Shift from Carbon Flow through the Microbial Loop to the Viral Shunt in Coastal Antarctic Waters during Austral Summer

The relative flow of carbon through the viral shunt and the microbial loop is a pivotal factor controlling the contribution of secondary production to the food web and to rates of nutrient remineralization and respiration. The current study examines the significance of these processes in the coastal waters of the Antarctic during the productive austral summer months. Throughout the study a general trend towards lower bacterioplankton and heterotrophic nanoflagellate (HNF) abundances was observed, whereas virioplankton concentration increased. A corresponding decline of HNF grazing rates and shift towards viral production, indicative of viral infection, was measured. Carbon flow mediated by HNF grazing decreased by more than half from 5.7 mu g C L-1 day(-1) on average in December and January to 2.4 mu g C L-1 day(-1) in February. Conversely, carbon flow through the viral shunt increased substantially over the study from on average 0.9 mu g C L-1 day(-1) in December to 7.6 mu g C L-1 day(-1) in February. This study shows that functioning of the coastal Antarctic microbial community varied considerably over the productive summer months. In early summer, the system favors transfer of matter and energy to higher trophic levels via the microbial loop, however towards the end of summer carbon flow is redirected towards the viral shunt, causing a switch towards more recycling and therefore increased respiration and regeneration.

Automated summary

In coastal waters of the Antarctic during the productive austral summer months, the relative flow of carbon through the viral shunt and the microbial loop plays a pivotal role in controlling secondary production and rates of nutrient remineralization and respiration. This study observed a general trend towards lower bacterioplankton and HNF abundances, but increased virioplankton concentration, with a corresponding decline in HNF grazing rates and a shift towards viral production indicating viral infection. Carbon flow through the viral shunt increased substantially from December to February, redirecting carbon flow towards recycling and increasing respiration and regeneration by the end of summer.