"The method is providing a new window into the microbial world. Now it's possible to determine both the identity and function of naturally occurring microbes, at the level of single cells. We don't even have to grow them in the laboratory to do it," said Ed DeLong, leader of the research group.
"Until recently no one knew which microbes were involved in the oxidation of methane in anoxic marine sediments," adds Victoria Orphan, first author of the Science paper. "By combining molecular and stable isotope techniques, we found a way to link specific microbes to this important ocean process."
In collaboration with Christopher House of Pennsylvania State University and researchers from Woods Hole Oceanographic Institution and the University of California, Los Angeles, the MBARI group used a remotely operated vehicle to collect marine sediments from deep sea methane seeps in the Eel River Basin off California. Molecular probes were used to identify archaea and sulfate-reducing bacteria in the sediments. These microorganisms, 0.5 to 2 micrometers in diameter, can live without oxygen and have not been grown in culture. First the scientists applied RNA probes, then they used secondary ion mass spectrometry to determine stable carbon isotope ratios of the individual microbe cells and cell aggregates. This method distinguished microbes that use methane as a carbon source from those using carbon derived from photosynthesis or other organic carbon sources.
The researchers showed that the archaea and bacteria cells could be identified individually within the cell aggregates. The two kinds of microbes form a partnership to extract energy from methane in the absence of oxygen. The methane-oxidizing archaea at the core of the aggregate transfer carbon compounds to their sulfate-reducing bacterial partners in the outer layers of the aggregate. Since nearly 80 percent of the methane in marine sediments is removed by these methane-consuming microbes, the discovery provides new insight into a critical process. Orphan and her colleagues are excited by the implications of this research, as these techniques can be used to simultaneously identify environmentally important microorganisms and characterize their metabolic activities in nature.
Note to media: Images available upon request.