Hydrographic fronts shape productivity, nitrogen fixation, and microbial community composition in the southern Indian Ocean and the Southern Ocean

Biogeochemical cycling of carbon (C) and nitrogen (N) in the ocean depends on both the composition and activity of underlying biological communities and on abiotic factors. The Southern Ocean is encircled by a series of strong currents and fronts, providing a barrier to microbial dispersion into adjacent oligotrophic gyres. Our study region straddles the boundary between the nutrient-rich Southern Ocean and the adjacent oligotrophic gyre of the southern Indian Ocean, providing an ideal region to study changes in microbial productivity. Here, we measured the impact of C and N uptake on microbial community diversity, contextualized by hydrographic factors and local physico-chemical conditions across the Southern Ocean and southern Indian Ocean. We observed that contrasting physico-chemical characteristics led to unique microbial diversity patterns, with significant correlations between microbial alpha diversity and primary productivity (PP). However, we detected no link between specific PP (PP normalized by chlorophyll-a concentration) and microbial alpha and beta diversity. Prokaryotic alpha and beta diversity were correlated with biological N-2 fixation, which is itself a prokaryotic process, and we detected measurable N-2 fixation to 60 degrees S. While regional water masses have distinct microbial genetic fingerprints in both the eukaryotic and prokaryotic fractions, PP and N-2 fixation vary more gradually and regionally. This suggests that microbial phylogenetic diversity is more strongly bounded by physical oceanographic features, while microbial activity responds more to chemical factors. We conclude that concomitant assessments of microbial diversity and activity are central to understanding the dynamics and complex responses of microorganisms to a changing ocean environment.

Automated summary

The cycling of carbon and nitrogen in the ocean is influenced by both biological communities and abiotic factors. Physical oceanographic features constrain microbial phylogenetic diversity, while microbial activity responds to chemical factors. Assessments of microbial diversity and activity are important for understanding the complex responses of microorganisms to a changing ocean environment.