The Relationship between Ribulose Bisphosphate Concentration, Dissolved Inorganic Carbon (DIC) Transport and DIC-Limited Photosynthesis in the Cyanobacterium Synechococcus leopoliensis Grown at Different Concentrations of Inorganic Carbon |
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| Authors: | Mayo W P Elrifi I R Turpin D H |
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| Affiliation: | Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6. |
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| Abstract: | To examine the factors which limit photosynthesis and their role in photosynthetic adaptation to growth at low dissolved inorganic carbon (DIC), Synechococcus leopoliensis was grown at three concentrations (as signified by brackets) of DIC, high (1000-1800 micromolar), intermediate (200-300 micromolar), and low (10-20 micromolar). In all cell types photosynthesis varied from being ribulose bisphosphate (RuBP)-saturated at low external DIC] to RuBP-limited at high external DIC]. The maximum rate of photosynthesis (Pmax) was achieved when the internal concentration of RuBP fell below the active site density of RuBP carboxylase/oxygenase (Rubisco). At rates of photosynthesis below Pmax, photosynthetic capacity was limited by the ability of the cell to transport inorganic carbon and to supply CO2 to Rubisco. Adaptation to low DIC was reflected by a decrease in the DIC] required to half-saturate photosynthesis. Simultaneous mass-spectrometric measurement of rates of photosynthesis and DIC transport showed that the initial slope of the photosynthesis versus DIC] curve is identical to the initial slope of the DIC transport versus DIC] curve. This provided evidence that the enhanced capacity for DIC transport which occurs upon adaptation to low DIC] was responsible for the increase in the initial slope of the photosynthesis versus DIC] curve and therefore the decrease in the half saturation constant of photosynthesis with respect to DIC. Levels of RuBP and in vitro Rubisco activity varied only slightly between high and intermediate DIC] grown cells but fell significantly (65-70%) in low DIC] grown cells. Maximum rates of photosynthesis followed a similar pattern with Pmax only slightly lower in intermediate DIC] grown cells than in high DIC] grown cells, but much lower in low DIC] grown cells. The changing response of photosynthesis to DIC] during adaptation to low DIC, may be explained by the interaction between DIC-transport limited and RuBP]-limited photosynthesis. |
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