Rapid shift in microbial community structure in a neutral hydrothermal hot spring from Costa Rica

In this work, we characterize the geochemistry and microbial community of Bajo las Penas, a neutral (pH6.5-7.4), hot spring (T = 62.0-68.0 C) located near Turrialba Volcano, Costa Rica. The microbialcomposition at its two sources belongs mainly to the family Aquificae, comprising OTUs closely related tothe genera Sulfurihydrogenibium, Thermosulfidibacter, Thermodesulfovibrio and Thermocrinis which isconsistent with the presence of moderate levels of sulfate (243-284 mg/L) along the stream. Wedetermined a dramatic shift in the microbial community just a few meters downstream of the sources ofthe hot spring (15-20 meters), with a change from sulfur related chemoautotrophic (e.g.,Sulfurihydrogenibium and an OTU closely related to Thermodesulfovibrio) to chemoheterotrophicprokaryotes (e.g. Meiothermus, Nitrososphaera, Thermoflexus, Thermus). Thus, in this neutral hot spring,the first level of the trophic chain is associated with photosynthesis as well other anaerobic CO 2 fixingbacteria. Then, thermotolerant chemoheterotrophic bacteria colonize the environment to degrade organicmatter and use fermentative products from the first level of the trophic chain. Our data demonstrate howquickly the microbial community of an ecosystem can change in response to environmental variables andsheds light on the microbial ecology of less common circumneutral pH hot springs.