the dataset. Sample comparisons were
performed at the same surveying effort, utilizing the lowest number of sequences
by random subsampling (20 290 reads for bacteria, 13 900 reads for archaea).
The paired-end reads of the 16S rRNA gene sequencing were deposited in the
National Center for Biotechnology Information (NCBI) in the Sequence Read
Archive SRP156750.
ResultsPompeia Province – geological settings
Pompeia Province is situated in the Gulf of Cádiz offshore of Morocco,
within the so-called middle Moroccan Field (Ivanov et al., 2000) at
water depths between 860 and 1000 m (Fig. 1). It encompasses the active Al
Gacel MV (Fig. 1c), another mud volcano that is extinct (further referred
as extinct MV) and two east–west elongated ridges (northern Pompeia Coral
Ridge and southern Pompeia Coral Ridge). CWCs occur on all of these
morphological features and scattered coral mounds surround the ridges with a
smooth relief (Fig. 1b). Detailed geological profiles and 3-D images of
these features are shown in Figs. 2 and 3.
Bathymetric and seismic maps showing morphological features in
northern Pompeia Province. (a–b) Bathymetric maps showing the Al Gacel MV,
the northern Pompeia Coral Ridge and the extinct MV. Yellow stars mark
sampling sites. (c) Ultra-high seismic profile of the Pompeia Escarpment,
west of the northern Pompeia Coral Ridge.
The Al Gacel MV is a cone-shaped structure, 107 m high and 944 m wide, with
its summit at 762 m depth and surrounded by a 11 m deep rimmed depression
(León et al., 2012) (Fig. 1c). It is directly adjacent to the northern
Pompeia Coral Ridge (Fig. 2a–b), which extends ca. 4 km in westward
direction (Fig. 2a–b) and it is terminated by the Pompeia Escarpment
(Figs. 1b and 2c). High-resolution seismic profiles of the Pompeia
Escarpment show CWC build-ups (R1 to R4) with steep lateral scarps of ca. 40 m height (Fig. 2c). The Al Gacel MV is of subcircular shape and exhibits
a crater at its top (Fig. 2a–b).
Ultra-high-resolution (a) and multichannel (b) seismic profiles
showing geological features in southern Pompeia Province. Note mud diapirism
has been described in this area (Vandorpe et al., 2017). OP represents the overpressure
zone.
Ultra-high-resolution sub-bottom seismic profiles crossing Pompeia
Province from the northwest (NW) to the southeast (SE) (Fig. 3a) show (i) the Al
Gacel MV surrounded by bottom-current deposits, (ii) an up to 130 m high CWC
framework growing on top the southern Pompeia Coral Ridge and (iii) semi-buried CWC mounds surrounding the ridge in areas of low relief. These
CWC mounds locally form smooth, up to 25–30 m high topographic reliefs
that are exposed but then taper downward below the seafloor (applying sound
speeds of 1750 m s-1 in recent sediments). Additionally, a multichannel
seismic profile following the same track but with higher penetration below
the seafloor (Fig. 3b) shows high-amplitude reflections inside the Al
Gacel cone and enhanced reflections at the top of the diapirs (dotted yellow line in Fig. 3b), pointing to the occurrence of
hydrocarbon-charged sediments. It furthermore exhibits breaks in seismic
continuity and diapiric structures at different depths below the southern
Pompeia Coral Ridge and the Al Gacel MV, showing the presence of a fault
system (Fig. 3b). These tectonic structures may promote the development of
overpressure areas (OP in Fig. 3b) and consequent upward fluid flow to the
surface.
ROV observation and measurements
Submersible ROV surveys at the Al Gacel MV (Fig. 1c) revealed the presence
of dispersed pockmark depressions at the eastern (Dive 10, 790 m) and
northern flanks (Dive 11, 760–825 m depth). These sites are characterized
by focused but low-intensity seafloor bubbling (e.g., Figs. 4b and 5a).
Analysis of water samples revealed CH4 concentration up to 171 nM
during Dive 10 and up to 192 nM during Dive 11 (Sánchez-Guillamón et
al., 2015).
ROV still frames from the Al Gacel MV (Dives 10 and 11). (a) The eastern side of the volcano, displaying a field of sponges, corals and
carbonates. (b–c) Active pockmark sites on the east side of the volcano,
displaying authigenic carbonate surrounded by shells of chemosynthetic
bivalves, fragments of scleractinian and octocorals, and
sulfide-oxidizing bacterial mats. (d) Meter-sized carbonate blocks located
on a slope at the summit of the volcano.
Pockmarks are typically characterized by olive-grey mud-breccia sediments
and authigenic carbonates appearing in the center and edges. The authigenic
carbonates are commonly associated with typical methane-seep-related
organisms (e.g., sulfide-oxidizing bacterial mats, chemosynthetic bivalves,
siboglinid tubeworms) (Figs. 4b–c and 5). Communities of
non-chemosynthetic organisms (e.g., sponges, corals) were also found at
pockmarks (Figs. 4b–c and 5c) but were more abundant in places where
no seepage was detected (Fig. 4a).
ROV still frames from the active pockmark site shown in Fig. 4b.
(a–b) Release of bubbles while sampling. (c) Detailed photograph of the
octocorals on top of the carbonate. (d) Detailed still frame from siboglinid
worms beneath the carbonate.
Observations with the submersible ROV at the northern Pompeia Coral Ridge
and the extinct MV (Dive 03) revealed widespread and abundant occurrences of
dead scleractinian corals (mainly Madrepora oculata and Lophelia pertusa) currently colonized by a few living
non-chemosynthetic organisms (e.g., Corallium tricolor, other octocorals, sea urchins) (Fig. 6b–d). Locally,
grey-black colored patches of sulfide-oxidizing bacterial
mats surrounded by dead chemosynthetic bivalves (Lucinoma asaphus and Thyasiravulcolutre) were
observed (Fig. 6a). CH4 seepage appeared to be lower than at the Al Gacel MV, with
concentrations of 80–83 nM.
ROV still frames from the northern Pompeia Coral Ridge and extinct
MV (Dive 03), where there is currently a diffused seepage of fluids.
(a) Abundant shells of chemosynthetic bivalves with sulfide-oxidizing bacterial
mats at the western site of the northern Pompeia Coral Ridge. b–d) Field of
dead scleractinian corals colonized by living corals. (d) Still frame from
the extinct MV.
Water parameters display homogenous values between the four sampling sites
(10 ∘C temperature, ca. 52 %–55 % dissolved oxygen, ca. 31 Kg m-3 density) (Table 2). At depths of 790 m (D10-N4, the same site as
carbonate D10-R7) and 760 m (D11-N9), the pH of seawater was 7.88 and 7.85,
respectively (Table 3). The same seawater samples exhibited ORP values of
136 mV (D10-N4) and 257 mV (D11-N9) (Table 3). Further analysis of these
seawater samples revealed Fe2+ concentration of 0.57 and 0.31 µM, while S2-
values were nearly absent (below detection limit) (Table 2). Fe2+ concentrations in porewaters ranged between
0.94 and 1.27 µM (D10-C5), 2.70 and 1.74 µM (D10-C8), and 2.39 and 5.32 µM (D11-C10). S2- concentrations in porewaters were below
detection limit (D10-C5), 50.23 µM (D10-C8) and 0.47 µM
(D11-C10) (Table 3).
In situ water variables measured during sampling with ROV sensors.
On-site measurements of soluble Fe2+ and S2+ values from
seawater and porewater. Please note that samples D10-C5, D10-C8 and D10-N4
were taken from the same site as the authigenic carbonate D10-R7 (see Fig. 2).
Petrography and stable isotopes signatures of carbonates (δ18O, δ13C)
Sample D10-R3 derives from a field of carbonates at the base of the Al Gacel
MV that is inhabited by sponges and corals (Fig. 4a). The sample is a
framestone composed of deep water scleractinian corals (Madrepora and
rarely
Lophelia) (Fig. 7a–b). The corals are typically cemented by microbial automicrite
(sensu Reitner et al., 1995) followed by multiple generations of aragonite. A
matrix of dark allomicrite (sensu Reitner et al., 1995) with oxidized framboidal
pyrites and remains of planktonic foraminifera is restricted to few
bioerosional cavities (ca. 5%) in the skeletons of dead corals (Fig. 8a–b). δ13C signatures of the matrix and cements range from
-26.68 ‰ to -18.38 ‰, while the embedded coral
fragments exhibit δ13C values between -5.58‰ and -2.09 ‰ (Fig. 7b, Table 4). The δ18O values
generally range from +2.35 ‰ to +3.92 ‰ (Fig. 9, Table 4).
Stable carbon and oxygen isotopes (δ13C, δ18O) of samples from the Al Gacel MV and the northern Pompeia Coral
Ridge.
Photographs of analyzed samples including sampling sites for
stable carbon and oxygen isotope (δ13C, δ18O)
analysis (crosses with numbers). Values of the stable isotopic analyses are
found in Table 2. (a–b) D10-R3 carbonate with embedded corals (c–d) D10-R7
carbonate with strong H2S odor. (e–f) D11-R8 carbonate with embedded
corals. (g) D03-B1 scleractinian-coral fragment, Madrepora oculata. Please note that we cannot
determine whether the corals were alive or dead at the time they were buried by
the carbonate.
Thin section photographs of MDACs. (a–b) D10-R3 consisting of a
micritic matrix with scattered foraminifers and oxidized framboidal pyrites
(reflected light). (c–d) D10-R7 consisting of micritic and micro-sparitic
carbonate with abundant unaltered framboidal pyrites (c transmitted
light and
d reflected light). Please note the open voids that represent potential
pathways for fluid seepage (yellow circle in c).
Sample D10-R7 was recovered from a pockmark on the eastern site of the Al
Gacel MV that is virtually influenced by active seepage (Fig. 3c). It
consists of black carbonate and exhibits a strong hydrogen sulfide
(H2S) odor (Figs. 5b and 7c–d). The top of this sample was
inhabited by living octocorals (Fig. 5c), while chemosymbiotic siboglinid
worms were present on the lower surface (Fig. 5d). The sample is
characterized by a grey peloidal wackestone texture consisting of
allomicrite with abundant planktonic foraminifers and few deep water
miliolids. The sample furthermore exhibits some fractured areas, which are
partly filled by granular and small fibrous cement, probably consisting of
Mg calcite. Locally, light brownish crusts of microbial automicrite similar
to ones in D10-R3 are present (see above). Framboidal pyrite is abundant and
often arranged in aggregates (Fig. 8c–d). The carbonate exhibits δ13C values ranging from -28.77 ‰ to -21.13 ‰ and
δ18O values from +2.37 ‰ to +3.15 ‰ (Fig. 9, Table 4).
Stable carbon and oxygen isotopes (δ13C, δ18O) of samples from the Al Gacel MV and the northern Pompeia Coral
Ridge (see Table 3 and Fig. 7 for precise sampling points).
Sample D11-R8 comes from an area with meter-sized carbonate blocks at the
summit of the Al Gacel MV and is mainly colonized by sponges and serpulid
worms (Fig. 4d). The sample generally exhibits a light grey mud–wackestone texture consisting of allomicrite with few scleractinian-coral
fragments and planktonic foraminifers (Fig. 7e–f). The carbonate
furthermore contains abundant quartz silt and, locally, pyrite enrichments.
A further prominent feature is the voids that are encircled by dark grey halos
and exhibit brownish margins (due to enrichments of very small pyrite
crystals and organic matter, respectively). δ13C signatures of
the matrix and cements range from -14.82 ‰ to -14.74 ‰, while embedded coral fragments exhibit δ13C
values of -4.91 ‰ to -2.99 ‰ (Fig. 7f, Table 4). δ18O
values generally range from +1.49 ‰ to +5.60 ‰ (Fig. 9, Table 4).
Sample D03-B1 is a necrotic fragment of a living scleractinian coral
(Madrepora oculata) recovered from the northern Pompeia Coral Ridge (Figs. 6d and 7g).
The coral carbonate exhibits δ13C values ranging from -8.08 ‰ to -1.39 ‰ and
δ18O values from -0.31 ‰ to +2.26 ‰ (Fig. 9, Table 4).
Lipid biomarkers and compound-specific carbon isotope signatures
The hydrocarbon fractions of the carbonate recovered from the active
pockmark (D10-R7) mainly consist of the irregular, tail-to-tail linked
acyclic isoprenoids 2,6,11,15-tetramethylhexadecane (C20, crocetane),
2,6,10,15,19-pentamethylicosane (C25, PMI), as well as of several
unsaturated homologues of these compounds (Fig. 10). Additionally, it
contains the regular, head-to-tail linked acyclic isoprenoid pristane
(C19).
Total ion current (TIC) chromatograms of the analyzed samples.
Isotopically depleted acyclic irregular isoprenoids such as Cr and PMI are
typically found in settings influenced by the anaerobic oxidation of methane
(AOM). Pr is pristane; Ph is phytane; Cr is crocetane; PMI is 2,6,10,15,19-pentamethylicosane; dots are n-alkanes; and crosses are siloxanes
(septum or column bleeding). Percentage values given on the vertical axes of
chromatograms relate peak intensities to highest peak (Cr in D10-R7).
The hydrocarbon fraction of the carbonate recovered from the summit of the
Al Gacel MV (D11-R8) is dominated by n-alkanes with chain-lengths ranging
from C14 to C33 (maxima at n-C16 and, subordinated, at
n-C20 and n-C31) (Fig. 10). The sample further contains pristane, a
mixture of crocetane and the head-to-tail linked acyclic isoprenoid phytane
(C20) (co-eluting), as well as traces of PMI.
In the carbonate from the active pockmark (D10-R7), crocetane and PMI
exhibited strongly depleted δ13C values (-101.2 ‰ and -102.9 ‰, respectively). In
the carbonate from the summit of the volcano (D11-R8), crocetane–phytane and
PMI showed less depleted δ13C values (-57.2 ‰ and -74.3 ‰, respectively).
δ13C values of n-alkanes in the carbonate D11-R8
(n-C17-22) ranged between -30.8‰ and -33.0 ‰ (Table 5).
Stable carbon isotopic composition (δ13C) of selected
lipid biomarkers (in Fig. 10).
a n.d. means not detected. b Please note that crocetane in D11-R8
coelutes with phytane.
DNA inventories (MiSeq Illumina sequences)
Bacterial DNA from samples D10-R3 (authigenic carbonate, base of the Al
Gacel MV) and D03-B1 (Madrepora oculata fragment, northern Pompeia Coral Ridge) mainly derives
from taxa that typically thrive in the water-column (e.g., Actinobacteria, Acidobacteria, Chloroflexi, Bacteroidetes, Woeseiaceae, Dadabacteria,
Kaiserbacteria, Poribacteria, Planctomycetes, Gemmatimonadetes) (Fig. 11a). The sample D10-R3 furthermore contains bacterial DNA of the
nitrite-oxidizing bacteria Nitrospira sp., while the sample D03-B1 contains DNA of the
bacterial taxa Verrucomicrobia, Enterobacteria and Nitrosococcus. Note that one
amplicon sequence variant (ASV_189) with a low number of
clustered sequences has been found in D03-B1, identified as a methanotrophic
symbiont of Bathymodiolus mauritanicus (see Rodrigues et al., 2013).
Bar chart representing relative abundances of prokaryotic taxa
detected in each sample: (a) bacterial taxa and (b) archaeal taxa. In “others”
an aggrupation of included taxa is related to ubiquitous organisms normally found
in sea- and seepage-related environments and unclassified organisms. The number
of reads per taxa is detailed in Table S1 (bacteria) and Table S2 (archaea).
Up to 50 % of bacterial DNA in sample D10-R7 (authigenic carbonate, top
of the Al Gacel MV) derive from taxa that are commonly associated with
fluid seepage and AOM, i.e., sulfide-oxidizing bacteria, sulfate-reducing
bacteria (SRB) and methane-oxidizing bacteria. The most abundant are SRB
taxa such as SEEP-SRB1, SEEP-SRB2, Desulfatiglans, Desulfobulbus and Desulfococcus, which typically form consortia with
ANME archaea.
Archaeal DNA (Fig. 11b) from samples D10-R3 and D03-B1 mainly consist of
Cenarchaeum sp., which represents 70 %–90 %. Candidatus Nitrosopumilus is the second most abundant in both
samples, representing 5 %–20 %. In contrast, around 90 % of archaeal
DNA in D10-R7 is related to ANME-1 and ANME-2 groups, in good concordance
with the relative abundances of SRB DNA.
Details of the number of reads per taxa are shown in Tables S1 and S2 in the Supplement.
DiscussionEvidence for hydrocarbon-rich seepage affecting Pompeia Province
Two-dimensional multichannel seismic images show that Pompeia Province
is affected by fluid expulsion related to compressional diapiric ridges and
thrust faults (Fig. 3b), as has been reported from other areas of the
Gulf of Cádiz (Somoza et al., 2003; Van Rensbergen et al., 2005;
Medialdea et al., 2009). There seems to be different types of fault–conduit
systems that link the overpressure zones (OP) with the seafloor (Fig. 3b),
controlling both the type and rate of seepage (e.g., eruptive, focused,
diffused or intermittent, the latter referred to as “dripping-like” in the
following). At the Al Gacel MV, conduits are, for instance, mainly linked to
faults and a dense hydro-fracture network, allowing the migration of
hydrocarbon-rich muds from the overpressure zone to the surface. During
active episodes, eruptions lead to the formation of mud-breccia flows as
observed in gravity cores (e.g., León et al., 2012). During rather
dormant episodes, focused and dripping-like seepage predominates, forming
pockmark features (Fig. 4b).
Currently, the Al Gacel MV is affected by continuous and focused
dripping-like seepages. These sites of active seepage are characterized by
carbonates that are suspected to be methane-derived (e.g., sample D10-R7,
Fig. 4b–c). In situ ROV measurements and subsequent water sample analysis
demonstrated high concentrations of CH4 in fluids that were escaping
upon removal of the carbonate D10-R7 from the active pockmark (171 nM, Fig. 5a) (Sánchez-Guillamón et al., 2015). This association suggests a
genetic relationship between hydrocarbon-rich seepage and the carbonate, as
also reflected in the low δ13C signatures of the carbonates analyzed
herein (down to ca. -30 ‰, Fig. 9, Table 3). Indeed,
the grey peloidal texture of this sample resembles that of AOM-derived
automicrites from the Black Sea that are related to micro-seepage of methane
(see Reitner et al., 2005). The isotopically depleted acyclic
isoprenoids observed here, such as crocetane and PMI (δ13C values between ca.
-103 ‰ and -57 ‰; see Fig. 10, Table 4), are typical
fingerprints of AOM-associated Archaea (Hinrichs et al., 1999; Thiel et al.,
1999, 2001; Peckmann et al., 2001; Peckmann and Thiel, 2004), which is also
in good accordance with the high abundance of DNA related to ANME. At the
same time, elevated concentrations of S2- and Fe2+ in porewaters
of D10-C8 micro-core (0.23 and 1.74 µM, respectively; see Table 2), abundant framboidal pyrite (Fig. 8c–d) and SRB-related DNA in the
carbonate (Fig. 11) evidence microbial sulfate reduction in the environment.
All these data clearly demonstrate that the carbonates have been formed via
AOM, fueled by fluids from the underlying mud diapir.
Other carbonate samples from the Al Gacel MV (i.e., D10-R3 and D11-R8)
have probably also been formed due to AOM, as they are isotopically depleted
as well (δ13C values between ca. -25 ‰ and -15 ‰; see Fig. 9 and Table 3). However, no active gas bubbling
was observed during sampling, even though both samples still contain open
voids that could form pathways for fluids. Several characteristics of these
voids (e.g., dark halos formed by pyrite, brownish margins due to organic
matter enrichments) are very similar to those of methane-derived carbonate
conduits (see Reitner et al., 2015). This could imply that the intensity of
hydrocarbon-rich seepage and consequently AOM may have fluctuated through
time. This in good accordance with the relatively low dominance of crocetane
and PMI in a carbonate sampled from the summit of Al Gacel MV (D11-R8; see Fig. 10). The moderately depleted δ13C values of crocetane/phytane
and PMI in this sample (-57.2 ‰ and -74.3 ‰, respectively; see Table 4) could be due to mixing
effects and are thus also in agreement with varying intensities of AOM in
the environment. The presence of only a few AOM-related DNA sequences (Fig. 11) and partly oxidized pyrites in the carbonate D10-R3 from the base of the
Al Gacel MV (Fig. 8a–b) are well in line with this scenario.
There is no evidence for eruptive extrusions of muddy materials at the coral
ridges. In the southern Pompeia Coral Ridge (Fig. 3), diapirs appear to
promote an upward migration of hydrocarbon-rich fluids in a divergent
way throughout a more extensive seabed area. This results in a continuous
and diffused seepage, which promotes the occurrence of AOM and the formation
of MDACs at the base of the ridges, related to the sulfate–methane
transition zone (SMTZ) (Boetius et al., 2000; Hinrichs and Boetius, 2002;
González et al., 2012a). This is in good accordance with the detection
of methane (80–83 nM) at the northern Pompeia Coral Ridge and the
presence of sulfide-oxidizing bacterial mats and shells of dead
chemosynthetic bivalves on the western part of the ridge (Fig. 6a).
Likewise, the CWC mound field surrounding the southern Pompeia Coral Ridge
(Fig. 3) is thoroughly characterized by micro-seeps, due to ascending fluids
from OPs through low-angle faults. This type of focused seepage may promote
formation of MDAC pavements in deeper layers of the sediments (Fig. 3),
similar to coral ridges along the Pen Duick Escarpment (Wehrmann et al.,
2011). The generation of MDAC hotspots at sites of such seepage also explain
the geometry of the downward tapering cones (Fig. 3).
Ecological meaning of hydrocarbon-rich seepage for CWCs
Our data suggests contemporaneous micro-seepage and CWC growth in Pompeia Province (e.g., Fig. 4b). This relationship has also been observed
elsewhere, e.g., in the North Sea offshore of central Norway (Hovland, 1990; Hovland and Thomsen, 1997) and on the Angola margin (Le Guilloux et al., 2009).
Corals utilize HCO3- deriving from both the environment and the
internal production of CO2 for skeleton biomineralization (Swart, 1983;
Zoccola et al., 2015; Nakamura et al., 2018). Hence, a potential utilization
of methane as a carbon source should be reflected in the δ13C signatures of their skeletons. However, scleractinian fragments recovered
from the Al Gacel MV (embedded in carbonates D10-R3 and D11-R8 from the
base and summit of the volcano, respectively) and the northern Pompeia Coral
Ridge (D03-B1, a necrotic part of a living Madrepora oculata) displayed barely depleted δ13C values (ca. -8 ‰ to -1 ‰; see Fig. 9 and Table 3),
close to the δ13C of marine seawater (0±3 ‰, e.g., Hoefs, 2015). These values do not support a
significant uptake of methane-derived carbon by the CWCs and thus a direct
trophic dependency as previously proposed (Hovland, 1990). Furthermore, the
only DNA in sample D03-B1 that could be attributed to a potential
methanotrophic endosymbiont (ASV_189, Rodrigues et al., 2013)
occurred in minor amounts and most likely represents contamination from the
environment or during sampling. Therefore, it appears more likely that the
CWCs feed on a mixture of phytoplankton, zooplankton and dissolved organic
matter as previously proposed for ones in other regions (Kiriakoulakis et
al., 2005; Duineveld et al., 2007; Becker et al., 2009; Liebetrau et al.,
2010). This is in good accordance with the presence of DNA from various
common archaeal and bacterial taxa (e.g., Acidobacteria, Actinobacteria,
Candidatus Nitrosopumilus, Cenarchaeum sp.) and some potential members of the corals' holobiont
(e.g.,
Enterobacteria, Verrucomicrobia, Nitrosococcus sp.) (Sorokin, 1995; Rädecker et al.,
2015; Webster et al., 2016) in sample D03-B1 (Fig. 11). Taken together,
there is no evidence that CWCs in the working area harbor microbial
symbionts that could potentially utilize the hydrocarbon-rich fluids.
However, future analyses on living coral tissue will be important to verify
this conclusion.
CWC development and hydrocarbon-rich seepage appear to be rather linked
via the formation of MDAC deposits, which provide the hard substrata needed for
CWC larval settlement (e.g., Díaz-del-Rio et al., 2003; Van Rooij et
al., 2011; Magalhães et al., 2012; Le Bris et al., 2016; Rueda et al.,
2016). However, if it is too severe, fluid flow and associated metabolic processes
can result in local conditions that are lethal to CWCs (see Sect. 4.3). Moreover,
AOM fueled by fluid flow can also cause an entombment of the CWCs by MDACs
(Wienberg et al., 2009, Wienberg and Titschack, 2015), as observed in
D10-R3 and D11-R8 carbonates from the Al Gacel MV (Figs. 7 and 9, Tables 3 and 4). It is therefore not surprising that large CWC systems in Pompeia
Province are always linked to structures that are affected by rather mild
noneruptive seepage (i.e., the extinct MV, the coral ridges and the CWC
Mound Fields; see Figs. 3 and 6). The observation that these systems are in
large part coral graveyards (Fig. 6b–d) similar to other areas in
the Gulf of Cádiz (see Foubert et al., 2008; Wienberg et al., 2009) may
be explained by a post-glacial decrease in current strength (Foubert et al.,
2008). In the light of our findings, however, they could also have been
negatively affected by periods of intensive seepage during higher tectonic
activity. Future studies are important to test this hypothesis in greater
detail.
Spatiotemporal coexistence of CWCs and chemosynthetic organisms –
the buffer effect
As discussed above, MDAC deposits are ecologically beneficial for CWCs, as
they serve as optimal substrata even when seepage is still present (e.g.,
Hovland, 1990; Hovland and Thomsen, 1997; Le Guilloux et al., 2009; this
study). Severe hydrocarbon-rich seepage, however, is ecologically stressful
for the corals. Particularly, fluid- and AOM-derived hydrogen sulfide is
considered problematic because of its role in coral necrosis (Myers and Richardson, 2009; García et al., 2016) and carbonate dissolution
effects (Wehrmann et al., 2011). Corals appear to be physiologically
tolerant to various environmental stressors, such as low oxygen
concentrations and acidification (e.g., Dodds et al., 2007; Form and Riebesell, 2012; McCulloch et al., 2012; Movilla et al., 2014).
Furthermore, hydrogen sulfides can be efficiently buffered through the
reaction with Fe-(oxyhydro)-oxides or Fe2+ dissolved in porewaters,
ultimately forming pyrite (Wehrmann et al., 2011). It appears that the
combination of these ecological capabilities plus certain environmental
factors allows CWCs to thrive in areas affected by hydrocarbon seepage.
Fe-(oxyhydro)-oxide nodules have previously been observed in the Iberian
and Moroccan margins (González et al., 2009, 2012b) but not in Pompeia Province. Instead, sulfide-oxidizing bacteria living in symbiosis
with invertebrates (e.g., siboglinid worms, Petersen and Dubilier, 2009)
(Fig. 5d) and thriving in mats (Figs. 4c and 6a) were particularly
prominent. These microbes withdraw reduced sulfur species through their
metabolic activity, thus forming a biological buffer. Likewise, microbially
mediated AOM substantially increases carbonate alkalinity at active sites,
thereby providing a buffer against acidification on a local scale (e.g., in
the active pockmark from the Al Gacel MV where seawater pH was 7.85; see
Sect. 3.2). We propose that such biological buffers provide a further
ecological linkage between hydrocarbon-rich seepage and cold-water corals
along Pompeia Province (“buffer effect model”; see Fig. 12). The impact
and exact capacity of this biological buffer, however, remains elusive and
must be evaluated in future studies.
The buffer effect model. (a) Buffer effect at pockmark sites
(e.g.,
sampling site of D10-R7) where carbonates are formed directly on the
bubbling site acting as a cap. (b) Buffer effect at diapiric ridges where
MDAC slabs are formed on the base of the ridge. (c) Buffer effect at coral
mounds where MDAC slabs are formed in deeper layers of the sediment. Py is pyrite, SMTZ is the sulfur-methane
transition zone.
Conclusions
Cold-water coral occurrences in Pompeia Province (Gulf of Cádiz) are
typically linked to hydrocarbon-seeping structures like mud volcanoes and
diapirs. The irregular topography of these structures affects bottom
water currents that supply nutrients to the corals. A further ecological
benefit is the seepage-fueled formation of authigenic carbonates, which
provide ideal substrates for coral larvae settlement. Cold-water corals
therefore indirectly take advantage of seepage-related conditions, instead
of feeding from the seeped fluids, such as sulfide and methane. However,
increased fluid seepage appears to be ecologically disadvantageous as
evidenced by corals embedded in some of the carbonates. Consequently,
cold-water coral growth in these habitats depends directly on seepage
intensity and how these fluids are drained onto the seafloor (i.e., eruptive,
focused, diffused or dripping-like). Cold-water coral growth appears to be
furthermore supported by the microbially mediated removal of seepage-related
toxic substances (e.g., reduced sulfur species through sulfide-oxidizing
bacteria) and shaping of environmental conditions (e.g., pH buffering
through AOM). This biological buffer is possibly crucial to keep conditions
favorable for the growth of cold-water corals in the studied area,
particularly in times of increased fluid seepage.
Data availability
All data used in this study are available on the SUBVENT-2 cruise
report in the Geological Survey of Spain archive
(http://info.igme.es/SidPDF/166000/941/166941_0000001.pdf, last access: 3 April 2019) (IGME, 2019) and in the Sequence
Read Archive (SAR) SRP156750 stored in the National Center for Biotechnology
Information (NCBI, 2019) (https://www.ncbi.nlm.nih.gov/sra/SRP156750, last access: 3 April 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-16-1607-2019-supplement.
Author contributions
BRT, DS and MH carried
out the microbial analysis. JPD carried out the biomarker
analysis. LS and TM processed seismic and bathymetric
data. PM processed ROV data. JG and JR carried out the petrographic analysis. ES and ELP carried out the porewater and seawater analysis. JR carried out the stable isotopic analysis. BRT
prepared the manuscript with contributions from all
co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors thank the captain and the crew onboard the R/V Sarmiento de Gamboa, as well as the UTM (Unidad de Tecnología Marina), who have been
essential for the success of this paper. Data obtained onboard are collected
in the SUBVENT-2 cruise, which can be found in the IGME archive. This work
was supported by the Spanish project SUBVENT (CGL2012-39524-C02) and the
project EXPLOSEA (CTM2016-75947) funded by the Spanish Ministry of Science,
Innovation and Universities. This open-access
publication was funded by the University of Göttingen.
Review statement
This paper was edited by Clare Woulds and reviewed by two
anonymous referees.
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