Free radical scavenging (antioxidant activity) of natural dissolved organic matter

•We measured the antioxidant activity of natural dissolved organic matter extracts.•All samples showed different antioxidant activity degree depending on their origin.•Samples associated with mangrove areas presented the highest antioxidant activity.•Marine samples associated with seagrass showed high antioxidant activity.

Free radicals are produced in aquatic environments through photochemical reactions. They can affect the concentration and composition of organic matter and have negative effects on aquatic organisms. Free radical scavengers (antioxidants) can remove these highly reactive species from the media. Some dissolved organic matter (DOM) constituents are widely known to present antioxidant properties (e.g. phenols and hydroquinones). However, little is known about the free radical scavenger capacity of DOM. Here we applied two simple, analytical assays (ABST and DPPH) to assess the antioxidant capacity of aquatic DOM, after their validation against a more complex electrochemical technique. These assays were applied to DOM from various environmental settings, including freshwater marshes, fringe mangrove estuaries and a coastal bay in Everglades National Park, Florida. All the samples presented different degrees of antioxidant activity depending on their origin and thus DOM quality. Samples associated with mangrove areas presented the highest antioxidant activity, possibly due to the presence of tannins, which are known to be powerful antioxidants. The free radical scavenging capacity or antioxidant properties of DOM may have important implications in aquatic photochemistry as well as in microbial processes.