Diversity and composition of cave methanotrophic communities

Methane oxidizing microorganisms (methanotrophs) are a major sink for the greenhouse gas methane (CH4), and have been investigated in several environments. Recent studies show that CH4 consumption in caves is pervasive and is a result of active methanotrophy. However, little is known about what controls the distribution and abundance of methanotrophs in caves. We sampled 42 sediments from 21 caves in North America to elucidate the factors shaping cave methanotroph communities. We hypothesized that cave methanotroph communities should be related to cave-air CH4 concentrations and exhibit dispersal-limited biogeographical patterns due to the insular nature of caves. Using 16S rRNA sequencing, we recovered methanotrophs from 88 % of samples collected, including locations in caves where CH4¬ concentrations were at or below detection limits (≤ 0.3 ppmv). Methanotrophs from the Methylocystaceae (Type II) were the dominant methanotrophs as has been seen in other environments with low CH4 concentrations. Despite being insular ecosystems, we found that the composition of methanotrophs did not vary with distance, both within and among caves. Instead, we found evidence for a core microbiome, perhaps suggesting that high-affinity methanotrophs are not dispersal limited. Additionally, we observed that the relative abundance of methanotrophs was positively related the proportion of gravel in cave sediments and the relative abundance of methylotrophs. Last, we found that the relative abundance of methanotrophs was inversely correlated with cave-air CH4 concentrations. Our results suggest that methanotrophs in caves have influences on cave biogeochemistry beyond CH4 oxidation and that high-affinity methanotrophs may disperse easily into caves.