| Purple sulfur bacteria (PSB) can dominate the nitrogen, carbon, and sulfur cycles in anoxic ecosystems via their anoxygenic photosynthetic metabolisms and are the only producers of the carotenoid, okenone, a biomarker targeted for photic zone euxinia. This is the first systematic report on the microbial physiology of okenone production in pure PSB cultures and their associated C, N, and S metabolisms. Four strains/species of PSB were exposed to various growth conditions to examine pigment production and associated metabolisms: Marichromatium purpuratum DSMZ 1591 (Mpurp1591), M. purpuratum DSMZ 1711 (Mpurp1711), Thiocapsa marina DSMZ 5653 (Tmar5653), and FGL21 (a strain isolated from Fayetteville Green Lake, New York). A novel in vivo technique for the quantification of pigments was developed. The ratio okenone to Bacteriochlorophyll a (Bchl a) significantly differed among species/strains of PSB, and was dependent on carbon metabolism. The broader implication is that concentrations of okenone in sediments are dependent on metabolism and species composition, and not solely cell density. Isotopic enrichments observed in anoxic basins may be due to the predominance of PSB. Cultures of Mpurp1591 had Delta13 Cbiomass -- CO2, via RuBisCO, ranging from -16.1 to -23.2‰ during exponential and early stationary phase growth. Cultures of Tmar5653 and Mpurp1591, had a Delta15Nbiomass -- NH4 of -15‰, via glutamate dehydrogenase, measured and recorded for the first time in PSB. The delta34S values and amount of intracellular S0 were dependent upon carbon metabolism. Continuous cultures of Mpurp1591 were used to explore the effects of light intensity and cell density on pigment production and isotopic fractionation. Cell density normalized values of okenone and Bchl a were not dependent upon light intensity and had a consistent relationship of approximately 2:3. The range of Delta13Cpigment -- CO2 for the production of pigments was expanded with the reported values for okenone and Bchl a of -15.5 +/- 1.2‰ and -21.8 +/- 1.7‰, respectively. The validity of okenone as a biomarker for water column euxinia has been confirmed, with the caveat that concentrations of okenone are dependent on metabolism and species composition. Furthermore, novel isotopic fractionations were observed for the C, N, and S metabolisms of PSB. |
