Factors affecting competition between type I and type II methanotrophs in two-organism, continuous-flow reactors

Abstract

Competition experiments were performed in a continuous-flow reactor using Methylosinus trichosporium OB3b, a type II methanotroph, and Methylomonas albus BG8, a type I methanotroph. The experiments were designed to establish conditions under which type II methanotrophs, which have significant cometabolic potential, prevail over type I strains. The primary determinants of species selection were dissolved methane, copper, and nitrate concentrations. Dissolved oxygen and methanol concentrations played secondary roles. M. trichosporium OB3b proved dominant under copper and nitratelimited conditions. The ratio of M. trichosporium to M. albus in the reactor increased ten-fold in less than 100 hours following the removal of copper from the reactor feed. Numbers of M. albus declined to levels that were below detection limits (<106/ml) under nitrogen-limited conditions. In the latter experiment, the competitive success of M. trichosporiumdepended on the maintenance of an ambient dissolved oxygen level below about 7.5 × 10⁻⁵ M, or 30% of saturation with air. The ability of M. trichosporium to express soluble methane monooxygenase under copper limitation and nitrogenase under nitrate limitation was very significant. M. albus predominated under methane-limited conditions, especially when low levels of methanol were simultaneously added with methane to the reactor. The results imply that nitrogen limitation can be used to select for type II strains such as M. trichosporium OB3b.