The first methane-oxidizing bacterium from the upper mixing layer of the deep ocean:Methylomonas pelagica sp. nov.

Methane enrichment of twenty-three 100-ml portions of seawater from three stations in the Sargasso Sea yielded the same obligate type I methanotroph. It is pigmented white, requires NaCl, grows well in seawater with either methane or methanol, but not on other C₁ compounds nor on C–C bonded organic matter, and it uses either ammonia or nitrate but not dinitrogen as a nitrogen source. Formaldehyde is produced in marked amounts from methanol. Growth occurs at 20° and 30°C but not at 10°C and is inhibited in natural sunlight. Representative isolates from each hydrographic station assimilate one-carbon units via the ribulose monophosphate pathway for formaldehyde fixation, and have a DNA base composition of 49 mol% guanine plus cytosine. The type strain, NCMB 2265, has been namedMethylomonas pelagica sp. nov. This upper ocean methanotroph may obtain its C₁ substrates in situ from particles of algal debris that become anoxic, ferment, and accumulate in the thermocline to form a false benthos.