Morris, William James2024-08-142024-08-1419791979https://hdl.handle.net/1828/19045The photosynthetic CO₂ assimilation rate of Isochrysis galbana was depressed by dissolved [O₂] greater than zero. The depression was linear between 80 and 945 µM o2 and independent of [HC03-J or [CO2] at high [02]. The depression caused by 330 µM O₂ compared to 115 µM 0₂ was relieved by 5.05 mM carbon. At least 10 µM CO₂ was required for half­-maximal photosynthesis, and 300 µM O₂ was sufficient to cause half­-maximal O₂ inhibition. The pH optimum of photosynthesis was narrow (7.25 to 7.50) and correlated with the [CO₂]. Glycidate inhibited photo­synthesis and the depression was independent of [O₂]. Optimal photosynthesis in Thalassiosira fluviatilis occurred at [O₂] between 220 and 500 µMat 2.0 mM carbon and between 125 and 225 µM at 0.2 mM carbon. A slight depression in photosynthesis was sometimes observed at [O₂] > 800 µM compared to [O₂] < 100 µM. This depression was sensitive to [carbon] and relieved by 2.0 mM carbon. Only 2.5 µM CO₂ was required for half-maximal photosynthesis and at least 700 µM O₂ was required for half-maximal O₂ inhibition. The pH optimum of photosynthesis was broad (< 7.00 to 8.25) and not closely correlated with [CO₂]. Glycidate stimulated photosynthesis and the stimulation was inversely related to [O₂]. These data are consistent with exclusive C-3 metabolism in I. galbana and the presence of photorespiration, although the absence of stimulation of photosynthesis by glycidate is not consistent with previous studies on C-3 terrestrial plants. In I. galbana the Warburg effect at high [O₂] was not relieved by elevated [CO₂] which indicated that O₂ inhibition can be caused by mechanisms other than photorespiration. The absence of a Warburg effect and the high affinity for CO₂ in T. fluviatilis are consistent with efficient recycling of photorespiratory CO₂ by C-4 metabolism. However, other mechanisms can explain these results; these include the dehydration of HC0₃⁻ in the chloroplast through the activity of carbonic anhydrase or absence of photorespiration per se resulting from a low intrinsic activity of RuDP oxygenase. Regardless of the causes of these differences, the high affinity for CO₂ broad pH tolerance, and absence of a Warburg effect in T. fluviatilis suggest that this species would have a considerable competitive advantage over a species like I. galbana in near-shore areas where high cell densities and high photosynthetic rates could result in alkaline pH values and O₂ supersaturation.106 pagesAvailable to the World Wide WebThesis