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Singlet Oxygen's Role in the Photochemical-Biochemical Degradation of Dissolved Organic Carbon

ABSTRACT<br/><br/>OCE-0527196<br/><br/>Dissolved organic matter (DOM) is the largest pool of organic carbon in aquatic ecosystems and constitutes a vital link in the global carbon cycle. Despite a large flux (ca. 2 x 1014g) of terrigenous DOM into the oceans every year, little of this material accumulates suggesting that it is rapidly decomposed. While the two main losses, photochemical and biochemical degradation, have been well studied, little work has examined the mechanisms responsible for degradation and chemical transformations, especially regarding photochemical processes.<br/><br/>In this research project, researchers at the University of Minnesota - Twin Cities will investigate the effects of a reactive oxygen species (ROS), singlet oxygen (1O2), on the chemical composition, reactivity and bioavailability of organic matter in natural waters. This project will use Lake Superior as a study site and the goals are: (1) To characterize the changes to the chemical and optical properties of DOM when exposed to 1O2; (2) To determine the changes in lability and nutritive quality of DOM following reaction with 1O2; (3) To examine the differential microbial utilization of 1O2-reactive amino acids-histidine, tryptophan, methionine, cysteine, and tyrosine-and their 1O2 reaction products, and (4) To quantify 1O2 production rates in Lake Superior.<br/><br/>To achieve these goals, DOM from two sources (soil and phytoplankton) and along a temporal-spatial gradient (river and offshore) will be exposed to a number of treatments including sunlight and well-defined sources of 1O2 with and without quenchers present. This study features the use of non-photochemical 1O2 sources that will enable examination of effects of 1O2 independent of sunlight. Effects on lability and chemical structure of DOM will be observed via bioassays and chemical analyses. The hypothesis that decreased microbial growth efficiency is observed in the presence of oxidized DOM, due to the reaction of 1O2 with specific amino acids present in DOM will be tested. This study is expected to be the first to demonstrate whether or not singlet oxygen is responsible for many of the changes in DOM bioavailability when it is exposed to sunlight. It will also produce some of the first measurements of singlet oxygen concentrations in large aquatic systems.<br/><br/>In terms of broader impacts, it ic certain that microbes and photochemical processes play a central role in environmental biogeochemistry. The processes examined in this project will facilitate understanding of global dynamics such as the carbon cycle, nutrient limitation and eutrophication. This project will support a young investigator (McNeill) and is collaboration between an ecologist and a chemist. In addition, several activities in the proposed project will promote teaching and training goals including: training and education of undergraduate students and graduate students and K-12 teachers. The PIs will participate in summer undergraduate research programs directed toward minority students (through the College of Biological Sciences LSSURP Program). The PIs will also participate in a K-12 teacher-training program focused on freshwater ecology. Additionally, this project will link with the NSF-RSEC program at the University of Minnesota by fostering an existing collaboration between one of the investigators and a faculty member at a Principally Undergraduate Institution.

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