Biogeochemical processes in the saline meromictic Lake Kaiike, Japan: implications from molecular isotopic evidences of photosynthetic pigments

Summary Stable carbon and nitrogen isotopic compositions were determined for individual photosynthetic pigments isolated and purified from the saline meromictic Lake Kaiike, Japan, to investigate species‐independent biogeochemical processes of photoautotrophs in the natural environment. In the anoxic monimolimnion and benthic microbial mats, the carbon isotopic compositions of BChls e and isorenieratene related to brown‐coloured strains of green sulfur bacteria are substantially (∼10‰) depleted in 13C relative to those found in the chemocline. In conjunction with 16S rDNA evidence reported previously, it strongly suggests that Pelodyctyon luteolum inhabited and photosynthesized in the anoxic monimolimnion and benthic microbial mats by using 13C‐depleted regenerated CO2. By contrast, both Chl a and BChl a in the monimolimnion and microbial mats have similar isotopic compositions as they do in the chemocline, implying that the source organisms live only in the chemocline. In the chemocline, the nitrogen isotopic compositions of BChl e homologues ranges from −7.7 to − 6.5‰, whereas that of BChl a is −2.1‰. These isotopic compositions suggest that green sulfur bacteria Chlorobium phaeovibrioides would conduct nitrogen fixation in the chemocline, whereas purple sulfur bacteria Halochromatium sp. and cyanobacteria Synechococcus sp. may assimilate nitrite.