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

Claire Evans,David W. Pond,Hugh J. Venables,Corina P. D. Brussaard,Joost Brandsma,Michael P. Meredith,David N. Thomas

doi:10.3390/microorganisms9020460

Abstract

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 µg C L−1 day−1 on average in December and January to 2.4 µg C L−1 day−1 in February. Conversely, carbon flow through the viral shunt increased substantially over the study from on average 0.9 µg C L−1 day−1 in December to 7.6 µg 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.

Highlights

The microbial loop describes the consumption of dissolved organic matter (DOM) by bacteria, which are grazed by protists providing a route for carbon transfer to higher trophic levels [1,2]
Over the course of the study the flow of carbon was observed to shift from a scenario where a large proportion passed through the microbial loop and little viral lysis occurred, towards a viral-shunt-dominated system where grazer-mediated transfer to higher trophic levels decreased by more than half
A better understanding of the microbial communities of the coastal Peninsula Antarctica will be key to determining the Southern

Summary

Introduction

The microbial loop describes the consumption of dissolved organic matter (DOM) by bacteria, which are grazed by protists providing a route for carbon transfer to higher trophic levels [1,2]. Occurring in parallel is the ‘viral shunt’ whereby microbial hosts are infected and subsequently lysed by viruses, which converts their cell biomass to progeny and DOM [5]. In this way carbon is directed back into the pool of potential bacterial substrates [6]. Bacterial consumption of the DOM generated by the action of bacteriophages represents a closed trophic loop, whereby matter is recycled at the base of the food chain [7]

Results

Discussion

Conclusion