Over the past 15 years, studies of Ocean Drilling Program (ODP) cores have consistently identified abundant bacteria in deeply buried oceanic sediment. Microorganisms have been recovered from depths as great as a half-mile below the seafloor and have been estimated to constitute one-tenth to one-third of Earth’s living biomass.
URI oceanographers Steven D’Hondt, Scott Rutherford, and Arthur J. Spivack are studying the activity of bacterial life deep in the sediments at the bottom of the ocean. The most recent issue of Science reports that they use the chemistry of the water in deep-sea sediments to show that these abundant organisms respire at far slower rates than organisms living at Earth’s surface.
Most visible animals breathe in free oxygen, gain energy by using the oxygen to oxidize the organic matter that they eat, and then breathe out carbon dioxide and water. However, oxygen is rare in marine sediments, so the bacteria that live in those sediments must rely on other chemicals for this process. Most microbial communities that are buried deep in marine sediments use sulfate, a common dissolved salt, instead of oxygen, to oxidize their food and garner energy.
The recent URI study relied on this knowledge to identify two broad oceanic provinces of deeply buried microbial activity. One province covers most areas of the open ocean. In this province, total microbial activity is very low and sulfate is abundant throughout the buried sediment. The second province occurs along continental margins. In this province, the total rate of microbial activity is high beneath the seafloor, so sulfate is destroyed at shallow sediment depths and the concentration of methane, a hydrocarbon gas, builds up to high levels at greater sediment depths.
The URI study showed that deep in the sediments of the open-ocean province the respiration rate of individual microorganisms may be more than 10,000 times slower than the respiration rate of bacteria that inhabit Earth’s surface. It also showed that the highest rates of microbial activity beneath the seafloor occur in a narrow zone along continental margins where the deeply buried methane comes into contact with the shallowly buried sulfate. In this narrow zone, the rate of microbial activity beneath the seafloor can be as high as the total rate of microbial activity at the sediment surface.
Contact: Lisa Cugini, (401) 874-6642, lcugini@gso.uri.edu
