Major Element Geochemistry of Biofilms in a Silica-Precipitating Hot Spring

Hydrothermal biofilm communities represent one of the best present-day representations of early microbial communities, dating back to 2.5 Ga, and possibly 3.8 Ga in the geologic record. Silica-precipitating hydrothermal springs have been thought to have great potential for biosignature preservation. The interactions of hydrothermal water, biofilms, and precipitated siliceous sinter, however, remain poorly constrained. To this end, we collected water and biofilm, as well as contextual sinter and rock samples from various hot springs in Yellowstone National Park. Here we focus on one hot spring in Sentinel Meadow (Lower Geyser Basin), with temperature and pH that vary from the source (93 C, pH 7.4) to the farthest of five collection points down channel (56 C, pH 8.2). Elemental analysis reveals that the biofilms are made up of from <1 to ~11 % dry wt. carbon and ~0.1 to 1% dry wt. nitrogen. Major element analysis via electron microprobe and complimentary x-ray fluorescence show that (excluding C and N from the total) SiO2 constitutes 86 to 94 % dry weight mass, with the rest made up of Al2O3 (3 to 8%), Na2O (1.7 to 3.7%), K2O (0.6 to 1.5%), and minor amounts of FeO, CaO, MgO, and TiO2 (<1%). Local sinter is SiO2 (97.5% dry wt.), Na2O (1.5%), and <1% Al2O3, FeO, K2O, CaO, MgO, and TiO2. In addition, sinter contains measurable amounts of carbon (1.4%) and nitrogen (0.2%). Discrepancies between the biofilm and sinter values show that the geochemical compositions of biofilms are not captured by the precipitating silica. If biofilms accumulated elements strictly from the water, then it would take as much as 440 L of water to supply 1 gram (dry wt) of biofilm with the elements contained therein, assuming complete uptake. This seems especially unlikely in the case of Al, which is quite dilute (~500 ppb), poses very little benefit nutritionally, and increases in concentration down channel. Other major element components also exhibit at least one, if not all, of these traits. A potential source of the elements found in biofilms is aeolian-deposited dust. Area country rock is dominated by siliceous volcanism, represented locally by rhyolite samples collected from Sentinel Meadow. With an average value of ~10 wt % Al2O3 for the surrounding country rock, it would take approximately 0.6 grams of the ground up rock as dust to account for the Al found in one gram of biofilm. The low Al2O3 content of the sinter indicates that the Al is not entombed from the biofilms. A hypothesis for the above discrepancies in Al (as well as other elements) is that dust deposited in the water is captured on the biofilm surfaces, and the biofilm community then breaks down the dust, utilizing any nutritionally or metabolically important elements, and either precipitating (for Si) or releasing (for Al) unnecessary elements.