The photosynthetic apparatus of Euglena gracilis: I. Adaptation to population density change

When Euglena gracilis is grown in batch culture, the chlorophyll content per cell increases in parallel with the population density; whereas, the protein content per cell remains constant. The quantitative relationships between the chlorophyll concentration, the level of some typical chloroplast components, and the photoactivities of chloroplast preparations were investigated. Chlorophyll formation is in concert with the synthesis of other lipidic components of the lamellae; in particular, the monogalactosyldiglycerides remain in a constant 2:1 molar ratio with chlorophyll. Plastoquinone A content increases in a parallel fashion, while total carotenoid exhibits a less marked increase. A different behavior is observed with some photosynthetic proteins: ferredoxin, cytochrome-552 and ferredoxin-NADP reductase remain constant and independent of chlorophyll concentration. On the contrary, the particulate b-type cytochrome, cytochrome-559, increases similarly to chlorophyll. Photoactivites were tested with isolated chloroplasts prepared from cells containing different amounts of chlorophyll. The rates of the Hill activities with ferricyanide, 2,6-dichlorophenolindophenol, or NADP as acceptor and NADP photoreduction, with DCIP and ascorbate, were found to increase with increasing chlorophyll concentration. On a cell basis, the photoreactions related to Photosystem II increased in a more pronounced fashion than chlorophyll. The P700 signal of isolated chloroplasts also increased, but to less marked extent than chlorophyll. The possible existence in vivo of control mechanisms to regulate independently the biosynthesis and the assemblage of Photosystem I and Photosystem II is considered.