Primary production by the purple nonsulfur bacterium <i>Rhodopila globiformis</i> in an acidic, moderately sulfidic warm spring

Populations of the acidophilic purple nonsulfur bacterium <i>Rhodopila globiformis</i> were identified in two geographically distinct thermal areas in Yellowstone National Park (Wyoming, USA), as confirmed by 16S rRNA gene sequencing and detection of characteristic methoxylated ketocarotenoids. Microcosm-based carbon uptake assays where oxygenic photosynthesis was excluded via addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea yielded a light-driven dissolved inorganic carbon (DIC) assimilation rate (7 ± 2 mg C g C^-1 h^-1) comparable to those of highly productive algal mats in acidic hot springs, suggesting that <i>R. globiformis</i> may be performing photoautotrophy at the time of the assay. Rates of acetate assimilation were more than two orders of magnitude lower than DIC assimilation and did not differ between light and dark treatments, indicating photoheterotrophic use of acetate was not occurring, though photoheterotrophic assimilation of other organic compounds cannot be excluded. The tepid (35°C) spring waters are acidic (pH = 3.7) with moderate dissolved hydrogen sulfide (0.2 mM) and abundant DIC (11 mM), an apparently rare set of conditions thought to arise from extremely shallow mixing of oxygenated meteoric water and volcanic gases. Though originally isolated and cultured photoheterotrophically, in nature, <i>R. globiformis</i> may grow photoautotrophically under the normal conditions of its habitat, utilizing a stable supply of DIC afforded by the injection of CO₂-rich volcanic gases. To our knowledge, these are the most acidic conditions under which light-driven DIC assimilation has been observed in the domain Bacteria.IMPORTANCEPurple nonsulfur bacteria are ecologically diverse and metabolically versatile anoxygenic phototrophs; however, only a few acid-tolerant species are known. We identified populations of the purple nonsulfur bacterium <i>Rhodopila globiformis</i> in warm, acidic springs with moderate (~0.2 mM) concentrations of dissolved hydrogen sulfide in two thermal areas of Yellowstone National Park (Wyoming, USA). Comprehensive geochemical analyses of the spring waters illustrate that they are formed by mixing of groundwater and CO₂-rich volcanic gases extremely close to the surface, relatively rare conditions that lead to characterization of <i>R. globiformis</i> as an endangered species. A high rate of light-driven assimilation of dissolved CO₂ that rivals rates for acidophilic algae was observed, indicating that <i>R. globiformis</i> is responsible for a significant amount of primary production and suggesting it may primarily grow photoautotrophically in nature. These observations constitute the first insights into the physiological ecology of the most acidophilic anaerobic anoxygenic phototroph presently known.