Regulation of dissolved organic carbon biogeochemistry by light: The impacts on bacterial productivity and community structure in aquatic systems

Dissolved organic carbon (DOC) represents one of the largest reservoirs of organic carbon on earth. Estuaries transport DOC from terrestrial ecosystems to the coastal oceans. During transit, light decreases DOC either through direct photochemical oxidation or indirectly through a coupled photochemical/bacterial pathway. Photochemical oxidation occurs when DOC chromophores absorb enough light to cleave chemical bonds, producing CO₂ and low molecular weight compounds. The coupled photochemical/bacterial pathway is mediated through bacterial transformation of DOC to biomass and CO₂. This thesis focuses on the role of light in DOC cycling with emphasis on the regulation of each pathway.; Photo-oxidation impacts both chemical and biological aspects of DOC cycling. Using the Parker River estuary (MA) as a model system, I showed that during the summer, light (320–800 nm) decreased the DOC pool by 10%. Using CO₂ production measured from each pathway, the half-life of DOC was faster than previous estimates. The addition of these critical parameters gave a more accurate understanding of the impact of light on DOC cycling.; Changes in bacterial activity and community structure were used to show that solar radiation is a selective force on bacteria involved in DOC cycling. Direct exposure to light (290–400nm) shifted bacterial metabolism from polysaccharides to low molecular weight, oxidized compounds as shown by changes in respiration and extracellular enzyme activity. Exposure to light (290nm–800nm) changed bacterial species composition as measured by 16S rDNA. However, the distribution of 16S rDNA may not reflect the active bacterial species. Therefore, a method that measures active species via 16S rRNA was assessed. Here, I show in studies with pure cultures, that reverse transcription (RT) of the 16s rRNA molecule is linear and reproducible. RT can be used to give an accurate representation of the richness and dominance of bacterial communities in nature.; Photo-oxidation plays a role in DOC cycling and influences bacterial metabolism, growth, and species composition in surface waters. Both the photochemical and coupled photochemical/bacterial pathway contribute to the turnover of the DOC pool and may give insight to global DOC cycling.