The photosynthetic cyanobacterium Trichodesmium is widely distributed in the surface low latitude ocean where it contributes significantly to N 2 fixation and primary productivity. Previous studies found nifH genes and intact Trichodesmium colonies in the sunlight-deprived meso-and bathypelagic layers of the ocean (200-4 000 m depth). Yet, the ability of Trichodesmium to fix N 2 in the dark ocean has not been explored. We performed 15 N 2 incubations in sediment traps at 170, 270 and 1 000 m at two locations in the South Pacific. Sinking Trichodesmium colonies fixed N 2 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 N 2 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 N 2 fixation in Trichodesmium. Finally, using a cell metabolism model we predict that Trichodesmium uses photosynthesis-derived stored carbon to sustain N 2 fixation while sinking into the mesopelagic. We conclude that sinking Trichodesmium provides ammonium, dissolved organic matter and biomass to mesopelagic prokaryotes.
Diatoms are a major phytoplankton group responsible for approximately 20% of carbon fixation on Earth. They perform photosynthesis using light-harvesting chlorophylls located in plastids, an organelle obtained through eukaryote-eukaryote endosymbiosis. Microbial rhodopsin, a photoreceptor distinct from chlorophyll-based photosystems, was recently identified in some diatoms. However, the physiological function of diatom rhodopsin remains unclear. Heterologous expression techniques were herein used to investigate the protein function and subcellular localization of diatom rhodopsin. We demonstrated that diatom rhodopsin acts as a light-driven proton pump and localizes primarily to the outermost membrane of four membrane-bound complex plastids. Using model simulations, we also examined the effects of pH changes inside the plastid due to rhodopsin-mediated proton transport on photosynthesis. The results obtained suggested the involvement of rhodopsin-mediated local pH changes in a photosynthetic CO 2 -concentrating mechanism in rhodopsin-possessing diatoms.
Crocosphaera watsonii
is as a key nitrogen (N) supplier in marine ecosystems, and it has been estimated to contribute up to half of oceanic N
2
fixation. Conversely, a recent study reported that
Crocosphaera
can assimilate combined N and proposed that unicellular diazotrophs can be competitors with non-N
2
fixing phytoplankton for combined N.
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