Dissolved organic matter uptake by Trichodesmium in the Southwest Pacific

Abstract : The globally distributed diazotroph Trichodesmium contributes importantly to nitrogen inputs in the oligotrophic oceans. Sites of dissolved organic matter (DOM) accumulation could promote the mixotrophic nutrition of Trichodesmium when inorganic nutrients are scarce. Nano-scale secondary ion mass spectrometry (nanoSIMS) analyses of individual trichomes sampled in the South Pacific Ocean, showed significant 13 C-enrichments after incubation with either 13 C-labeled carbohydrates or amino acids. These results suggest that DOM could be directly taken up by Trichodesmium or primarily consumed by heterotrophic epibiont bacteria that ultimately transfer reduced DOM compounds to their host trichomes. Although the addition of carbohydrates or amino acids did not significantly affect bulk N 2 fixation rates, N 2 fixation was enhanced by amino acids in individual colonies of Trichodesmium. We discuss the ecological advantages of DOM use by Trichodesmium as an alternative to autotrophic nutrition in oligotrophic open ocean waters. Nitrogen is recognized as the proximate limiting nutrient for primary production in the oceans 1. The oceanic nitrogen reservoir is controlled by a balance between fixed nitrogen gains (via dinitrogen-N 2-fixation) and losses (denitrification) 2. While global nitrogen budget estimations determine that denitrification exceeds N 2 fixation considerably 3 , recent improvements in the 15 N 2 isotope tracer method used to measure biological N 2 fixation have evidenced that formerly published rates could be underestimated by a factor of ~2 to 6 4–8 , and thus could be high enough to balance denitrification on a global basis. However, the variability among N 2 fixation rates obtained when using the two different methods (adding 15 N 2 as a bubble or pre-dissolved in seawater) 4,9 can be high 7 and at times not significant 10–12. A mechanistic understanding of which factors determine the degree of discrepancy between the two 15 N 2 methods is currently lacking. Moreover, marine pelagic N 2 fixation had been long attributed to the tropical and subtropical latitudinal bands of the ocean, e.g. 13 , while other ecological niches such as high latitude waters, oxygen minimum zones and the vast dark realm of the ocean (below the euphotic zone) are now recognized as active N 2 fixation sites 14–16. It is likely that the inclusion of these previously unaccounted for active N 2 fixation sites will be more important in equilibrating denitrification and N 2 fixation rates than the underestimation of rates due to discrepancies between isotopic tracer methods. In chronically stratified open ocean regions such as the vast subtropical gyres, primary production depends largely on external fixed nitrogen inputs provided by N 2 fixation performed by prokaryotes termed 'diazotrophs'. Diazotrophic cyanobacteria are photosynthetic prokaryotes (with the exception of the photoheterotrophic Candidatus Atelocyanobacterium thalassa which cannot photosynthesize) 17 that thrive in oligotrophic tropical and subtropical waters of the oceans where they provide an important source of fixed nitrogen for other phyto-plankton 13. Despite being classically regarded as photoautotrophs, some unicellular diazotrophic cyanobacteria like Cyanothece are able to take up dissolved organic matter (DOM) molecules photoheterotrophically 18. As well, various filamentous diazotrophic cyanobacteria such as Anabaena bear genes for amino acids transport, which may be used to incorporate amino acids from the in situ DOM pool, or to assimilate amino acids self-produced during diazotrophic growth 19 .
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Scientific Reports, Nature Publishing Group, 2017, 7, pp.41315 〈10.1038/srep41315〉
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Mar Benavides, Hugo Berthelot, Solange Duhamel, Patrick Raimbault, Sophie Bonnet. Dissolved organic matter uptake by Trichodesmium in the Southwest Pacific. Scientific Reports, Nature Publishing Group, 2017, 7, pp.41315 〈10.1038/srep41315〉. 〈hal-01621711〉

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