Photosynthesis and the intracellular inorganic carbon pool in the bluegreen alga Anabaena variabilis: Response to external CO2 concentration

Influence of carbon dioxide concentration during growth on fluorescence induction characteristics of the Green Alga Chlamydomonas reinhardii

Carbon dioxide concentration during growth is commonly not considered to be a factor influencing the photochemical properties of plants. It was observed that fluorescence induction in Chlamydomonas reinhardii cells grown at air levels of CO2 was both qualitatively and quantitatively different from that of cells grown at 5% CO2. In the two cell types, measured at equivalent chlorophyll and irradiance levels, the fluorescence intensity and the ratio of the levels of peak fluorescence (Fp) to that of the initial fluorescence (Fo) were much lower in the air-adapted than in the 5% CO2 adapted cells. The maximum fluorescence (Fmax) in the presence of diuron was also lower for air-adapted cells. Roughly twice the light input was required for the air-adapted cells to give a fluorescence induction transient and intensity equivalent to that of the 5% CO2-adapted cells. Similar properties were observed in several other unicellular green algae and in cyanobacteria. Chlamydomonas grown under variable CO2 concentrations exhibit significant differences in photosynthetic carbon metabolism and are presumed to have altered energy requirements. The observed variation in fluorescence induction may be due to changes in the properties of the thylakoid reactions (e.g. cyclic electron flow) of Chlamydomonas cells, which may, in turn, be due to a response to the altered energy requirements.

Glyoxylate decreases the oxygen sensitivity of nitrogenase activity and photosynthesis in the cyanobacterium Anabaena cylindrica

Raising the pO2 reduced nitrogenase activity (C2H2 reduction) of Anabaena cylindrica for both glyoxylate-treated (5 mM) and untreated cells. The stimulation caused by glyoxylate, however, increased with increases of pO2 from 2 to 99 kPa. As the pO2 increased the net CO2 fixation was lowered (Warburg effect) while the CO2 compensation point increased. Glyoxylate partly relieved this sensitivity of net photosynthesis to oxygen and reduced the compensation point considerably. The cells used were preincubated in the dark to exhaust photosynthetic pools. A more pronounced reduction in sensitivity of nitrogenase to oxygen for glyoxylate-treated cells was evident when a preincubation in air with reduced pCO2 (13 μl l(-1)) was used. This was, however, not evident until after a 10-h incubation in air. Before this point 2 kPa O2 sustained the highest nitrogenase activity. Addition of 0.5 and 5 mM of HCO 3 (-) to Anabaena cultures preincubated at low CO2 levels (29 μl l(-1)) abolished the stimulatory effect of glyoxylate on the nitrogenase. Thus, the results sustain the suggestion that glyoxylate may act as an inhibitor of photorespiratory activities in cyanobacteria and can be used as a means of increasing their nitrogen and CO2 fixation capacities.

Uptake and efflux of inorganic carbon in Dunaliella salina

The apparent photosynthetic Km (CO2) of air-grown Dunaliella salina is 2 μM as measured both by the filtering centrifugation technique and by O2 electrode. These cells are capable of accumulating inorganic carbon (Cinorg) up to 20 times its concentration in the medium. It is suggested that air-grown Dunaliella cells are able to concentrate CO2 within the cell. Analysis of the efflux of Cinorg from cells previously loaded with H(14)CO 3 (-) demonstrated the existence of an internal pool which has an half-time of depletion of 2.5-7 min depending on the conditions of the experiment. This finding indicates that the internal Cinorg pool is not readily exchangeable with the external medium. Furthermore, the influence of the presence or absence of unlabelled Cinorg in the medium during the efflux experiment on the half-time observed indicate that efflux of Cinorg is not a simple diffusion process but is rather carrier-mediated.