Photosynthetic properties of a cyanobacterium, Gloeobacter violaceus PCC7421.

Abstract

A cyanobacterium Gloeobacter violaceus PCC7421 is assigned to the species that holds ancestral properties in photosynthesis, for example, lacking thylakoid membranes, thus photosynthetic apparatus sits on cell membranes together with a respiratory system. Photosynthetic properties of this alga were examined in order to elucidate the original oxygenic photosynthetic reaction system. G. violaceus grown autotrophically in the BG11 medium contains Chl a, R-phycoerythrin, phycocyanin, allophycocyanin and carotenoids; a unique carotenoid, oscillaxanthin was present. A relative content of Chl a was low. Cells showed a low oxygen evolution activity; the maximum activity was less than 30% of that of Synechocystis sp. PCC6803. A content of RC, that is P700 and P680 on Chl a basis, was very close to that of Synechocystis when grown at 25°C, on the other hand, it varied when grown at 35°C; a content of P680 was decreased, and that of P700, increased. An activity of P680 under the two temperature conditions was confirmed by delayed fluorescence in the nano-second time scale, even though the charge recombination time was prolonged at high temperature. This coincided with a low oxygen evolution activity at high temperature. Energy transfer from phycobilin to Chl a was inefficient as evidenced by the emission from the terminal pigment of phycobilin. A characteristic PS I fluorescence at cryogenic temperature was not detected, and this might be linked to a very low homology in the sequence of psaA/B. Based on these results, photosynthetic properties were discussed from the evolutionary point of view.