Bacteria in sea ice and underlying water of the eastern Weddell Sea in midwinter

Bacteria in the water beneath the sea ice of the eastern Weddell Sea were homogeneously distributed. Direct counts resembled values from spring and autumn, whereas viable cell counts, total ATP concentrations, as well as heterotrophic assimilation and extracellular enzymatic activities were very low, implying a metabolic inactive bacterioplankton. The consolidated sea ice had a very heterogeneous horizontal distribution of microbes on large as well as small scales but vertical profiles in low and densely populated ice cores exhibited similar patterns. A close relation between bacterial colonization of sea ice and genetic ice classes was revealed. Sea ice of the 'predominantly congelation ice' had the lowest bacterial biomass and displayed very low heterotrophic activities which were comparable to those of the water column. Samples of older sea ice belonging to the 'mainly frazil' and 'mixed ice' had maximal numbers of bacteria. They often included high proportions of culturable cells and dividing cells as well as large bacteria. The bacteria of these ice classes were active and contributed significantly to the productivity in the Weddell Sea during winter. 'Predominantly frazil ice' was less colonized; however, selective bacterial growth was also indicated in this typical winter ice by an increase in the proportions of culturable and psychrophilic bacteria with advancing age of the ice. Psychrophilic bacteria dominated in consolidated sea-ice whereas facultative psychrophiles prevailed in young sea-ice and water, corroborating a strict partitioning in a microbial sea-ice and a seawater regime. Generally, temperature does not appear to be the significant factor for the development of bacterial communities in the surface layer of the eastern Weddell Sea in winter since the metabolically active bacterial flora develops in the very cold sea-ice environment. The organic matter supply and its improved usability obviously controls bacterial activity as well as the selective enrichment of psychrophiles.