SPATIAL VARIATIONS IN MICROBIAL COMMUNITY COMPOSITION BETWEEN WATER AND SEDIMENT SAMPLES IN LITTLE HOT CREEK, CALIFORNIA: IMPLICATIONS FOR THE ROLE OF TEMPERATURE IN CONTROLLING HOT SPRING DIVERSITY

In hot springs, metabolic capability tends to shift from hotter chemosynthetic (>72°C) to cooler photosynthetic zones (<72°C) away from the source. Microbial diversity in this chemo- to photoautotrophic transitional zone is still poorly constrained. We studied a hot spring in Long Valley Caldera near Little Hot Creek, California as part of the 2015 Geobiology Summer Course to characterize microbial community composition, diversity, and function along a ~23 m transect of decreasing temperature (~82-71°C). 16S rRNA gene sequencing for the planktonic composition across the temperature gradient yielded a consistent microbial community composition dominated by Thermocrinis, Sulfurihydrogenibium, EM3 group Thermotoga, Caldimicrobium, and Anaerolineaceae. However, sediment community compositions were notably more heterogeneous. Sediment near the source was dominated by chemolithoautotrophic Sulfurihydrogenibium and Caldiarchaeum, while larger contributions from candidate phyla and unclassified archaea were observed in sites possibly representing the metabolic transitional zone; photosynthetic organisms such as Synechococcus dominated sediment after ~15 m. Although Sulfurihydrogenibium and Thermocrinis were both prevalent in the water of Little Hot Creek, these genera were also found in >15% abundance in one sediment site each. Despite the large shifts in sediment community composition down transect, richness did not inversely correlate with temperature. Additionally, the co-presence of Sulfurihydrogenibium and Thermocrinis in the water is unusual, as Sulfurihydrogenibium typically colonizes high sulfide springs, whereas Thermocrinis is common in low sulfide springs [Takacs-Vesbach et al. 2013]. The distribution of these genera in the sediment could be controlled by chemical parameters such as dissolved oxygen or sulfide concentration, although the mechanism for this shift from a planktonic to a sediment community remains unclear. Compositional trends in Little Hot Creek communities differ from previously surveyed hot spring environments. The lack of obvious covariation between temperature and community diversity is particularly interesting and might lead to a better understanding of processes responsible for metabolic shifts observed in hot spring environments.