The biological role and enzymatic source of extracellular reactive oxygen species in marine phytoplankton
Within the marine environment, reactive oxygen species (ROS) are abundant and participate in geochemical reactions that shape the fate and availability of metals, carbon, and oxygen due to their reactive nature. Phytoplankton are major sources of the ROS superoxide (O2-) and hydrogen peroxide (H2O2). Indeed, by exporting electrons to surrounding oxygen via enzymes, phytoplankton generate extracellular O2- (eO2-) which can then dismutate to extracellular H2O2 (eH2O2). ROS are commonly associated with stress; however, they also serve beneficial biological functions. Despite the environmental importance of eROS, their enzymatic source and ecophysiological role in phytoplankton has remained mysterious. In phytoplankton, several biological functions have been proposed for eROS production, yet a consensus has not been reached. Additionally, a class of enzymes that catalyzes electron transfers called flavoenzymes mediates production of eROS in many organisms. However, pathways of eROS production by phytoplankton are poorly understood. Here, I interrogate the ecophysiological role(s) of eROS production and its enzymatic source in a diversity of phytoplankton in laboratory and field settings. In Chapter I, results demonstrate that eO2- production is stress-independent and dynamically regulated as a function of cell abundance and growth phase consistent with a signaling role, as well as light availability in the globally-relevant coccolithophore E. huxleyi. Chapter II reveals that eO2- production is light-driven, regulated by flavoenzymes, and promotes health by serving a photoprotective role in many phytoplankton. Further, results support my hypothesis that many phytoplankton form eO2- to dissipate excess energy from light stress. Also, I estimate that light-driven eO2- production by phytoplankton will increase in future ocean conditions where mixing layer light levels are predicted to increase due to climate change. In Chapter III, field results demonstrate that eH2O2 production is dynamically regulated consistent with a signaling role and influences phytoplankton growth and microzooplankton grazing. Indeed, eH2O2 production, phytoplankton growth, and grazing were inversely correlated. Moreover, incubations show that increasing eH2O2 production decreases phytoplankton growth and grazing. Overall, my work helps illuminate the ecophysiological role and enzymatic source of eROS production by phytoplankton, thereby advancing understanding of biogeochemical cycling, redox states, plankton web dynamics, and health of current and future oceans.