Experimental verifications of metabolic potential in deeply-sourced springs in western Turkey

This study investigated microbial metabolism in fault-associated cool and hydrothermal springs in western Turkey in an effort to bridge thermodynamic predictions of favorable metabolic pathways with observations in vitro and the presence of diagnostic functional genes. Doing so will help constrain the utility of calculations of Gibbs free energies of reaction as predictive tools for microbial ecology, aiding future studies investigating the habitability of astrobiological targets such as Mars or Europa. Modern-day serpentinization and related weathering processes occur in the subsurface of ophiolite-hosted spring systems in Turkey, prompting two exploratory field surveys of several springs in both northwest and southwest Turkey. Both hydrothermal activity and serpentinization provide a source of reduced compounds and are thought to be important for energy generation on early Earth, as well as other rocky planets. The geochemistry of spring fluids is impacted by both hydrothermal alteration and the weathering of ultramafic reservoir rocks, producing chemically distinct systems with a diverse suite of metabolic options. Cultivation of native microorganisms from sampled spring fluids in targeted growth media, in conjunction with taxonomic affiliations with known organisms based on 16SrRNA bacterial phylogeny, is used to assess the metabolic diversity in each system. These findings are compared with calculated Gibbs free energies of reaction for several chemosynthetic metabolic pathways to assess whether or not they are consistent with actual metabolic capacity. Nitrogen-cycling functional genes are detected and related to growth in targeted media. Evidence for partial denitrification within a biofilm supported by fluid discharging from an actively burning methane vent in the Tekirova ophiolite (Yanartaş, southwest Turkey) is substantiated by geochemical and isotopic evidence, as well as observations in culture and the presence of functional nitrogen-cycling genes, substantiating thermodynamic predictions that some exergonic metabolic pathways are utilized in situ. Data obtained on taxonomic relationships and observations in vitro of heterotrophic, thermophilic enrichment cultures from hot springs in northwest Turkey sheds new insight onto the functional diversity of fault-associated hydrothermal and cool spring systems. These findings support a correlation between thermodynamics and metabolic potential, though environmental factors (e.g. availability of organic carbon and nitrogen) may limit metabolic functioning in situ.