Extracellular superoxide production by Porites species provides insight into controls on coral physiology

Reactive oxygen species (ROS), including superoxide, are central molecules in eukaryotic growth, function, and immunity. Some coral species, such as Porites sp., have been associated with high extracellular superoxide concentrations, yet we lack a clear understanding of the role of controls on superoxide production in corals. Here, we combine extracellular superoxide concentration and decay rate measurements with bioinformatics to better constrain the controls and mechanisms underlying ROS formation by Porites species. Consistent with previous studies, we find that extracellular superoxide concentrations are significantly higher for Porites species compared with other coral species. We further find that superoxide decay rates are not significantly different across species, indicating that changes in production rather than decay control steady-state concentrations. Extracellular superoxide is produced by Porites astreoides across life stages from larval to newly settled polyps to adult colonies. Our bioinformatic analysis reveals that Porites lobata has genes that encode for two types of NADPH oxidase (NOX), enzymes that produce exclusively extracellular superoxide. In fact, we find widespread presence of NOX genes within genomes across scleractinian species despite species-specific variation in superoxide production. Together, these findings indicate that corals regulate extracellular superoxide levels and point to an important role for extracellular superoxide in coral physiology, such as cell signaling, cell differentiation, and growth regulation. These findings add to a growing appreciation for the beneficial role of ROS in marine organisms and provide a foundation to investigate the role of ROS in coral physiology that may provide insight into coral health and subsequent approaches for coral restoration and preservation.