PHOTOTROPHIC COMMUNITY DISTRIBUTION AND MORPHOLOGY ALONG A GEOTHERMAL STREAM: INSIGHTS INTO BOTH MODERN AND ANCIENT MICROBIAL ECOLOGY

Modern geothermal systems are often studied in conjunction with the rock record to determine ancient microbial ecology. Filaments, microbial mats and stromatalites have been recovered from rocks dating back to the Archean, but evidence for the origin of these structures is often contradictory. Phototrophic bacteria form filaments, mats and stromatalies in a variety of environments given the ubiquity of light. However, the relationship between phototrophic morphology and geochemistry in geothermal systems is poorly constrained.

Rabbit Creek (RC), a circum-neutral geothermal stream in the Midway Geyser Basin of Yellowstone National Park, provides a natural temperature gradient ranging from a near boiling source to ~45ºC 250 meters downstream. Near the source, the creek is host to yellow and orange phototrophic mats at the upper temperature limit of photosynthesis (72ºC). In contrast, green and orange filamentous phototrophs are more common downstream in the cooler water where they form long ropes. In addition to sulfide and temperature, other features of the stream change with distance from the source. For example, oxygen increases with distance from the source, pH increases as CO₂outgasses, and the width and depth of the stream bed change leading to variable flow.Thus, RC is a suitable environment to glean the adaptive methods to changing environmental conditions and community shift distribution of ancient and modern bacterial phototrophs.

Based on reports of abundant Cyanobacteria in phototrophic mats in circum-neutral YNP hot springs, we expected RC mats and filaments to be comprised largely of Cyanobacteria. Further, we expected community composition to change over the geochemical gradients observed in RC along with the changes observed morphology. 16s rRNA data revealed a relatively consistent phototrophic community structure at the phylum level (3.8:1 ratio of Chloroflexi:Cyanobacteria) whereas alpha diversity increases with decreasing temperature. At higher taxonomic resolution, there is a 1:1 ratio of Roseiflexaceae(Chloroflexi)to Leptococcaceae(Cyanobacteria)across sites.This is indicative of a transcriptional shift in the community population, not necessarily a shift in community structure, suggesting that the microbial community is adapting morphologically.