Extracellular organic carbon dynamics during a bottom-ice algal bloom (Antarctica)

Abstract

Antarctic fast ice provides a habitat for diverse microbial communities, the biomass of which is mostly dominated by diatoms capable of growing to high standing stocks, particularly at the icewater interface. While it is known that ice algae exude organic carbon in ecologically significant quantities, the mechanisms behind its distribution and composition are not well under- stood. This study investigated extracellular organic carbon dynamics, microbial characteristics, and ice algal photophysiology during a bottom-ice algal bloom at McMurdo Sound, Antarctica. Over a 2 wk period (November to December 2011), ice within 15 cm from the icewater interface was collected and sliced into 9 discrete sections. Over the observational period, the total concen- trations of extracellular organic carbon components (dissolved organic carbon (DOC) and total carbohydrates (TCHO) — the sum of monosaccharides (CHOMono) and polysaccharides (CHOPoly)) increased, and were positively correlated with algal biomass. However, when normalised to chlorophyll a, the proportion of extracellular organic carbon components substantially decreased from initial measurements. Concentrations of DOC generally consisted of <20% TCHO, typically dominated by CHOMono, which decreased from initial measurements. This change was coincident with improved algal photophysiology (maximum quantum yield) and an increase in sea-ice brine volume fraction, indicating an increased capacity for fluid transport between the brine channel matrix and the underlying sea water. Our study supports the suggestion that microbial exudation of organic carbon within the sea-ice habitat is associated with vertical and temporal changes in brine physicochemistry.