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

Abstract

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 ( approximately 10 per thousand) depleted in (13)C 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 (13)C-depleted regenerated CO(2). 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 per thousand, whereas that of BChl a is -2.1 per thousand. 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.

Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

“…In a similar speculative vein, the basic idea that sulphate-reducing bacteria spent an appreciable fraction of their evolutionary past in an environment experiencing bioavailable nitrogen stress is circumstantially supported by the observation that sulphate reducers are capable of facultative nitrogen fixation 33 . It is even conceivable that nitrogen fixation under euxinic conditions could produce a negative d 15 N signature 34 which future geochemical data may record, and which would work in the same direction as our semiquantitative isotopic predictions. If our arguments are correct, it would imply that global productivity occurred in the context of two distinct N-cycle states, fixation/euxinic and non-fixation/non-euxinic, between which an interchange occurred in time and/or space until the Neoproterozoic oxygenation event caused anoxic conditions to become a much more localized/transient phenomenon, permitting the sustainable build of a nitrate pool and making nitrogen fixation itself a more spatially contextual phenomenon.…”

The importance of diazotrophic cyanobacteria as primary producers during Cretaceous Oceanic Anoxic Event 2

“…However, all these processes except for demetallation (Mg loss) and metal insertion (Ni chelation) are not related to the nitrogen atoms in tetrapyrrole structure. During the Mg-loss process, we observed no isotopic discrimination in a modern meromictic lake (Ohkouchi et al, 2005b). In our laboratory experiments, we have recently observed little isotopic fractionation associated with Ni chelation in octaethylporphyrin (Kashiyama et al, 2006).…”

Osmium evidence for synchronicity between a rise in atmospheric oxygen and Palaeoproterozoic deglaciation

“…Then, they were washed with distilled water several times to remove calcium salt and dried in an oven. Measurements for TOC contents and their carbon isotopic compositions were performed by an on-line system of Finnigan Delta Plus XP isotope-ratio mass spectrometry at JAMSTEC 33,35 . Total organic carbon content in the samples was determined by calibrated ion currents of m/z 44, 45, and 46 in isotope ratio mass spectrometry.…”