Glass, J. B.; Warters, L.; Alajlan, A.

Methanol, the simplest alcohol, has long been recognized as a key energy and carbon source for soil and plant-associated bacteria and fungi, and is increasingly recognized as an important substrate for marine bacteria. Lanthanide-dependent methanol dehydrogenases (encoded by the gene xoxF) are now recognized as the key catalysts for methylotrophy in many environments, yet the identity of the most transcriptionally active methylotrophs (\"Clade X\") has remained elusive. Here we show that \"Clade X\" methylotrophs belong to the deep-branching alphaproteobacterial order TMED127, which we propose be renamed \'Methylaecorales\'; \'methyl\' for \'methylotrophic metabolism\' and \'aecor\' for \'ocean surface\', as these bacteria are most transcriptionally active near the sea surface. TMED127/Methylaecorales have small, streamlined genomes (~1.5 Mb) and appear to be obligate methylotrophs that use the serine cycle for carbon assimilation. TMED127/Methylaecorales showed a diel pattern of xoxF5 and glucose dehydrogenase (gdh) transcription, peaking in the late afternoon, in oligotrophic surface water of the Sargasso Sea. Several other highly transcribed genes of unknown function had no homologs outside of TMED127/Methylaecorales genomes. Our findings illuminate an overlooked marine methylotrophic bacterium and predict a diel cycle of methanol production in surface seawater by an unknown pathway.