Seminar: Beatrix Heinze

Groundwater is the largest terrestrial reservoir of freshwater and harbors much of the world’s prokaryotic biomass. High methane concentrations as well as genetic surveys suggest a strong contribution of groundwater microbes to methane oxidation, but little is known about the amounts of methane removed by microbial activity in such ecosystems. Using an ultra-low level 14C-labelling approach, we quantified in situ microbial methane oxidation rates in carbonate and sand aquifers from central and northern Germany. Rates in carbonate aquifers resembled measurements in oligotrophic marine systems ranging from 0.002 to 0.02 μg CH4 L-1 d-1. In contrast, rates in sand aquifers exceeded those measured in lake water columns, being as high as 116.7 μg CH4 L-1 d-1. As electron acceptors such as nitrate, sulfate, iron, or, at few sites, oxygen, were available, groundwater methane concentrations appeared to be the most important rate limiting factor. Methane was mainly derived from microbial production, shown by high methane to [ethane + propane] ratios, but elevated 13C and low 14C contents suggested fossil gas inputs at several sites. Our results further suggest that methane serves as an energy rather than carbon source for the groundwater microbiome, as >90% of methane was oxidized to CO2 and not incorporated into microbial biomass. Metagenomics revealed the presence of methane monooxygenase genes in members of the Methylomirabilales and Rokubacteriales, suggesting the potential use of the intra-aerobic pathway for methane oxidation in the mostly anoxic to suboxic carbonate groundwaters. Extrapolation of our findings to annual consumption revealed fast methane turnover times of <1 year in both high and low rate wells, underlining the rapid methane consumption by groundwater microbes. These results indicate that microbial methane removal in groundwater is as efficient as in marine systems or subglacial lakes, where microbial oxidation is estimated to remove >90% of all methane before it reaches the atmosphere.