Sinking Trichodesmium fixes nitrogen in the dark ocean

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.

Fig. 1. Trichodesmium-specific ¹⁵N atom % enrichment.

A, B Boxplots showing the range, average and outliers of ¹⁵N/¹⁴N ratios measured in natural (non ¹⁵N₂-labeled) and ¹⁵N₂-labeled samples from 170 m, 270 m and 1 000 m depth at stations S05M and S10M, respectively. The number of trichomes scanned per depth is shown over each box (n values). All Trichodesmium filaments analyzed were significantly enriched in ¹⁵N. Examples of nanoSIMS ¹⁵N/¹⁴N ratio images of Trichodesmium filaments sampled at station S05M showing the ¹⁵N/¹⁴N isotopic ratio enrichment of Trichodesmium filaments according to the color bar. Filaments shown in panel (C) are those not incubated with ¹⁵N₂ (natural). The filaments shown in panels (D–F) are those incubated with ¹⁵N₂ and collected from sediment traps deployed at 170, 270, and 1000 m, respectively. The same pattern is repeated for station S10M in panels (G–J) The scale bar in nanoSIMS ¹⁵N/¹⁴N ratio images is 5 µm.

Fig. 2. Diazotroph community expression profiles.

Diazotroph community nifH gene expression profiles. Relative abundance of nifH cDNA reads originating from sediment traps at 170 m, 270 m, and 1000 m at stations S05M (A) and S10M (B). Relative abundances were calculated based on 146,000 ± 18,000 reads per sample.

Fig. 3. Trichodesmium culture sinking simulation experiments.

A ¹⁵N atom % enrichment values of triplicate Trichodesmium cultures submitted to a 12 h:12 h dark:dark light cycle under increasing hydrostatic pressure (0–3 MPa) and decreasing temperature (27–14 °C) on a sinking particle simulator for 192 and 360 h (simulating 160 and 300 m depth, respectively). B ¹⁵N atom % enrichment values of triplicate Trichodesmium cultures submitted to a 12 h:12 h dark:dark at ambient laboratory pressure and decreasing temperature (27 to 14 °C) for 192 and 360 h.

Fig. 4. Relationship between the initial carbon storage to biomass ratio (initial R_Sto) of Trichodesmium cells and the depth at which cell carbon storage is depleted.

The x-axis represents the initial R_Sto, i.e., the carbon storage to biomass ratio of Trichodesmium at 40 m (the bottom of the mixing layer, where cells start to sink). The y-axis represents the depletion depth, i.e., the depth at which carbon storage becomes zero. The blue line represents the threshold depth below which cell carbon storage is depleted (and thus N₂ fixation is no longer possible) for various initial R_Sto values. The shaded area indicates the depletion depth range considering the variability in empirical and theoretical sinking velocities of Trichodesmium (Supplementary Methods; Table S2). The yellow and pink dots indicate the depletion depth for initial R_Sto values of 1 and 1.3, respectively. The blue line represents the simulation based on the median sinking velocity. The dashed line indicates the depletion depth of 1000 m depth. The depletion depth must be below 1000 m for cells to fix N₂ at 1000 m depth and explain our field observations.