極限生物圏 地球生物圏のフロンティア

Abstract

The search for life on the edges (frontiers) of the global biosphere bridges earth-bound biology and exobiology. This communication reviews recent microbiological studies on selected “frontiers”, i.e., deep-sea, deep subsurface, and Antarctica. Deep-sea is characterized as the aphotic (non-photosynthetic) habitat, and the primary production is mostly due to the chemosynthetic autotrophy at the hydrothermal vents and methane-rich seeps. Formation of the chemosynthesis-dependent animal communities in the deep leads to the idea that such communities may be found in the “ocean” of the Jovian satellite, Europa. An anoxic (no-O2), as well as aphotic, condition is characteristic of the deep subsurface biosphere. Microorganisms in the deep subsurface biosphere exploit every available oxidant for anaerobic respiration. Sulfate, nitrate, iron (III) and CO2 are the representative oxidants in the deep subsurface. Below the 3000 m-thick glacier on Antarctica, >70 lakes having liquid water are entombed. One of such sub-glacial lakes, Lake Vostok, has been a target of “life in extreme environments” and is about to be drill-penetrated for microbiological studies. These biospheric frontiers will provide new knowledge about the diversity and the potential of life on Earth and facilitate the capability of astrobiologial exploration.