Isolation and characterisation of oxygen evolving thylakoids from the marine prokaryote Prochloron didemni

The present study describes the first successful attempt to isolate oxygen evolving thylakoids and thylakoid fragments from the marine prokaryote Prochloron didemni, a member of the recently discovered group of prochlorophytes. Oxygen evolving thylakoid membranes and fragments were isolated from seawater suspended cells of Prochloron didemni by passage of the cells through a Yeda press and subsequent differential centrifugation of the broken material. Three fractions were collected at 1000 x g, 5000 x g, and 3000 x g and identified by light microscopy as cells (and their fragments), thylakoids and membrane fragments, respectively. Pigment content, oxygen evolution rate and 77 K fluorescence spectra of these fractions were virtually identical. This finding indicates that the membrane fragments obtained are not enriched in photosystem II. The P680+* reduction kinetics of thylakoid membrane fragments were determined by monitoring flash induced absorption changes at 830 nm and analysing the time course of their decay. The multiphasic relaxation kinetics and their modification by NH2OH were found to be similar to those observed in cyanobacteria and plants. These findings provide an independent line of evidence for the idea of a high conservation of the basic structural and functional pattern of the water oxidising complex in all organisms that perform oxygenic photosynthesis.