Abstract
Many studies link the compositions of microbial communities to their environments, but the energetics of organism-specific biomass synthesis as a function of geochemical variables have rarely been assessed. We describe a thermodynamic model that integrates geochemical and metagenomic data for biofilms sampled at five sites along a thermal and chemical gradient in the outflow channel of the hot spring known as “Bison Pool” in Yellowstone National Park. The relative abundances of major phyla in individual communities sampled along the outflow channel are modeled by computing metastable equilibrium among model proteins with amino acid compositions derived from metagenomic sequences. Geochemical conditions are represented by temperature and activities of basis species, including pH and oxidation-reduction potential quantified as the activity of dissolved hydrogen. By adjusting the activity of hydrogen, the model can be tuned to closely approximate the relative abundances of the phyla observed in the community profiles generated from BLAST assignments. The findings reveal an inverse relationship between the energy demand to form the proteins at equal thermodynamic activities and the abundance of phyla in the community. The distance from metastable equilibrium of the communities, assessed using an equation derived from energetic considerations that is also consistent with the information-theoretic entropy change, decreases along the outflow channel. Specific divergences from metastable equilibrium, such as an underprediction of the relative abundances of phototrophic organisms at lower temperatures, can be explained by considering additional sources of energy and/or differences in growth efficiency. Although the metabolisms used by many members of these communities are driven by chemical disequilibria, the results support the possibility that higher-level patterns of chemotrophic microbial ecosystems are shaped by metastable equilibrium states that depend on both the composition of biomass and the environmental conditions.
Highlights
The structures of microbial communities emerge from a combination of environmental, ecological, and evolutionary interactions
The gradients of temperature, pH and redox conditions along the outflow channel of the hot spring coincide with changes in composition of microbial communities that are apparent from metagenomic sequencing
The relative abundances of the model proteins in metastable equilibrium are calculated as functions of a redox variable, log aH2ðaqÞ, but temperature and chemical activities of basis species including pH are taken from field measurements
Summary
The structures of microbial communities emerge from a combination of environmental, ecological, and evolutionary interactions. Gradients of temperature are apparent in hot springs in Yellowstone National Park, and chemical properties such as pH, oxidation-reduction potential, and concentrations of dissolved sulfide and inorganic carbon show great variation among sites, providing the foundation for delineating different possible chemotrophic metabolisms that may take advantage of the disequilibrium among inorganic chemical species [1]. An example is the transition between chemotrophy and the onset of phototrophic metabolisms at lower temperatures [2,3,4,5]. This transition is sometimes referred to as the ‘‘photosynthetic fringe’’ [2] and is regarded as an ecotone [4], i.e. a transition between ecosystems with different metabolic and taxonomic characteristics. Larger-scale geochemical variation across hot springs in Yellowstone can be correlated with phylogenetic trends determined from 16S RNA or metagenomic sequencing, for example using ordination methods such as principal components analysis [8]
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