Ecology of bacteria from Antarctic hypersaline lakes

There exists a unique and characteristic bacterial biota in Antarctic hypersaline lakes and 15 new bacterial species have been described from the lakes of the Vestfold Hills, Antarctica. In this study, antibodies were raised against the type strains of the four bacterial species which have been most consistently isolated from the oxylimnion of the hypersaline lakes of the Vestfold Hills. The abundance of these serogroups was measured throughout the austral spring and summer in four hypersaline lakes and a seawater site by indirect immunofluorescence microscopy. Maximum abundances of Halomonas serogroups (up to 40% of the total bacteria) and the Flavobacterium gondwanense serogroup (up tO 10% of the total bacteria) were observed at discrete depths within the water column in the two most hypersaline lakes at about midsummer, coincident with the time of maximum sunlight and the commencement of the summer thaw. Change in species abundance was not reflected in the total bacterial count, indicating compositional change of the total bacterial population. Concurrently, radioisotope incorporation methods were used to estimate heterotrophic bacterial productivity and metabolism. In the coastal marine site, incorporation rates were within the range previously reported for Antarctic ecosystems, ranging up to 2.1 pM thymidine h‚ÄövÖ¬™¬¨œÄ and 11.0 pM leucine h‚ÄövÖ¬™¬¨œÄ . Incorporation rates in the hypersaline lakes ranged up to 4.6 pM thymidine h‚ÄövÖ¬™¬¨œÄ and 94 pM leucine h‚ÄövÖ¬™¬¨œÄ . Generation times throughout all sample sites were fastest after the summer thaw at a depth of 2 m. Even at these peak times productivity in the hypersaline lakes was more than 100 times lower than average productivity in temperate aquatic environments. Optimum temperature for radioisotope incorporation was close to in-situ temperature in most cases, confirming the adaption of in-situ bacteria to the low water temperatures. Dissolved organic carbon (DOC) concentrations reflected the strong stratification and seasonal variation of other lake parameters. The ratio of [¬¨‚â•H]leucine incorporation to [¬¨‚â•H]thymidine incorporation (a measure of bacterial physiological state or species composition) correlated with DOC concentrations. DOC concentrations correlated well with total bacterial numbers but not with changes in species composition. In the meromictic hypersaline lakes, reduction of sulphur compounds is also an important means of energy generation. The Halomonas and Flavobacterium species isolated so far do not appear to be involved in sulphur cycling. However, a novel species of Brevibacterium, isolated and partly described in this study, reduces dimethylsulfoxide to dimethylsulphide and utilises methionine and cysteine as sole energy sources.