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.