text: Photodegradation involves the transformation of CDOM into smaller and less colored molecules (e.g., organic acids), or into inorganic carbon (CO, CO), and nutrient salts (NH, HPO). Therefore, it generally means that photodegradation transforms recalcitrant into labile DOC molecules that can be rapidly used by prokaryotes for biomass production and respiration. However, it can also increase CDOM through the transformation of compounds such as triglycerides, into more complex aromatic compounds, which are less degradable by microbes. Moreover, UV radiation can produce e.g., reactive oxygen species, which are harmful to microbes. The impact of photochemical processes on the DOC pool depends also on the chemical composition, with some studies suggesting that recently produced autochthonous DOC becomes less bioavailable while allochthonous DOC becomes more bioavailable to prokaryotes after sunlight exposure, albeit others have found the contrary. Photochemical reactions are particularly important in coastal waters which receive high loads of terrestrial derived CDOM, with an estimated ~20–30% of terrestrial DOC being rapidly photodegraded and consumed. Global estimates also suggests that in marine systems photodegradation of DOC produces ~180 Tg C yr of inorganic carbon, with an additional 100 Tg C yr of DOC made more available to microbial degradation. Another attempt at global ocean estimates also suggest that photodegradation (210 Tg C yr) is approximately the same as the annual global input of riverine DOC (250 Tg C yr;), while others suggest that direct photodegradation exceeds the riverine DOC inputs.