| Hot spring biofilms are of obvious biological origin, but of surprising composition. Organic carbon makes up only a minor percentage of the total mass of thermophilic chemotrophic and phototrophic biofilms. In this study it was found that the majority of biofilm mass is inorganic material, largely silica, with measurable quantities of dozens of other elements, and that the distribution of major elements mimics that of surrounding rock and soil far more closely than the hot spring fluids. A three-component major-element mixing model involving biomass (carbon and nitrogen), silica precipitation (silicon), and aeolian deposition of local country-rock-derived particulates (aluminum, sodium, potassium, iron, calcium, and silicon), best explains the compositions. Comparisons of biofilms with the composition of their geochemical surroundings help identify trace elements that are anomalously enriched or depleted. These anomalies provide insight into the processes of active or passive elemental accumulation by biofilms, which could be used to identify evidence for life in hydrothermal deposits in the rock record. |
