Microbial Loop Dynamics in Antarctic Sea-Ice

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 ice and in cultures spiked with DCMU, photosynthate limitation restricted bacterial growth and significantly influenced community structure. In contrast, the bottom of the ice is characterised by a high concentration of DOM and bacterial metabolic activity was shown to be higher and DCMU was less influential with respect to changes in bacterial abundance or community composition. To examine in situ microbial dynamics, a series of cores were extracted from Antarctic sea-ice and reinserted into the ice matrix upside down to expose resident microbial assemblages to a significantly different light, temperature and salinity regime. Limited assimilation of algal-derived DOM by bacteria in ice cores that were flipped illustrated a malfunction in the microbial loop after a period of 18 days. Bacteria originally at the bottom of the sea ice appeared to be temperature-limited, while a lack of growth in cells originally at the top of the ice profile was attributed to a community dominated by slow-growing psychrophilic species. A stronger physiological response to disturbance was elicited by microalgae and significant growth was contrasted with severe bleaching and cell death. This reciprocal transplant is the first of its kind to examine the in situ sea ice community and illustrats that although microbial assemblages are similar with respect to trophic dynamics, they are also attuned to distinct regions within the ice. The bacterial assimilation of algal-derived DOM is of fundamental importance to the microbial loop and by confirming that photosynthate is a major stimulus for bacterial growth, these results provide a new and unique insight into microbial dynamics in Antarctic sea-ice.