| Research Abstract |
1. The Lake Katanuma, a strongly acid lake, was surveyed during February 1998 to December 2000. The thermal pattern in the lake water showed dimictic, whereas the difference in water temperature between epilimnion and hypolimnion in summer was relatively small due to the volcanic heat supply from the lake bottom. During stratification in summer, the occasional circulations of whole lake water in a short term were observed in 1998 and 1999. Consequently, the condition of high concentration of hydrogen sulfide and oxygen depletion in hypolimnion spread over the whole lake. From September to December, the lake water circulated from the surface to the bottom. In 2000, however, the short-term circulation during summer stagnation was not observed. The occurrence of short-term circulations depends on the weather of that year.
2. The changes in the vertical distribution of bacterial biomass were influenced by the seasonal change in stagnation and circulation of lake water. In stratification period, high densities of filamentous bacteria and low abundance of short rods were observed. After the overturn in September, the biomass of filamentous bacteria decreased and that of short rods increased in the whole lake water.
3. Phylogenetic analyses of the sequences of 16S rRNA gene show that the strains isolated from lake water fell into proteobacteria alpha subdivision and gamma subdivision. The partial sequences of the strains belonging to alpha proteobacteria were closely related to that of Methylobacterium fujisawaense. The strains of gamma proteobacteria show distinct similarities to sequences of Lysobacter enzymogenes and the iron oxidizing lithotrophs, suggesting that these strains may act as autotrophs.
Further studies in terms of the phylogenetic analyses by rDNA sequences directly retrieved from the lake water are needed.
|
