Immunogold Localization of the Phycobiliprotein of a Cryptophyte Alga to the Intrathylakoidal Space

Abstract

The cryptophyte algae, or cryptomonads, are unicellular eukaryotes which form a distinct taxonomic group. Several features of the cryptomonad chloroplast make it both structurally and biochemically unique among photosynthetic bacteria, algae and land plants. Cryptomonads possess two chlorophylls (chls), chl a and chl c₂, and employ phycobiliprotein as their major light-trap** or “antenna” pigment (3). Other photosynthetic species with two chls do not possess phycobiliproteins; red algae and cyanobacteria, which possess phycobiliproteins, have only chl a. Moreover, the phycobiliproteins of red algae and cyanobacteria are arranged into round or disc-shaped structures called phycobilisomes, which are attached to the outer surface of the thylakoid membrane (4). Each phycobilisome is composed of three phycobiliproteins: phycoerythrin (PE), phycocyanin, and allophycocyanin. These pigments are layered in such a way that energy is efficiently transferred from PE to phycocyanin to allophycocyanin, which in turn transfers light energy to the photosynthetic reaction centers located in the thylakoid membrane (5). in contrast, each species of cryptomonad possesses only one phycobiliprotein, either PE or phycocyanin, and no allophycocyanin (3).