Abstract
Two large-celled bacterial species, Macromonas sp. and Chromatium sp., bloom at an upper boundary of the H2S layer in Lake Kaiike throughout the year (bacterial plate). Large pearl white refractile inclusions of Macromonas sp., which were composed of CaCO3, disappeared from the cells when the bacterial plate sample was acidified below a pH 6.3. But, as soon as the pH was returned to an initial level (8.5), these cells rapidly deposited them again, indicating that CaCO3 deposition is one of the essential activities in maintaining cell integrity. The inclusions also disappeared with exposure of live bacteria to a H2S solution. It was suggested that Macromonas sp. inhabits the H2S boundary using CaCO3-inclusions as protection from environmental H2S.
