The utilization of organic matter from tree logs, fallen twigs, dead roots and leaves by bacteria as food has revealed a wealth of information about the roles of these tiny bugs in energy and nutrient recycling, but it continues to perplex and intrigue the scientists who analyze this complex interactions.
Image 1 Most scientists agreed that some components of tree leaves and root exudates are sugars, organic acids and amino acids, that is, labile parts of the trees that are readily used as energy and nutrients by microbes. But the part of the tree composed of phenolics, lignin, cellulose or similar complex polymers, that is, the ‘hard’ (non-labile) part of organic matter has long been considered recalcitrant (hard to eat) to bacterial utilization.
Researchers from the University of Alabama, Stroud Water Research Center (Avondale, PA) and Michigan State University have been thinking out of the box. These researchers have been examining the incorporation of terrestrial organic matter into microbial biomass by incubating stream sediment in recirculating mesocosms (bioreactor) with natural stream water to which labeled tree tissues leachate (extracts) were added. The tree tissues leachate consists of both labile and non-labile parts of the organic matter. “The goal of our study was to elucidate microbes within stream sediments that actively utilize terrestrial organic matter, thereby controlling energy flux to higher trophic levels and to downstream reaches,” says Dr. Philips Akinwole, whose PhD work at the University of Alabama focused mainly on this National Science Foundation funded innovative project. “It was generally accepted that most of the turnover of dissolved organic carbon is accomplished via the metabolism of a small pool of labile component of the total dissolved organic matter. We argued that this interpretation may not be entirely correct, and that there is increasing evidence that bacteria can grow efficiently on non-labile substances,”
In 2009 and 2010, in collaboration with Stroud Water Research Center, Dr. Akinwole collected stream sediments from forested White Clay Creek and placed them in recirculating bioreactors. To determine the microbes actively using streamwater dissolved organic matter, streambed sediments placed in bioreactor chambers were amended with Carbon-labeled organic matter and incorporation into microbial fatty acids was examined.
Elucidating the actual microbial groups that utilize recalcitrant organic matter was done for the first time by carbon isotope analysis of the microbial fatty acids at the Stable Isotope Biogeochemistry Laboratory, Michigan State University. The research team found out that consistently labeled fatty acids occurred in bacteria often associated with aerobic or facultative anaerobic metabolism (e.g. Firmacutes, Acidobacteria, Nitrospirae, etc). In addition, incorporation into microeukaryotic fatty acids suggested that protozoans consumed bacteria that utilized 13C-labeled dissolved organic matter. “Our data support the hypothesis that streamwater terrestrial dissolved organic matter is utilized by stream bacteria, and substantially contributes to the energy flow in aquatic ecosytems,” says Dr. Akinwole.
For policy makers, this study has important implications for protection of forested streams where much of the organic matter is derived from the surrounding terrestrial ecosystem. As organic matter from trees is an important source of carbon and energy for stream microbes, any human activity (such as deforestation) that disrupts or accelerates the delivery of terrestrial organic matter to streams may need regulation.
“It becomes very difficult to predict with any certainty what’s going to happen with too much or little organic matter in streams, which is why an advanced experiment like this is so important,” says Dr. Akinwole.
A key message for policy makers is that interactions between aquatic and terrestrial systems are complex systems; active management of forested streams and rivers is going to be a crucial part of nations’ strategies for keeping a lid on anthropogenic activities along riverine banks.
Researchers like Akinwole have been studying dissolved organic matter and microbes interactions for more than two decades in the White Clay Creek. What they’ve learned has important implications, not only for the future of streams, but also for the biggest problem facing our planet; climate change, because deforestation could release millions of tons of carbon dioxide and other greenhouse gases into the atmosphere.
Cover image: Bioreactor setup for 13C leachate uptake measurement, including streamwater-fed bioreactors chambers containing sediments and water jackets
References
Amon RMW, Benner R. 1996. Bacterial utilization of different size classes of dissolved organic matter. Limnol. Oceanogr. 41: 41- 51.
Bano N, Moran MA, Hodson RE. 1997. Bacterial utilization of dissolved humic substances from a freshwater swamp. Aquatic Microbial Ecology, 12: 233-238.
Wiegner TN, Kaplan LA, Newbold JD, Ostrom PH. 2005a. Synthesis of a 13C-labeled tracer for stream DOC: labeling tulip poplar carbon with 13CO2. Ecosystems 8: 501–511.
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