CHEMICAL AND STABLE ISOTOPIC COMPOSITION OF MICROBIAL MATS FROM HOT SPRINGS OF UZON CALDERA, KAMCHATKA

Hot springs of the Uzon Caldera were chemically and isotopically analyzed to better understand the relationship(s) between thermophilic microorganisms and the environments in which they live.  These springs reach temperatures in excess of 95°C and contain abundant microbial mats that grow on siliceous sinters and fine-grained hydrothermal deposits.  Gases that emanate from the springs are composed predominantly of CO₂ (20 to 90%), with lesser amounts of CH₄, (£ 20%), H₂, NH₃ and SO₂.  Because the springs are acidic (pH 4 to 6), they contain little dissolved inorganic carbon (DIC: mmol L^-1) and sulfide (< 200 ppb), yet in some cases where microbial activity and pH are relatively high, these constituents can reach the millimol L^-1 and ppm range, respectively.

There was a tight positive relationship between the %C and %N of bulk solid samples (C/N ratio of 5.3), which is similar to that for mats from other hot spring environments.  In addition, the samples displayed a relatively large range of stable isotope compositions, from 0 to 25‰ for both d¹³C and d¹⁵N.  The wide range of d¹³C values is probably due to the variety of microbial carbon fixation pathways (e.g., Calvin, Acetyl Co-A, reductive TCA, 3-HP) that are utilized by autotrophic microorganisms in these springs.  The wide range in d¹⁵N values is probably related to the fact that the springs are not nitrogen limited.  Because the springs are acidic, NH₃ is easily hydrolyzed to NH₄, and this in turn is readily available for microbial uptake.

There was a significant positive relationship between the d¹³C value of bulk solid samples and the CO₂/CH₄ ratio of free gas emanating from the springs. This CO₂ and CH₄ may originate from the release of abiotic volcanic gases or it may be related to microbial processes such as respiration and methanogenesis.  The d¹³C value of the bulk solid may be reflecting the predominant source of carbon being incorporated in the biomass of each spring.  Of those spring containing sufficient DIC for carbon isotope analysis, d¹³C values were relatively high at +8 to +14‰, which implies the gas source was probably not atmospheric CO₂ (~ -7‰). The origin of C-bearing components in the springs and the relationship(s) between them can be clarified through the carbon isotope analysis of gas samples collected from the springs.