Diversity and Abundance of Methanotrophic Bacteria in a Permanently Stratified Lake.

Methanotrophic bacteria are globally significant due to their role as a major biological sink for methane, a greenhouse gas. These microorganisms are especially active at interfaces where methane and oxygen meet. Methanotrophs are also of environmental interest because of their ability to co-metabolize many common organic pollutants including halogenated and aromatic hydrocarbons. Lake Mishawum in Woburn, MA, is a shallow, permanently stratified lake with an anoxic, methane-rich hypolimnion and an oxic epilimnion. This unique stratification is maintained because the bottom water continually receives inputs of salty groundwater, while the upper layer is fed by rainwater runoff. Modeling of methane concentration profiles in the lake showed high rates of methane oxidation at the oxic-anoxic interface. Hence, methanotrophs have been postulated to be a significant and active population of the bacteria residing in the lake's metalimnion. The diversity, abundance, and seasonal variation of methanotrophic bacteria at this interface was investigated using PCR gene diversity studies, quantitative PCR (QPCR), and cultures of methane-oxidizing isolates from metalimnion water samples. Gene diversity and isolate analyses revealed that as in other saline lakes, both Type I and Type II methanotrophs were present at the interface. QPCR studies using the methanol dehydrogenase gene showed that their populations comprised up to 15 percent of the total bacterial population in the summer months. Year long QPCR studies of methanotrophs populations at the Lake Mishawum interface indidcated that, while total bacteria numbers do not change over time, methanotroph population size varied seasonally and correlated with temperature changes in the lake. These results suggest that methanotrophs are a diverse and highly active population in the oxic-anoxic interface of Lake Mishawum. The finding of both Type I and Type II methanotrophs at the interface calls into question the role of these two populations and the potential for different metabolic strategies for methane oxidation and growth in this unique environment.