Disentangling the drivers of taxonomic and functional community structure along the Agulhas current system

Abstract

Global climate change is expected to disproportionately affect marine ecosystems, due to increases in atmospheric CO2 which will lead to changes such as lower ocean pH. The factors which influence the structure of microbial communities remain unclear, and crucially on how they will respond to these environmental changes. The Indian-sector of the Southern Ocean (SO) is noted for its high level of variability in mesoscale water movements due to the of the Agulhas current system (Agulhas and Return Current). The influence of the Agulhas leakage on global oceanic circulation makes this area a sensitive lever in climate change scenarios [1]. However, due to a number of reasons, we know very little regarding the correlation between microbial diversity and functional processes in this ecosystem and more specifically, how water mass stratification and the Agulhas current systems may influence this relationship. To reduce this knowledge deficit we applied lllumina based amplicon sequencing and Shotgun metagenomic analysis to assess microbial diversity and potential functional capacity, respectively. Ocean water samples (27 in total) from the Crossroads (CR) monitoring line were collected during the SANAP/DEA Marion Island Relief cruise (15th April - 9th May 2015) on the SA Agulhas II polar research equipped with a CTD/bottle and rosette sampler. Samples were collected at pre-determined depths: (a) 9 deep (10 m above seafloor), (b)9 middle (oxygen minimum) and (c) 9 surface (fluorescence maximum).

We found high taxonomic richness in surface and deep samples, with generally low numbers for middle samples, corresponding to oxygen minimum zones. Adonis analysis revealed marked differences between the three sample types (i.e. surface, middle, and deep) dominated by marine Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes. Our data showed the first evidence of extensive biogeochemical capacity (C, N, S), with a large proportion showing homology to those of Alphaproteobacteria (Rhizobiales), Gammaproteobacteria (genus Pseudoalteromonas) and Cyanobacteria (genus Synechoccus). Taken together, our results reveal important functional cues for biogeochemical cycling in the SO and provide a solid baseline for understanding future perturbations and consequent impacts on biogeochemical cycling.