Proximity to Propagule Sources Affects the Response of Soil Methane Oxidizer Community to Increased Salinity

The responsiveness of microbially-mediated processes such as methane oxidation to environmental change is likely to be dependent on the correct species or strains of methane oxidizers being present to exploit the new conditions. It is unclear whether the slow dispersal of microbes will limit the availability of the system to respond. Here, we compared the ability of methanotroph community in lakeshore soils of two lakes to respond to an experimental increase in salinity. One set of samples was taken from lakeshore soils of a freshwater lake (Yang Lake), the other from a slightly brackish lake (Qinghai Lake), both on the Tibetan Plateau. Samples were incubated in microcosms by adding ~5% 13 CH 4 or 12 CH 4 and different concentrations of NaCl solution. DNA stable-isotope probing (DNA-SIP) followed by high-throughput sequencing was used to determine how the active methanotrophic populations differed in lakeshore soils with different salinity levels. Samples from both saline and freshwater lake initially showed much-reduced methane oxidation ability and methanotrophic activity at increased salinity. For the freshwater samples with the salinity of 25 and 50 g L -1 after NaCl addition, there was no adaptation and increase in methanotrophy after 7 days, while by contrast samples from the brackish lake showed an initial depression of methane oxidation, followed by greatly increased rates after several days. Sequencing showed that this recovery of methanotrophy in the brackish lake samples was associated with a major switchover in composition of active methanotroph community. In particular, the relative abundance of Type Ia methanotrophs became more abundant at increased salinity. Methylomicrobium came to dominate over Methylovulum from before. It appears that in the freshwater lake environment, isolation from any nearby sources of high-salinity-tolerant bacteria has prevented adaptation to a high salinity change in the environment. This is an apparent instance of propagule limitation preventing adaptation to changed conditions