Occurrence and Carbon Metabolism of Green Nonsulfur-like Bacteria in Californian and Nevada Hot Spring Microbial mats as Revealed by Wax Ester Lipid Analysis

Green nonsulfur-like bacteria (GNSLB) in Yellowstone hot spring microbial mats have been extensively studied and are thought to operate both as photoheterotrophs and photoautotrophs. Here we studied the occurrence and carbon metabolisms of GNSLB by analyzing the distribution and isotopic composition of their characteristic wax ester lipids in four Californian and Nevada hot spring microbial mats at a range of temperatures (37–96°C). The distribution of wax esters varied strongly with temperature. At temperatures between 50–60°C the wax ester composition in each of the four hot spring microbial mats was dominated by C₃₀ to C₃₆ wax esters, consisting of mixtures of C₁₅-C₁₈ n-alkyl and branched fatty acids and alcohols, typical for GNSLB. Stable carbon isotopic analysis showed that these wax esters were only depleted by 5 to 10‰ compared to dissolved inorganic carbon in the overlying water, suggesting that these GNSLB were mainly autotrophic. However, analysis of different depth layers of one microbial mat showed that these GNSLB wax esters were increasingly depleted in ¹³C with depth, suggesting that photoautotrophy mainly occurred in the top layer of the mat. ¹³C-depleted C₃₆-C₄₄ wax esters were found in one hot spring at high temperatures (77–96°C) and are likely derived from allochtonous plant waxes. At several lower temperature sites (35–40°C) the wax esters were predominantly composed of C₂₈, C₃₀ and C₃₂ wax esters consisting of mixtures of C₁₄-C₁₆ fatty acids and n-alkanols and were depleted in ¹³C by 15–20‰ relative to dissolved inorganic carbon, suggesting they may be derived from heterotrophic organisms. Our results indicate that autotrophic GNSLB occur widely in hot springs and that diverse groups of organisms contribute to the pool of wax ester lipids in hot spring environments.