Sinking Trichodesmium fixes nitrogen in the dark ocean
Document Type
Article
Date of Original Version
10-1-2022
Abstract
The photosynthetic cyanobacterium Trichodesmium is widely distributed in the surface low latitude ocean where it contributes significantly to N2 fixation and primary productivity. Previous studies found nifH genes and intact Trichodesmium colonies in the sunlight-deprived meso- and bathypelagic layers of the ocean (200–4000 m depth). Yet, the ability of Trichodesmium to fix N2 in the dark ocean has not been explored. We performed 15N2 incubations in sediment traps at 170, 270 and 1000 m at two locations in the South Pacific. Sinking Trichodesmium colonies fixed N2 at similar rates than previously observed in the surface ocean (36–214 fmol N cell−1 d−1). This activity accounted for 40 ± 28% of the bulk N2 fixation rates measured in the traps, indicating that other diazotrophs were also active in the mesopelagic zone. Accordingly, cDNA nifH amplicon sequencing revealed that while Trichodesmium accounted for most of the expressed nifH genes in the traps, other diazotrophs such as Chlorobium and Deltaproteobacteria were also active. Laboratory experiments simulating mesopelagic conditions confirmed that increasing hydrostatic pressure and decreasing temperature reduced but did not completely inhibit N2 fixation in Trichodesmium. Finally, using a cell metabolism model we predict that Trichodesmium uses photosynthesis-derived stored carbon to sustain N2 fixation while sinking into the mesopelagic. We conclude that sinking Trichodesmium provides ammonium, dissolved organic matter and biomass to mesopelagic prokaryotes.
Publication Title, e.g., Journal
ISME Journal
Volume
16
Issue
10
Citation/Publisher Attribution
Benavides, Mar, Sophie Bonnet, Frédéric A. Le Moigne, Gabrielle Armin, Keisuke Inomura, Søren Hallstrøm, Lasse Riemann, Ilana Berman-Frank, Emilie Poletti, Marc Garel, Olivier Grosso, Karine Leblanc, Catherine Guigue, Marc Tedetti, and Cécile Dupouy. "Sinking Trichodesmium fixes nitrogen in the dark ocean." ISME Journal 16, 10 (2022). doi: 10.1038/s41396-022-01289-6.