Connectivity among Photosystem II centers in phytoplankters: Patterns and responses.

Fast Repetition and Relaxation chlorophyll fluorescence induction is used to estimate the effective absorption cross section of PSII (σPSII), to analyze phytoplankton acclimation and electron transport. The fitting coefficient ρ measures excitation transfer from closed PSII to remaining open PSII upon illumination, which could theoretically generate a progressive increase in σPSII for the remaining open PSII. To investigate how ρ responds to illumination we grew marine phytoplankters with diverse antenna structures (Prochlorococcus, Synechococcus, Ostreococcus and Thalassiosira pseudonana) under limiting or saturating growth light. Initial ρ varied with growth light in Synechococcus and Thalassiosira. With increasing actinic illumination PSII closed progressively and ρ decreased for all four taxa, in a pattern explicable as an exponential decay of ρ with increasing distance between remaining open PSII reaction centers. This light-dependent down-regulation of ρ allows the four phytoplankters to limit the effect of increasing light upon σPSII. The four structurally distinct taxa showed, however, distinct rates of response of ρ to PSII closure, likely reflecting differences in the spacing or orientation among their PSII centers. Following saturating illumination recovery of ρ in darkness coincided directly with PSII re-opening in Prochlorococcus. Even after PSII had re-opened in Synechococcus a transition to State II slowed dark recovery of ρ. In Ostreococcus sustained NPQ slowed dark recovery of ρ. In Thalassiosira dark recovery of ρ was slowed, possibly by a light-induced change in PSII spacing. These patterns of ρ versus PSII closure are thus a convenient probe of comparative PSII spacings.