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京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第17893号 / 工博第3802号 / 新制||工||1581(附属図書館) / 30713 / 京都大学大学院工学研究科合成・生物化学専攻 / (主査)教授 跡見 晴幸, 教授 森 泰生, 教授 濵地 格 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18304号 / 工博第3896号 / 新制||工||1598(附属図書館) / 31162 / 京都大学大学院工学研究科合成・生物化学専攻 / (主査)教授 跡見 晴幸, 教授 森 泰生, 教授 濵地 格 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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The archaeal branch of life represents some of the oldest life forms on Earth. Archaea are believed to have diverged from Prokaryotes roughly 3.5 billion years ago and it’s theorized that biological methane production started around this time as well. This would make methanogenesis one of the oldest metabolisms on our planet. Methanogenesis is a process that, so far, is known to be unique to archaea. Since their evolution, methanogens have had massive impacts on Earth’s climate and biology. Methane is an important part of the global carbon cycle, but is also a major greenhouse gas, making it a vital area of research. The methanogen studied in this thesis is referred to as the wild type (WT) and was isolated from an active serpentinization system at Chimaera seep in Çıralı, Antalya Gulf, Turkey. The Chimaera seep is a geological formation analogous to mid-ocean ridges, but exposed and above land. The research in this thesis focuses on the description of the WT, which is believed to be a new species. Describing the WT consisted of the characterization of extremes as well as optimal growth conditions. The WT was found to grow at initial pH levels of 9.0 and 9.5. It grew from 15 °C to 45 °C, but not at 47 °C and not at 12 °C, and had an optimum at 39 °C. The WT had measurable growth up to 40 g/L NaCl, and had its optimum at 0 g/L. The WT grew best with H2/CO2 substrate, but also grew well on formate.

This dissertation research aims to show that there are deterministic microbial distribution patterns based on quantifiable environmental thresholds by determining and rationalizing the relative abundances of hyperthermophilic methanogens, autotrophic iron(III) oxide reducers, and heterotrophic sulfur reducers within deep-sea hydrothermal vents. Organisms of these metabolisms are predicted to be relatively more abundant in different regions depending on environmental conditions such as reduction potential, organic carbon, and hydrogen availability. The relative abundances of these metabolic groups within samples from the Endeavour Segment and Axial Volcano in the northeastern Pacific Ocean were determined. Iron(III) oxide reducers were detected in nearly all samples while methanogens were generally not present or present in concentrations lower than those of the iron(III) reducers. To determine growth constraints and the effect of hydrogen concentration on hyperthermophilic methanogen growth kinetics, Methanocaldococcus jannaschii and two new Methanocaldococcus field isolates were grown at varying hydrogen concentrations. The hydrogen-dependent growth kinetics for all three strains were statistically indistinguishable, exhibiting longer doubling times and lower maximum cell concentrations with decreasing hydrogen concentrations until growth ceased below 17-23 μM. This minimum hydrogen concentration for hyperthermophilic methanogenesis was correlated with field microbiology and fluid geochemistry data from the Endeavour Segment and Axial Volcano. Anomalously high methane concentrations and thermophilic methanogens were only observed in fluid samples where hydrogen concentrations were above this predicted threshold. Aside from anomalous sites, methanogens are predicted to be hydrogen limited, and may rely on hydrogen produced by heterotrophs as suggested by in situ sampling and co-culture experiments. Models and kinetic experiments suggest that iron(III) oxide reducers are not hydrogen limited under the same conditions. A Methanocaldococcus strain that we isolated from Axial Volcano and used in our hydrogen threshold experiments was bioenergetically modeled over its range of growth temperatures, pH, NaCl concentrations, and NH4Cl concentrations. Its methane production rates and growth energies were largely constant but increased at superoptimal temperatures and when nitrogen was limiting. The results of this research demonstrate that the rates of and constraints on metabolic processes can be used to predict the distribution and biogeochemical impact of hyperthermophiles in deep-sea hydrothermal vent systems.

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