Abstract:
Understanding the impact of oceanographic features on marine microbial ecosystems remains a major
ecological endeavour. Here we assess microbial diversity, community structure and functional capacity
along the Agulhas Current system and the Subtropical Front in the South Indian Ocean (SIO). Samples
collected from the epipelagic, oxygen minimum and bathypelagic zones were analysed by 16S rRNA
gene amplicon and metagenomic sequencing. In contrast to previous studies, we found high taxonomic
richness in surface and deep water samples, but generally low richness for OMZ communities. Betadiversity
analysis revealed significant dissimilarity between the three water depths. Most microbial
communities were dominated by marine Gammaproteobacteria, with strikingly low levels of
picocyanobacteria. Community composition was strongly influenced by specific environmental factors
including depth, salinity, and the availability of both oxygen and light. Carbon, nitrogen and sulfur
cycling capacity in the SIO was linked to several autotrophic and copiotrophic Alphaproteobacteria
and Gammaproteobacteria. Taken together, our data suggest that the environmental conditions in
the Agulhas Current system, particularly depth-related parameters, substantially influence microbial
community structure. In addition, the capacity for biogeochemical cycling of nitrogen and sulfur is
linked primarily to the dominant Gammaproteobacteria taxa, whereas ecologically rare taxa drive
carbon cycling.
