A local upwelling controls viral and microbial community structure in South Australian continental shelf waters

Despite the increasing awareness of the role of viruses and heterotrophic bacteria in microbial dynamics and biogeochemical cycles, there is still a critical lack of information on their community composition and dynamics, especially in relation to upwellings. We investigated, within surface waters and the Deep Chlorophyll Max, the community composition and dynamics of flow cytometrically defined sub-populations of heterotrophic bacteria and virus-like particles in nearby water masses that were affected and unaffected by a localised wind-driven coastal upwelling. In contrast to previous studies we uniquely identified a 4-fold increase in total viral abundance and a decrease in bacterial abundance, from upwelled to offshore waters. Individual viral sub-populations were seen to correlate significantly to both bacterial populations and chlorophyll a, suggesting the possibility of individual viral populations infecting multiple host species rather than the often assumed single host species. The percentage of HDNA bacteria was high (84.3-93.4%) within upwelled waters, in accordance with the highest recorded values within an upwelling system, and decreased down to 35.5-42.6% away from the upwelling. Additionally, changes in the community composition of individual bacterial sub-populations suggest individual populations might be better adapted to distinct environments. We suggest that each flow cytometrically defined bacterial population may possess its own environmental niche where favourable conditions for that population result in an increase in abundance, cellular activity and productivity.