The effects of manipulations of the concentrations of O₂, CO₂, total inorganic carbon, and glycidate on the Warburg effect in two species of marine unicellular algae
Date
1979
Authors
Morris, William James
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The 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.