News on the dinitrogen fixation mechanisms across the North-South Indian Ocean

9 December 2024

Projections of net primary production increasingly diverge in Earth system models in part due to the parameterization of dinitrogen (N₂) fixation under climate change, with the tropical Indian Ocean being among the most uncertain regions. Based on samples collected during the SWINGS cruise (GEOTRACES GS02), Chowdhury and his colleagues (2024, see reference below) challenge this gap by providing an extensive study of the N₂ fixation mechanisms from the tropical to the austral Indian Ocean. They observe a sharp contrasting distribution of diazotroph groups across the frontal system. North of the fronts and mostly near the South African coast, cyanobacterial diazotrophs dominated, driving high N₂ fixation rates. South of the fronts non-cyanobacterial diazotrophs prevailed without significant N₂ fixation activity being detected. The authors also question the paradigm of the importance of dissolved iron in constraining diazotrophy while dissolved manganese seems to be an important driver too. They also propose a discussion on the respective roles of the nifH and nifD genes, the later looking as a better descriptor of non-cyanobacterial diazotrophs taxonomy compared to the more commonly used nifH.

**Figure 1:** This region is characterized by sharp biogeochemical divides (fronts), clearly seen in the surface distribution of temperature (A) and chlorophyll concentrations (B). N₂ fixation rates were highest north of the fronts, coinciding with the warmest waters (C), while primary production rates were highest south of the fronts with few exceptions in localized points off the South African coast (D).
STF = subtropical front, SAF = subantarctic front, PF = polar front, AC = Agulhas current, ARC = Agulhas return current

**Figure 2:** The biogeochemical divide observed in temperature and chlorophyll (Fig. 1) also imprinted sharp contrasts in the distribution of diazotroph species. *Trichodesmium,* a filamentous cyanobacterium mostly found in sub/tropical regions, was more abundant in warmer and trace metal rich waters close to coasts or islands (A), while unicellular diazotrophs were fairly homogenously distributed (D), and non-cyanobacterial groups were more prevalent south of the fronts (E, F). Unexpectedly, UCYN-A (recently found to be an organelle of an eukaryotic algae, rather than an independent species; https://www.science.org/doi/10.1126/science.adk1075) was more abundant in warmer waters north of the fronts, whereas elsewhere in the ocean they tend to prefer nutrient-rich and colder waters. This may be due to unique distribution and availability of limiting trace metals in this region.

Reference:

Chowdhury, S., Berthelot, H., Baudet, C., González-Santana, D., Reeder, C. F., L’Helguen, S., Maguer, J.-F., Löscher, C. R., Singh, A., Blain, S., Cassar, N., Bonnet, S., Planquette, H., & Benavides, M. (2024). Fronts divide diazotroph communities in the Southern Indian Ocean. FEMS Microbiology Ecology, 100. Access the paper: 10.1093/femsec/fiae095

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