Do Photochemical Transformations of Dissolved Organic Matter Produce Biorefractory as well as Bioreactive Substrates

Photochemical processes appear to have an important impact on the cycling of dissolved organic matter (DOM) in a wide variety of aquatic environments. Numerous photoproducts of DOM, including dissolved inorganic carbon and low-molecular-weight organic compounds, have been identified, and the types of photoproducts and extent of phototransformation of DOM is dependent upon its chemical composition. Heterotrophic bacteria are the primary consumers of DOM, and photochemical transformations of DOM can affect its bacterial utilization in a variety of ways. Numerous studies spanning fresh and marine waters have demonstrated enhanced bacterial growth following exposure of DOM to solar radiation. These studies indicate that photochemical processes can enhance the microbial utilization of biorefractory DOM. Other studies in fresh and marine waters, however, have demonstrated that exposure of DOM to solar radiation can result in a reduction in bacterial growth rates, suggesting that photochemical and microbial processes compete for the same substrates. It appears that some photoproducts of DOM are resistant to microbial degradation, suggesting that photochemical processes could be involved in the production as well as destruction of biorefractory DOM.