Department of Earth System Science

Phytoplankton in high-light, low-nutrient ocean environments are challenged with maintaining high photosynthetic efficiency and simultaneously preventing photodamage that results from low levels of electron acceptors downstream of photosystem II (PSII). Here, we identify a process in open ocean picophytoplankton that preserves PSII activity by diverting electrons from the photosystem I (PSI) complex-mediated carbon assimilation to oxygen via a propyl gallate-sensitive oxidase associated with the photosynthetic electron transport chain. This process stabilizes diel photochemical efficiency of PSII, despite midday photoinhibition, by maintaining oxidized PSII reaction centers. Although measurements of the maximum photochemical efficiency of PSII, F_v : F_m  show midday photoinhibition, midday CO₂ fixation is not depressed. Moreover, CO₂ fixation saturates at low irradiances even though PSII electron flow is not saturated at irradiances of 1,985 mmol photons m^-2 s^-1. This disparity between PSII fluorescence and CO₂ fixation is consistent with the activity of an oxidase that serves as a terminal electron acceptor, maintaining oxidized PSII reaction centers even when CO₂ fixation has saturated and the total number of functional reaction centers decreases because of photoinhibition (reflected in lower midday F_v : F_m  values). This phenomenon is less apparent in coastal phytoplankton populations, suggesting that it is a strategy particularly distinctive of phytoplankton in the oligotrophic ocean. Spatial variability in features of photosynthetic electron flow could explain biogeographical differences in productivity throughout the ocean and should be represented in models that use empirical photosynthesis and chlorophyll fluorescence measurements from a limited number of ocean sites to estimate the productivity of the entire ocean.