Assessment of photosynthetic performance of Prochloron in Lissoclinum patella in hospite by chlorophyll fluorescence measurements

Abstract

Two new PAM fluorometers (pulse amplitude modulated) were used in an investigation of photosynthetic performance of Prochloron resident as a symbiont in the ascidian Lissoclinum patella, growing in a coral reef of Heron Island on the Great Barrier Reef. With a new DIVING-PAM in situ measurements of effective PSII quantum yield (ΔF/Fm') as a function of quantum flux density (rapid light curves) were carried out in 2.5 m depth in the reef and in a seawater tank. Photosynthetic electron transport rates were measured on in hospite Prochloron both in situ and in collected material. Both light-limited and light-saturated yields were exceptionally high. Maximal yields (Fv/Fm) were ~0.83. A new TEACHING-PAM was employed for analysing dark-light induction and light-dark relaxation kinetics in collected samples with Prochloron in hospite. Considerable variability in kinetic responses was observed which was found to be at least in part due to differences in O2 concentration. It is suggested that endogenous reductants feed electrons into the intersystem transport chain, which normally is reoxidized by O2 (chlororespiration), and that in the dark, the reduction level of PSII acceptors is increased due to a decline in O2 concentration. The pattern of fluorescence responses differed markedly from those found in cyanobacteria and provides new insights into light-harvesting responses of a photosynthetic prokaryote with a membrane bound light-harvesting system, as contrasted with an extrinsic light-harvesting system.