| Summary: | Sea-ice is a predominant feature of polar oceans and exerts a unique influence on marine ecosystems. The annual circumpolar expansion of sea-ice around Antarctica provides a stable platform for the in situ colonisation and growth of a diverse assemblage of microbes that are integral to the energy base of the Southern Ocean. An active microbial loop has been proposed to operate within the ice matrix connecting bacteria, microalgae and protozoa, but validating this metabolic pathway has historically relied on bulk correlations of chlorophyll a (a surrogate for microalgal biomass) and estimates of bacterial production or abundance. I investigate the microbial loop using a range of physiological, genetic, and ecological techniques to determine whether the photosynthate exuded by phototrophic microalgae serves as a growth substrate for heterotrophic bacteria. This link is examined at a range of spatial (in vitro and in situ experiments) and temporal (8 hours to 18 days) scales by manipulating the supply of algal-derived photosynthate and documenting the subsequent change in bacterial metabolic activity, cell abundance and community composition. Single-cell analysis of both bacterial membrane integrity and intracellular activity revealed that sea ice is among the most productive microbial habitats. In short-term in vitro experiments, increased availability of dissolved organic matter (DOM) was shown to elicit a rapid metabolic response in sea ice bacteria, however single-activity was significantly reduced in treatments where photosynthate was restricted by either removing the majority of algal cells or inhibiting photosynthesis with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). To verify this metabolic response, microcosm simulations were conducted over a period of 9 days with microbes derived from two regions of the ice (bottom layer and high-salinity surface region) with presumed differences in the concentration of DOM. Metabolic activity was relatively low in bacteria derived from the high-saline region of the ...
|
|---|