Production and consumption of nitric oxide by three methanotrophic bacteria

We studied nitrogen oxide production and consumption by methanotrophs il Me thylobacter luteus (group I), Methylosinus trichosporium OB3b (group II), a nd an isolate from a hardwood swamp soil, here identified by 16S ribosomal DNA sequencing as Methylobacter sp, strain T20 (group I). All could consume nitric oxide (nitrogen monoxide, NO), and produce small amounts of nitrous oxide (N2O). Only Methylobacter strain T20 produced large amounts of NO (> 250 parts per million by volume [ppmv] in the headspace) at specific activi ties of up to 2.0 x 10(-17) mol of NO cell(-1) day(-1), mostly after a cult ure became O-2 limited. Production of NO by strain T20 occurred mostly in n itrate-containing medium under anaerobic or nearly anaerobic conditions, wa s inhibited by chlorate, tungstate, and O-2, and required CH4. Denitrificat ion (methanol-supported N2O production from nitrate in the presence of acet ylene) could not be detected and thus did not appear to be involved in the production of NO. Furthermore, ed, and Cu nitrite reductases, NO reductase, and N2O reductase could not be detected by PCR amplification of the nirS, nirK, norB, and nosZ genes, respectively. M. luteus and nl, trichosporium p roduced some NO in ammonium-containing medium under aerobic conditions, lik ely as a result of methanotrophic nitrification and chemical decomposition of nitrite, For Methylobacter strain T20, arginine did not stimulate NO pro duction under aerobiosis, suggesting that NO synthase was not involved. We conclude that strain T20 causes assimilatory reduction of nitrate to nitrit e, which then decomposes chemically to NO. The production of NO by methanot rophs such as Methylobacter strain T20 could be of ecological significance in habitats near aerobic-anaerobic interfaces where fluctuating O-2 and nit rate availability occur.