An Obligate Methylotrophic, Methane-oxidizing Methylomicrobium Species from a Highly Alkaline Environment

A new, obligately methylotrophic, methane-oxidizing bacterium, strain AMO 1, was isolated from a mixed sample of sediments from five highly alkaline soda lakes (Kenya). Based on its cell ultrastructure and high activity of the hexulose-6-phosphate synthase, the new isolate belongs to the type I methanotrophs. It differed, however, from the known neutrophilic methanotrophs by the ability to grow and oxidize methane at high pH values. The bacterium grew optimally with methane at pH 9-10. The oxidation of methane, methanol, and formaldehyde was optimal at pH 10, and cells were still active up to pH 11. AMO 1 was able to oxidize ammonia to nitrite at high pH. A maximal production of nitrite from ammonia in batch cultures at pH 10 was observed with 10% of CH4 in the gas phase when nitrate was present as nitrogen source. Washed cells of AMO 1 oxidized ammonia most actively at pH 10-10.5 in the presence of limiting amounts of methanol or CH4. The bacterium was also capable of oxidizing organic sulfur compounds at high pH. Washed cells grown with methane exhibited high activity of CS2 oxidation and low, but detectable, levels of DMS and DMDS oxidation. The GC content of AMO 1 was 50.9mol%. It showed only weak DNA homology with the previously described alkaliphilic methanotroph "Methylobacter alcaliphilus" strain 20Z and with the neutrophilic species of the genera Methylobacter and Methylomonas. According to the 16S rRNA gene sequence analysis, strain AMO 1 was most closely related to a neutrophilic methanotroph, Methylomicrobium pelagicum (98.2% sequence similarity), within the gamma-Proteobacteria.