Antioxidants are a class of compounds that provide a protective function against oxidative stress in biological systems by out competing physiologically important molecules (e.g., DNA, lipids and proteins) for reactive oxygen species (ROS) oxidation. The balance of ROS production and cellular antioxidant capacity has been extensively studied, and increased attention has been placed on the (methods of measuring) antioxidant content of foods. Less research, however, has been conducted on the role of antioxidants in aquatic environments where they likely also help determine the fate and distribution of reactive oxidants by providing an important sink for the photochemical and biological sources of ROS. In this study, antioxidant activity was measured in natural waters through the competition between ascorbic acid, an antioxidant, and MCLA, a chemiluminescent probe for superoxide. Antioxidant activity detection limits were below 0.1 nM in pure and seawater matrices. Oligotrophic surface seawater samples collected at solar noon contained over 0.4 nM of antioxidants and exhibited first-order decay with a half-life of 3-7 min, consistent with a reactive species capable of scavenging superoxide. In coastal waters the antioxidant activity was two orders of magnitude higher suggesting enhanced redox cycling of ROS. These high activities, combined with varying superoxide production rates, can help explain the spatial heterogeneity of superoxide concentrations measured in the literature. Further spatial and temporal coverage of these measurements will continue to build our understanding of the biogeochemical significance of ROS and antioxidants in aquatic environments, and will improve our ability to model steady state ROS concentrations.

Measuring antioxidant activity toward superoxide in natural waters

ACS Spring 2022

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