Microbial Diversity in Hydrothermal Systems and Their Influence on Geological Environments

Microorganisms found at terrestrial and seafloor hydrothermal systems are classified based on their optimum growth temperatures. Prokaryotes exhibiting optimum growth temperatures above 45degreesC are defined as thermophiles, while those with optimum growth temperatures above 80degreesC are called hyperthermophiles. Hyperthermophiles have chemically and thermodynamically stable cytoplasmic-membrane consisting of ether-lipids. Hyperthermophiles observed at temperature up to 113degreesC in seafloor hydrothermal areas should have thermostable proteins. Presumably, the thermostable proteins are replacing certain amino acid residue by proline instead of alanine in order to increase hydrogen bonds and salt bridges. Microorganisms inhabiting terrestrial and seafloor hydrothermal areas obtain energy mainly by use of chemical disequilibrium, so that their habitat may be restricted to the redox boundary between ambient water and reduced hydrothermal fluids. Among more than 200 species of cultivable thermophiles and hyperthermophiles, hyperthermophiles mainly obtain energy by sulfur reduction, because this reaction can produce energy efficiently at >80degreesC. Some hyperthermophilic sulfur reducers can reduce Cr(VI), Mn(IV), Fe(III), Co(III), As(V), Se(VI), Tc(VII), Au(III) and U(VI) through hydrogen or organic compounds that act as reducing agents for yielding energy. Some hyperthermophiles use W instead of Mo for activating their enzymes, Bacteria can pump out toxic metal ions from their cells, however toxic metal resistance systems of archaea, including hyperthermophiles, have not yet been studied. Methane producing hyperthermophiles can yield energy by the autotrophic reaction, when sufficient carbon dioxide and molecular hydrogen can be supplied and methane can be removed from hydrothermal systems. If nitrogen can be supplied into a hydrothermal system, hydrogen oxidization by nitrogen species (e.g. NO3-, NO2-, NO and N2O) may be expected through the metabolism of some hyperthermophiles.