Solar UVR at the Earth's surface (280-400 nm) has wide-ranging impacts on biological and chemical processes that affect the cycling of elements in aquatic environments. This chapter uses recent field and laboratory observations along with models to assess these impacts on carbon, nitrogen, sulfur and metals cycles. Much emphasis is placed on the interactions of UVR with carbon capture and storage, decomposition, and trace gas exchange. UV exposure generally inhibits phytoplankton photosynthesis and also affects microbial processes both through direct inhibition of bacterial activity as well as through effects on the biological availability of carbon and nitrogen substrates. One important aspect of UV interactions with carbon cycling involves the formation and decomposition of UV-absorbing organic matter, principally chromophoric dissolved organic matter (CDOM). CDOM controls UV exposure in the sea and in many freshwater environments. It can be directly photodecomposed to dissolved inorganic carbon (DIC), carbon monoxide, and various carbonyl-containing compounds. UV can potentially affect nitrogen and sulfur cycling in a variety of ways such as alterations in nitrogen fixation, effects on the biological availability of dissolved organic nitrogen (DON), UV photoinhibition of phytoplankton, bacterioplan-kton and zooplankton that affect sources and sinks of dimethyl sulfide (DMS) and UV-initiated photoreactions that oxidize DMS and produce carbonyl sulfide. Metal cycling also interacts in many ways with UVR via photoinhibition of microbial redox cycling, direct photoreactions of dissolved metal complexes and metal oxides and indirect reactions that are mediated by photochemically-produced reactive oxygen species (ROS). Photoreactions can affect the biological availability of essential trace nutrients such as iron and manganese, transforming the metals from complexes that are not readily assimilated into free metal ions or metal hydroxides that are available.
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