Functional organization and plasticity of the photosynthetic unit of the cyanobacterium Anacystis nidulans

Anacystis nidulans grown under high and low light, 100 and 10 μE m^?2 s^?1, respectively, was analyzed with respect to chlorophyll/P700, phycobiliproteins/P700, chlorophyll/cell, and oxygen evolution parameters. The photosynthetic unit sizes of this cyanobacterium, measured as the ratio of total chromophores (chlorophyll and bilin) to P700, were shown to be similar to those of higher plants and green algae. High light grown cells possessed a photosynthetic unit consisting of a core of 157 ± 6 chlorophyll a molecules per P700 associated with a light harvesting system of 95 ± 3.5 biliprotein chromophores. Low light grown cells had substantially more biliprotein chromophores per P700 (125 ± 3.1) than high light cells, but showed no significant difference in the numbers of chlorophyll a molecules per P700 (149 ± 4). Analyses of aqueous biliprotein extracts indicate that low light grown cells produce proportionately more phycocyanin relative to allophycocyanin than high light cells. Calculations of the molecular weight of biliproteins per P700 suggest that there is less than one phycobilisome per reaction center I under both growth conditions. Differences in chlorophyll/cell ratios and oxygen evolution characteristics were also observed. High light cells contain 6.3 × 10^?12 mg chlorophyll cell^?1, while low light grown cells contain 12.8 × 10^?12 mg chlorophyll cell^?1. Photosynthetic oxygen evolution rate vs. light intensity curves indicate that high light grown cells reach maximal levels of oxygen evolution at higher light intensity than low light grown cells. Maximal rates of oxygen evolution were 16.6 μmol oxygen min^?1 (mg chlorophyll)^?1 for high and 8.4 μmol oxygen min^?1 (mg chlorophyll)^?1 for low light cells. Maximal oxygen evolution rates per cell were equivalent for both cell types, although the amount of P700 per cell was lower in high light cells. High light grown cells are therefore capable of producing more oxygen per reaction center I than low light grown cells.