Biochemical and structural characterization of Photosystems from Galdieria sulphuraria

Abstract

Red alga (Rhodophyta) is one of the most ancient eukaryotic algae and its photosynthetic apparatus is in a transitional state between cyanobacteria and higher plants. Under rhodophyta, cyanidiales are group of asexual, unicellular red algae which thrive in acidic pH (0.5 - 3.0) and high temperature (50 to 55°C). Cyanidiales are classified into three genera, Cyanidium, Cyaniodioschyzon and Galdieria. Within cyanidiales, Galdieria has been a considerable debate among researchers about its systematic position and it's an outlier in terms of habitat, reproduction and sequence similarity. There is also considerable difference in photosystems of cyanidiales. In case of photosystem I (PSI), cyanidium has a monomeric PSI with an intrinsic light harvesting complex attached to it. Also, in photosystem II (PSII), different cyanidiales have different lumenal PSII subunits: Cyanidium has PsbV not PsbP, whereas PsbV is replaced by PsbP in cyaniodioschyzon. But there is only minimal knowledge of PSI and PSII in Galdieria. In our study, we addressed these questions by use of high resolution mass spectrometry to identify the different subunits of PSI and PSII in Galdieria sulphuraria. For structural and functional aspects of both photosystems, we had studied the isolated complexes by electron microscopy and time resolved fluorescence spectroscopy. Initial results from these studies showed that PSI is a monomer and there are several pools of red-shifted chlorophyll with potentially complex kinetic relationships. Our work is supported from grants of National Science Foundation (MCB-0417142).