Hot Spring Microbial Community Elemental Composition: Hot Spring and Soil Inputs, and the Transition from Biocumulus to Sinter

Abstract:

Hydrothermal microbial communities contain some of the most deeply branching members of the tree of life, and hydrothermal environments have been present on the Earth’s surface since the condensation of the ocean over four billion years ago. Hydrothermal microbial communities are a potential source for biosignatures across nearly all of Earth’s history, and the most likely mode of life (past and/or present) if it had developed on other bodies in the solar system. While there are general patterns of element enrichment for hydrothermal water, the elemental composition of bulk hydrothermal microbial communities (here termed biocumulus, encompassing biomass and non-biomass material) are largely unexplored. In order to elucidate the elemental composition of hot spring biocumulus and explore the sources of those elements, we sampled 87 hot spring biocumulus in 19 hot springs along with dozens of associated soil, rock, sinter, and autochthonous biomass samples and analyzed them for 41 elements, in conjunction with a larger sampling campaign (> 1000 hot spring water samples from 11 hydrothermal areas within Yellowstone National Park). While biocumulus are of obvious biological origin, they have surprising elemental compositions. Organic carbon makes up a minor percentage of the total mass of thermophilic chemotrophic and phototrophic biocumulus. We have found that the majority of hot spring biocumulus 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. Analyses indicate a consistent pattern of elemental composition for biocumulus across varying hydrothermal geochemical compositions, and a systematic loss of biologically-associated elements during diagenetic transformation of biocumulus to siliceous sinter.