Is the distribution of Prochlorococcus and Synechococcus ecotypes in the Mediterranean Sea affected by global warming?
1CNRS, Observatoire Océanologique, UMR7144, Groupe Plancton Océanique, 29680 Roscoff, France
2UPMC-Université Paris 06, Station Biologique, Place Georges Teissier, 29680 Roscoff, France
3School of Life Sciences, University of Warwick, Coventry CV4 7AL, UK
4Dept. Chemistry and Biomolecular Science, Macquarie University, North Ryde, NSW 2109, Australia
5CNRS/INSU and UPMC-Université Paris 06, Laboratoire Arago, UMS2348, Observatoire Océanologique, 66651 Banyuls-sur-mer, France
6CNRS and UPMC-Université Paris 06, UMR7093, Laboratoire d'Océanographie de Villefranche, 06234 Villefranche-sur-mer, France
7Laboratoire d'océanographie physique et biogéochimique, Centre d'océanologie de Marseille, case 901, campus de Luminy, 13288 Marseille cedex 09, France
*These two authors contributed equally to this work
Abstract. Biological communities populating the Mediterranean Sea, which is situated at the northern boundary of the subtropics, are often claimed to be particularly affected by global warming. This is indicated, for instance, by the introduction of (sub)tropical species of fish or invertebrates that can displace local species. This raises the question of whether microbial communities are similarly affected, especially in the Levantine basin where sea surface temperatures have significantly risen over the last 25 years (0.50 ± 0.11 °C in average per decade, P < 0.01). In this paper, the genetic diversity of the two most abundant members of the phytoplankton community, the picocyanobacteria Prochlorococcus and Synechococcus, was examined during two cruises through both eastern and western Mediterranean Sea basins held in September 1999 (PROSOPE cruise) and in June–July 2008 (BOUM cruise). Diversity was studied using dot blot hybridization with clade-specific 16S rRNA oligonucleotide probes and/or clone libraries of the 16S-23S ribosomal DNA Internal Transcribed Spacer (ITS) region, with a focus on the abundance of clades that may constitute bioindicators of warm waters. During both cruises, the dominant Prochlorococcus clade in the upper mixed layer at all stations was HLI, a clade typical of temperate waters, whereas the HLII clade, the dominant group in (sub)tropical waters, was only present at very low concentrations. The Synechococcus community was dominated by clades I, III and IV in the northwestern waters of the Gulf of Lions and by clade III and groups genetically related to clades WPC1 and VI in the rest of the Mediterranean Sea. In contrast, only a few sequences of clade II, a group typical of warm waters, were observed. These data indicate that local cyanobacterial populations have not yet been displaced by their (sub)tropical counterparts.