During the project period, we have conducted intensive studies in both marine and freshwater systems. Investigations in marine systems included oceanic oxygen minimum zones (OMZs; oceanic waters where oxygen is depleted and anaerobic processes prevail) as well as coastal waters subject to permanent and seasonal oxygen depletion. We have studied methane cycling in the eastern tropical North Pacific and in the adjacent Golfo Dulce, Costa Rica, where waters with OMZ-like conditions are easily accessed. Our studies provide the first evidence for active methane consumption by anaerobic oxidation in the OMZ waters. The eastern tropical North Pacific holds the largest accumulation of methane in the open ocean, and our results imply that this methane pool is highly dynamic, and that consumption by anaerobic methane-oxidizing microbes is the main methane sink, substantially attenuating the transport of methane to surface water and eventually to the atmosphere.
In coastal waters subject to seasonal oxygen depletion, we similarly demonstrated how a highly efficient microbial community acts as a filter towards the release of the methane that accumulates in anoxic bottom waters. This is an important finding as these coastal “dead zones” are spreading and could represent a strong source of methane if such a filter did not exist.
This work required the development of highly sensitive methods to measure the microbial processes in the water column. While our measurements were conducted on samples recovered to the laboratory, we have also developed a system for incubation in situ, i.e. directly within the water column, in order to avoid potential artefacts resulting from sample recovery. The system will be of wide use in future studies of microbial processes in anoxic waters.
We have identified several different types of microbes that likely contribute to methane consumption in these systems. One surprising finding is that in oxygen-depleted coastal waters, methane oxidation is carried out by bacteria that are known as obligate aerobes, i.e. they are assumed to require oxygen for their metabolism. How these organisms manage to thrive in the absence of oxygen is a puzzle that remains to be solved. Our genomic investigations suggest that they may be able to use nitrogen compounds for their respiration as an alternative to oxygen. However, in a collaborative study, we also discovered that some microbes that are abundant in marine systems are able to produce oxygen under anoxic conditions. This may have very important implications for our understanding of the biogeochemistry of oxygen-depleted systems.
Freshwater systems make up a very important source of methane to the atmosphere, yet the mitigating impact of anaerobic methane oxidation on methane emissions was not known. We have determined rates of anaerobic methane oxidation at a range of different locations and explored how process rates depend on environmental parameters. We find that methane is oxidized anaerobically in diverse habitats and with an array of different oxidants including nitrate, nitrite, ferric iron, and sulfate. We also find that anaerobic methane oxidation in some settings efficiently traps methane and prevents its emission to the atmosphere. We have identified a group of microbes that appears to be of particular importance for oxidizing methane with iron and/or sulfate as electron acceptor, and we explored the ecophysiology of these organisms with both biogeochemical and molecular biological techniques. Based on this, we developed kinetic relationships that allows the inclusion of these processes in modellling of methane emissions from freshwater systems.
