Stable carbon and nitrogen isotopic compositions of photosynthetic pigments as tracers for elemental cycle in the modern- and paleo-environments

Chloropigments drive photosynthesis, an essentially important process in the biogeochemical cycles on Earth. Eukaryotic phototrophs have chlorophylls (Chl) a, b, and c, while prokaryotic phototrophs except for cyanobacteria, have bacteriochlorophylls (BChl) a, b, c, d, e, and g. In this study we report carbon and nitrogen isotopic compositions of chloropigments in the saline meromictic Lake Kaiike [e.g., Nakajima, 2004; Ohkouchi et al., 2005]. Since Lake Kaiike is characterized by a dense chemocline population of photoautotrophic bacteria including purple sulfur bacteria (PSB), green sulfur bacteria (GSB), and cyanobacteria [Nakajima et al., 2003], it could be “model ocean” for oceanic environments during Cretaceous Oceanic Anoxic Events and Early Earth when the atmospheric oxygen level should have substantially lower than the present. Here, in conjunction with that molecular genetic information [Koizumi et al., 2004], we will discuss biogeochemical processes associated with species-independent microbial activities in this lake based on both carbon and nitrogen isotopic compositions of individual photosynthetic pigments. Furthermore, we will mention about the implication of the information for the studies of paleoenvironments.