[Research progress of atmospheric methane oxidizers in soil]

Microbial oxidation in soil is the only biological sink for atmospheric methane (about 1.8 ppmv). Two groups of atmospheric methane oxidizing bacteria are existed in aerobic soils: obligate and alternative atmospheric methane oxidizing bacteria. The former, such as upland soil cluster alpha (USCalpha) and upland soil cluster gamma (USCgamma), are widely distributed in a variety of aerobic upland soils, and their particulate methane monooxygenase (pMMO) have very high affinity for methane in low concentration. Bacteria in this group are probably genuine oligotrophs. However, so far, there is still no cultivated strain of this group. The latter (Methylocystis/Methylosinus) belongs to traditional methane-oxidizing bacteria, and are widely distributed in soil environments with periodic high methane emission. Most strains of these two genera are known to possess two pMMO isozymes with low and high affinity to methane respectively, and these strains can keep atmospheric methane oxidizing activity for relative long periods ( > 3 months) relying on the high affinity pMMO (pMMO2). However, the growth and reproduction of bacteria in this group are still dependent on endogenous high-concentration methane which is periodically produced within the soils. We reviewed the research progress of these two groups of atmospheric methane-oxidizing bacteria, their possible living strategies, and the effects of several key environmental factors (e. g. soil temperature and moisture, soil pH, vegetation, land use, nitrogen input) on their community composition and methane oxidizing activity. Several important research directions of atmospheric methane oxidizing bacteria have also been proposed.