2000 年 34 巻 3 号 p. 135-141
Microbial respiration is versatile in exploiting terminal electron acceptors (TEAs). In addition to dioxygen (O2), prokaryotic microorganisms utilize nitrate, sulfate, carbon dioxide, etc, as TEAs. Respiration using nitrate as a TEA, or denitrification, results in the release of dinitrogen (N2). Oceanic denitrification is largely limited to the oxygen minimal layer, which is developed below photosynthetic zone; bottom waters are relatively rich in dioxygen, thus 'peak minimum' in dioxygen concentration is formed. However, denitrification is thought to have been a predominant process during the past oceanic anoxic events (OAEs), and denitrification must have been closely coupled with microbial nitrogen fixation. Therefore, a 'short-circuit' cycling of nitrogen is presumed for the biogeochemistry during the past OAE periods. Respiration using sulfate as a TEA, or sulfate reduction, is also thought to have been a major process during the OAEs. Sulfate reduction using methane results in carbonate rock formation, as seen in modern methane-rich cold seeps. This serves as an effective methane-trap, lowering the amount of methane to be released into the hydro-atmosphere. Influence of microbial versatility in TEA utilization on biogeochemical cyclings, along with other examples, is discussed.
