The Characterization of Microbiomic and Geochemical Compositions Across the Photosynthetic Fringe

Hot spring outflow channels provide geochemical gradients that are reflected in microbial community compositions. In many hot spring outflows, there is a distinct visual demarcation as the community transitions from predominantly chemotrophs to having visible pigments from phototrophs. It has been hypothesized that this transition to phototrophy, known as the photosynthetic fringe, is a result of the pH, temperature, and/or sulfide concentration gradients in the hot spring outflows. Here, we explicitly evaluated the predictive capability of geochemistry in determining the location of the photosynthetic fringe in hot spring outflows. A total of 46 samples were taken from 12 hot spring outflows in Yellowstone National Park that spanned pH values from 1.92 to 9.04 and temperatures from 28.9 to 92.2 C. Sampling locations were selected to be equidistant in geochemical space above and below the photosynthetic fringe based on linear discriminant analysis. Measurements of pH, temperature, sulfide, dissolved oxygen, total ammonia, nitrate, and dissolved inorganic and organic carbon concentrations trended with respect to the position of the photosynthetic fringe, but no trends were statistically significant. In contrast, there are correlations between pH, temperature, dissolved inorganic carbon, dissolved organic carbon, and total ammonia concentrations and overall microbial community composition and individual cliques, or groups of co-occurring microorganisms, according to co-occurrence network analyses. Although total sulfide concentrations have previously been cited as a determining factor for microbial community composition, sulfide concentrations are only correlated with individual nodes within the co-occurrence network analysis and are not significantly correlated with the overall microbial community composition. Dissolved inorganic and organic carbon concentrations and N-containing compounds may be controlling factors on the co-occurrence of taxa relating to nitrogen and carbon cycling. These results indicate that there is a complex interplay between geochemical variables and the position of the photosynthetic fringe that cannot be fully explained by statistical correlations with the individual geochemical variables included in this study.