A temperate river estuary is a sink for methanotrophs adapted to extremes of pH , temperature and salinity

Summary River T yne ( UK ) estuarine sediments harbour a genetically and functionally diverse community of methane‐oxidizing bacteria (methanotrophs), the composition and activity of which were directly influenced by imposed environmental conditions ( pH , salinity, temperature) that extended far beyond those found in situ . In aerobic sediment slurries methane oxidation rates were monitored together with the diversity of a functional gene marker for methanotrophs ( pmoA ). Under near in situ conditions (4–30° C , pH 6–8, 1–15 g l −1 NaCl ), communities were enriched by sequences affiliated with M ethylobacter and M ethylomonas spp. and specifically a M ethylobacter psychrophilus ‐related species at 4–21°C. More extreme conditions, namely high temperatures ≥ 40°C, high ≥ 9 and low ≤ 5 pH , and high salinities ≥ 35 g l −1 selected for putative thermophiles ( M ethylocaldum ), acidophiles ( M ethylosoma ) and haloalkaliphiles ( M ethylomicrobium ). The presence of these extreme methanotrophs (unlikely to be part of the active community in situ ) indicates passive dispersal from surrounding environments into the estuary.