Prokaryotic and diazotrophic population dynamics within a large oligotrophic inverse estuary

The ecology of microbial assemblages inhabiting classical (positive) estuaries has been well documented. However, we know relatively little about the microbial ecology of inverse (negative) estuaries, which exhibit different physical and hydrodynamic properties, including oligotrophy and hypersalinity. We investigated the dynamics of bacterioplankton communities in Spencer Gulf, an inverse estuary in temperate South Australia. We characterised patterns in the overall diversity and composition of the resident microbial assemblage, and tested the hypothesis that pelagic nitrogen-fixing bacteria (diazotrophs) could be an important functional group in the nutrient limited waters of the region. Prokaryotic and diazotrophic communities were evaluated using 16S ribosomal DNA and nifH amplicon tag pyrosequencing, respectively. Significant heterogeneity in microbial community composition and diazotrophic population structure was observed, which was driven by shifts in the relative importance of temperate vs. subtropical and oceanic vs. coastal ecotypes of Cyanobacteria throughout the inverse estuary. The globally significant unicellular cyanobacterium, UCYN-A ‘Candidatus Atelocyanobacterium thalassa’, was the dominant diazotrophic phylotype. Temperature, chlorophyll a and nitrogen availability were all significant drivers of bacterioplankton dynamics within the gulf. These results demonstrate the heterogeneous microbiology of inverse estuaries, indicating that specific abiotic and biotic characteristics select for discrete microbial communities, and that pelagic nitrogen fixation may be important in this temperate oligotrophic system.