Fractionation of Stable Carbon Isotopes by Photoautotrophically Growing Anoxygenic Purple and Green Sulfur Bacteria

Fractionation of stable carbon isotopes 12C and 13C by three pure cultures of photoautotrophic purple sulfur bacteria (Ectothiorhodospira shaposhnikovii, Lamprocystis purpureus, and Thiocapsa sp.) (PSB) and the green sulfur bacterium Prosthecochloris sp. (GSB) was investigated in 13–15-day experiments. The cultivation was carried out in a luminostat (2000 lx) on mineral media with 1–1.5 g/l NaHCO3 (inoculum) with the subsequent transfer to the medium with up to 10 g/l NaHCO3. For PSB, the difference in the quantitative characteristics of the isotopic composition of suspended carbon (including bacterial cells) and mineral carbon of the medium (Δ13C = δ13Csubstrate − δ13Cbiomass) changed from 15.0 to 34.3‰. For GSB, the range of Δ13C changes was significantly less (18.3–22.7‰). These data suggested the possibility of a pool of soluble mineral carbon in PSB cells. The pool of intracellular mineral carbon was calculated; depending on the PSB species and growth stage, it varied from 0 to 68% of the total cell carbon. The α coefficients reflecting the carbon isotope fractionation by PSB and GBS and calculated from the changes of the bicarbonate carbon isotopic composition in the medium depending on its consumption were 1.029 ± 0.003 and 1.019 ± 0.001, respectively. These α values did not depend on the growth rate. CO2 fixation on ribulose-bisphosphate was shown to be the major factor determining the carbon isotope fractionation by PSB; at the stage of CO2 penetration into the cell, fractionation was insignificant. In GSB, fractionation occurred mostly at CO2 penetration into the cell, while it was insignificant at the stage of carbon dioxide fixation in the reverse TCA cycle. Analysis of the isotopic data of the photosynthesis by PSB and GSB in meromictic lakes also revealed that in PSB-dominated natural communities suspended organic matter was more enriched with light 13C (Δ13C = 23.4−24.6‰) than in the communities with more active GSB (Δ13C = 10.2−14.0‰)