"What we report was completely unexpected," said oceanographer and biogeochemist Samantha Joye. "This temperature-driven decoupling short-circuits organic matter recycling and will be of interest to a broad spectrum of biologists, geochemists and environmental scientists."
The research will be published the week of Nov.14 in the Proceedings of the National Academy of Sciences. Co-author of the study is Nathaniel B. Weston, who worked with Joye as a doctoral student at UGA. He is now at the Patrick Center for Environmental Research at the Academy of Natural Sciences in Philadelphia.
"These surprising results show that temperature dependence strongly affects the efficiency of organic matter breakdown and need to be taken into account in models of the role of sediments in the global carbon cycle," said Paul Kemp, program director in the National Science Foundation's biological oceanography program, which supported the research.
Scientists have long known that buried organic carbon in marine sediments plays a crucial role in many terrestrial and atmospheric processes. The number of anaerobic microorganisms that chew away at this carbon is vast, and they can hydrolyze, ferment or terminally oxidize organic compounds.
"The microbes responsible for all but the final step of organic matter degradation in sediments are often ignored, and we were interested in opening the microbial 'black box' in sediments and clarifying the temperature controls on different microbial groups," said Weston.
Weston and Joye studied sediment cores from Umbrella Creek near the mouth of the Satilla River on the coast of Georgia. By sampling at different times of the year--when temperatures of the sediment were different--they found a variable temperature regulation of the sequential processes that lead to the breakdown of organic carbon. This meant that functional groups of microbes at work in the sediments have different optimal temperature ranges and thus operate at variable rates as a function of local temperature.
Despite the obvious importance of processes such as hydrolysis and fermentation in the mineralization of organic matter, relatively little has been known until now about how temperature affects the processes, which are responsible for the initial breakdown of complex organic matter in sediments.
"We have shown that the balance and efficiency of coupling between successive microbial processes involved in organic carbon breakdown is extremely sensitive to even small changes in temperature," said Joye. "These results suggest that global climate change (warming) may influence the efficiency of organic carbon recycling, and thus organic carbon burial, as well as the type and magnitude of material fluxes to the overlying water column. This could impact patterns of coastal primary production."
The team used two experimental approaches to confirm their results. One involved analyzing a slurry mix of the sediments, and this work reflected the response of a single sediment microbial community to changes in temperature. To investigate whether a similar response was observed over a seasonal temperature cycle, the team used flow-through bioreactor experiments, using sediments collected from the same site, four times over a year.
In these experiments, Weston and Joye documented for the first time a greater temperature sensitivity of sulfate reduction, an important process in marine sediment degradation, than of hydrolysis and fermentation.
It remains unclear whether the temperature-driven decoupling the team documented for temperate marine sediments applies to other geographic zones, such as tropical environments.
"Still, the data presented here show clearly that small changes in temperature can impact the efficiency of organic-matter turnover in anoxic [living without oxygen] marine sediments," said Joye.
The research was also supported by the Georgia Sea Grant Program and the National Science Foundation's Long Term Ecological Research Program through Biological Oceanography.