Microbial Evolution In Sea Ice: Communities To Genes

Abstract

Microbial communities encased in growing sea ice must contend with the combined stresses of low temperature and high salinity, environmental pressures that only intensify over the course of the winter. This harsh physical environment was expected to negatively impact both the abundance and diversity of the microbial community entrained within the ice, hypotheses which were tested in Chapters 1 and 2, respectively. While the overall abundance of microorganisms decreased in the coldest ice, extracellular polymeric substances were produced throughout the winter in all measured horizons. Microbial communities entrained from seawater into sea ice were preserved in the ice, with communities dominated by SAR11 Alphaproteobacteria (Bacteria) and Marine Group I Crenarchaeota (Archaea) found essentially unchanged throughout the winter. These results informed further hypotheses on the potential for increased lateral gene transfer by conjugation, transduction, or natural transformation in sea ice, addressed in Chapter 3 by measurement of the concentrations of bacteria, viruses, and extracellular free DNA in natural sea ice. These hypotheses were supported by the measurement of up to 100× more extracellular free DNA in sea ice brine than in the underlying seawater and extremely high virus-to-bacteria ratios (up to 2820), with predicted virus-to-bacteria contact rates up to 844× those expected in the underlying seawater. In Chapter 4 a comparative analysis of the genome of a model psychrophilic γ- proteobacterium, Colwellia psychrerythraea strain 34H, was used to examine the potential for the exchange of genes of particular utility in permanently cold habitats. Phylogenetic analysis and G+C content were used to identify a genomic island in C. psychrerythraea strain 34H containing a number of genes encoding proteins involved in the degradation of abundant compatible solutes like glycine betaine. Furthermore, the positive growth of C. psychrerythraea strain 34H on sarcosine (a derivative of glycine betaine) as a sole carbon and nitrogen source suggested that the laterally transferred genes were expressed in vitro.