Anaerobic Microbial Communities

Although our environment is rich in free oxygen (O2), there are many geologic environments that lack O2, such as the sediments underlying essentially all the Earth's oceans. While no animals live in these anaerobic conditions, there are abundant microorganisms, such as bacteria, that thrive without oxygen essentially by "breathing" inorganic substances, like sulfate or iron, in place of oxygen (Dianne Newman's group studies the biochemical details of how bacteria pull off this amazing feat. Visit their site to learn more). We are interested in learning how quickly and how efficiently these microbes can degrade organic matter. The answer has far-reaching importance, from the accumulation of CO2 in our atmosphere (which leads to global greenhouse warming) to the accumulation of natural gas and petroleum in sediments (on which our society depends for its economic prosperity). For example, some scientists have proposed that we could counteract greenhouse warming by fertilizing the oceans, causing more phytoplankton to grow and thereby transforming CO2 into organic matter that is ultimately buried at sea. But will this new organic material remain buried, or will the microbes that live in the seafloor convert it back into CO2? We don't yet know the answer.

Ocean sediments are essentially big compost piles that never get stirred. Like your compost pile (if you don't have a compost pile, visit here) the breakdown of organic matter consumes oxygen, and so without stirring the compost pile goes anaerobic. What happens next? Lots of smelly gases are produced and the breakdown of organic materials slows to a crawl. At this point we can (and often do) ask several questions: What organisms are still working to break down my compost? Will they ever finish, and how long will it take? Why is some stuff (lettuce) degraded much more quickly than others (oak leaves)? Is there something we could add to speed the process up without having to stir up the compost pile every few days?

We study analogous aspects of the microbial communities that operate in natural anaerobic environments. The goal is to understand in detail how, and how quickly, they decompose organic matter. A variety of metabolic processes are involved, including fermentation, inorganic respiration, and methanogenesis. The figure at the top of this page outlines how some of those processes are linked together. Molecular hydrogen plays a key role in many of these links, and so we are using studies of hydrogen isotopes to investigate them further. Currently, the National Science Foundation has funded a collaborative study between myself and David Valentine (at UCSB) to study hydrogen isotopes in the important group of sulfate-reducing bacteria, represented by the circle at the bottom of the diagram.