Carbon Dioxide and the Regulation of Photosynthesis: Activities of Photosynthetic Enzymes and Carbonate Dehydratase (Carbonic Anhydrase) in Chlorella after Growth or Adaption in Different Carbon Dioxide Concentrations

Abstract

The unicellular green alga Chlorella pyrenoidosa, after growth in air or 5 % (v/v) CO*2, has markedly differing rates of photosynthesis in the presence of very low concentrations of CO*2. It was proposed that the differences could be accounted for by differences in the activities of ribulosebisphosphate carboxylase or other enzymes of the reductive pentose phosphate pathway. This hypothesis was tested and it was shown that between the two types of cells there were no significant differences in the activities of ribulosebisphosphate carboxylase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (EC 4.1.1.31), phosphoenolpyruvate carboxykinase (ATP) (EC 4.1.1.49), glyceraldehyde-phosphate dehydrogenase (NADP) (EC 1.2.1.9), hexosebisphosphatase (EC 3.1.3.11), fructose-bisphosphate aldolase (EC 4.1.2.13), ribosephosphate isomerase (EC 5.3.1.6) and phosphoribulokinase (EC 2.7.1.19). There was also no difference between air-grown and 5% CO*2-grown cells in the content of fraction I protein isolated by disc gel electrophoresis and measured by a colorimetric procedure. A fivefold or sixfold increase in rate of photosynthesis occurs during about 90 min when 5% CO*2-grown Chlorella is illuminated in the presence of a very low CO*2 concentration (approximately 1 µM free CO*2, which is about one-tenth the concentration of CO*2 in distilled water in equilibrium with air). During this increase, the only photosynthetic enzyme to change in activity was carbonate dehydratase (carbonic anhydrase, EC 4.2.1.1). This enzyme was absent from cells grown with 5% CO*2 but was present in air-grown cells. The enzyme appears to be induced by low CO*2 concentrations and repressed by high concentrations. It was concluded that carbonate dehydratase plays a major regulatory role in photosynthesis in Chlorella at low CO*2 concentrations.