Singular physiological behavior of the scleractinian coral Porites astreoides in the dark phase

Unlike most other corals that have been declining since the 1980s, the population of Porites astreoides, one of the dominant species of coral in Caribbean reefs, appears to be resilient. We investigated the physiological regulation of the electron transport chain of Symbiodiniaceae chloroplasts during the light / dark transition in P. astreoides compared to nine other common scleractinian corals. Protocols were applied to coral samples in seawater tanks and in situ. The maximum quantum yield (Fv/Fm) in the dark and the effective photochemical efficiency (Fq’/Fm’) in the light were measured during light-dark transitions, and alternative electron flow (AEF) mechanisms were evaluated using fluorescence variation in response to serial irradiation pulses (SIP-protocol). The variation in Fv/Fm (ΔYIImax) was calculated after 3 min or 2 h of dark acclimation (ΔYIImax(2h); ΔYIImax(3min)). The three species that belong to the genus Porites (P. astreoides, P. divaricata, P. furcata) showed plastoquinone reduction (PQ) in response to the SIP protocol, unlike all the other species tested. A marked decrease in Fv/Fm (ΔYIImax(2h) = 47.79%) was observed in P. astreoides in the dark whereas the average ΔYIImax(2h) of the other species tested was 0.677%. The decrease in ΔYIImax in P. astreoides was due to a significant increase in Fo (ΔFo(2h) = -108.64% ± SD 21.48) whereas Fm remained relatively stable. The increase in Fo was attributed to reduction of the PQ pool through a chlororespiration-like mechanism known to reduce the production of reactive oxygen species (ROS). This mechanism was triggered immediately after exposure to the dark, while a brief and moderate light exposure reversed it. Given the ecological success of P. astreoides, we suggest that the high antioxidant capability of this species in the dark phase could be one of the factors favoring its survival in the face of various environmental and anthropogenic threats.