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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 1 | Copyright
Biogeosciences, 15, 1-12, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 02 Jan 2018

Research article | 02 Jan 2018

Effect of light on N2 fixation and net nitrogen release of Trichodesmium in a field study

Yangyang Lu1,2, Zuozhu Wen1,3, Dalin Shi1,3, Mingming Chen1,2, Yao Zhang1,2, Sophie Bonnet4, Yuhang Li5, Jiwei Tian6, and Shuh-Ji Kao1,2 Yangyang Lu et al.
  • 1State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
  • 2College of Ocean and Earth Sciences, Xiamen University, Xiamen, China
  • 3College of Environment and Ecology, Xiamen University, Xiamen, China
  • 4IRD, Aix Marseille Université, CNRS/INSU, Université de Toulon, Mediterranean Institute of Oceanography (MIO), New Caledonia, France
  • 5Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China
  • 6Physical Oceanography Laboratory, Ocean University of China, Qingdao, China

Abstract. Dinitrogen fixation (NF) by marine cyanobacteria is an important pathway to replenish the oceanic bioavailable nitrogen inventory. Light is the key to modulating NF; however, field studies investigating the light response curve (NF-I curve) of NF rate and the effect of light on diazotroph-derived nitrogen (DDN) net release are relatively sparse in the literature, hampering prediction using models. A dissolution method was applied using uncontaminated 15N2 gas to examine how the light changes may influence the NF intensity and DDN net release in the oligotrophic ocean. Experiments were conducted at stations with diazotrophs dominated by filamentous cyanobacterium Trichodesmium spp. in the western Pacific and the South China Sea. The effect of light on carbon fixation (CF) was measured in parallel using the 13C tracer method specifically for a station characterized by Trichodesmium bloom. Both NF-I and CF-I curves showed a Ik (light saturation coefficient) range of 193 to 315µEm−2s−1, with light saturation at around 400µEm−2s−1. The proportion of DDN net release ranged from  ∼ 6 to  ∼ 50%, suggesting an increasing trend as the light intensity decreased. At the Trichodesmium bloom station, we found that the CFNF ratio was light-dependent and the ratio started to increase as light was lower than the carbon compensation point of 200µEm−2s−1. Under low-light stress, Trichodesmium physiologically preferred to allocate more energy for CF to alleviate the intensive carbon consumption by respiration; thus, there is a metabolism tradeoff between CF and NF pathways. Results showed that short-term ( < 24h) light change modulates the physiological state, which subsequently determined the CN metabolism and DDN net release by Trichodesmium. Reallocation of energy associated with the variation in light intensity would be helpful for prediction of the global biogeochemical cycle of N by models involving Trichodesmium blooms.

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Short summary
We investigated the light response of field Trichodesmium N2 fixation and net dissolved nitrogen release behavior. Our results suggest that N2 fixation was a function of light intensity, and the light requirement of Trichodesmium nitrogen fixation was high relative to its photosynthetic light demand. Meanwhile, light is a crucial parameter driving the physiological state of Trichodesmium, which subsequently determined the C / N metabolism and net dissolved nitrogen release.
We investigated the light response of field Trichodesmium N2 fixation and net dissolved nitrogen...