Carbon isotopic composition of the tetrapyrrole nucleus in chloropigments from a saline meromictic lake: A mechanistic view for interpreting the isotopic signature of alkyl porphyrins in geological samples

We report carbon isotopic compositions of tetrapyrrole nuclei (chlorophyllides) and side chains (phytol or farnesol) of chlorophyll a, bacteriochlorophyll a, and bacteriochlorophylls e isolated from several depths in the saline meromictic Lake Kaiike, Japan. The chlorophyllides are substantially enriched in 13C relative to the side chains in all the depths of the lake, namely oxic mixolimnion, chemocline, anoxic monimolimnion, and benthic microbial mat. The isotopic compositions of chlorophyllide moieties are strongly related to those of the whole molecule (chloropigments; r2 = 0.94) with the former being 1.8 ± 0.8‰ (1σ, n = 18) enriched in 13C relative to the latter. No significant difference was observed between chlorophyll species. The heterogeneity of 13C in the chloropigments can be ascribed to differences between the biosynthetic pathways of chlorophyllides and side chains. The alkyl porphyrins in the geological samples are derived from the chlorophyllides, whose carbon skeleton is ultimately inherited from glutamate, one of the major amino acids in the cell. Together with the isotopic relationship between chlorophylls and cell previously reported, we estimate that the carbon isotopic composition of DPEP, a major C32 alkyl porphyrin in the geological samples, is 1.8‰ enriched in 13C relative to the cell.