High single-cell metabolic activity in Antarctic sea ice bacteria

Antarctic sea ice bacteria have historically been examined by techniques that measure bulk community properties. In recognition of the complexity of microbial community structure and function, methods are now available to assess the physiological status of individual bacterial cells. We employed 3 assays: Nucleic Acid Double Staining (NADS), modified Vital Stain and Probe (mVSP) and 5-cyano-2, 3-ditolyl tetrazolium chloride (CTC) to (1) quantify the cell-specific metabolic status of bacteria present in the bottom of Antarctic fast-ice and (2) determine whether physiological probes can be used to quantify short-term changes in bacterial metabolic activity. Each assay was used in manipulative experiments (<8 h) conducted on bacteria from melted ice cores with dark/light (5 μmol photons m-2 s-1) and the presence/absence of microalgae (filtered, unfiltered samples) as treatment variables. Estimates of cell viability (intact membranes) were >68% (NADS) and >95% (mVSP) of the post-incubation cell count. High cellular rRNA content was observed using fluorescence in situ hybridisation with an average of 80% of DAPI-stained cells hybridising with the general bacterial probe EUB338 (mVSP). An average of 32% of the bacterial cells were undergoing active respiration (CTC+) which is significantly higher than the 2 to 10% level of activity typical for most marine systems. The short-term effect of incubation irradiance or manipulation of microalgal photosynthate was not significant for any assay. Single-cell analysis of melted ice cores suggests that sea ice is among the most productive microbial habitats, while CTC is favoured for establishing a real-time bacterial response in the sea ice ecosystem. © Inter-Research 2008.