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The CO2-concentrating mechanism (CCM) was induced in the green unicellular alga Chlorella when cells were transferred from high (5% CO2) to low (0.03%) CO2 concentrations. The induction of the CCM correlated with the formation of a starch sheath specifically around the pyrenoid in the chloroplast. With the aim of clarifying whether the starch sheath was involved in the operation of the CCM, we isolated and physiologically characterized a starchless mutant of Chlorellapyrenoidosa, designated as IAA-36. The mutant strain grew as vigorously as the wild type under high and low CO2 concentrations, continuous light and a 12 h light/12 h dark photoperiod. The CO2 requirement for half-maximal rates of photosynthesis [K0.5(CO2)] decreased from 40 µM to 2–3 µM of CO2 when both wild type and mutant were switched from high to low CO2. The high affinity for inorganic carbon indicates that the IAA-36 mutant is able to induce a fully active CCM. Since the mutant does not have the pyrenoid starch sheath, we conclude that the sheath is not involved in the operation of the CCM in Chlorella cells.

Arginase, ornithine carbamoyl transferase (OCT) and arginine deiminase activities were found in cell-free extracts of NostocPCC 73102, a free-living cyanobacterium originally isolated from the cycad Macrozamia. Addition of either arginine, ornithine or citrulline to the growth medium induced significant changes in their in vitro activities. Moreover, growth in darkness, compared to in light, induced higher in vitro activities. The in vitro activities of arginase and arginine deiminase, two catabolic enzymes primarily involved in the breakdown of arginine, increased substantially by a combination of growth in darkness and addition of either arginine, or ornithine, to the growth medium. The most significant effects on the in vitro OCT activities where observed in cells grown with the addition of ornithine. Cells grown in darkness exhibited about 6% of the in vivo nitrogenase activity observed in cells grown in light. However, addition of external carbon (glucose and fructose) to cells grown in darkness resulted in in vivo nitrogenase activity levels similar to, or even higher than, cells grown in light. Growth with high in vivo nitrogenase activity or in darkness with the addition of external carbon, resulted in repressed levels of in vitro arginase and arginine deiminase activities. It is suggested that nitrogen starvation induces a mobilization of the stored nitrogen, internal release of the amino compound arginine, and an induction of two catabolic enzymes arginase and arginine deiminase. A similar and even more pronunced induction can be observed by addition of external arginine to the growth medium.

Photosynthetic characteristics of Dunaliella salinawith high (red form) and low β-carotene (green form) concentrations were studied. D. salinagrowing in brine saltworks exhibited a high level of β-carotene (15 pg cell−1). The rate of oxygen evolution as a function of irradiance was higher in the red than in the green form (on chlorophyll basis). Photosynthetic inhibition of the green form was observed above 500 µmol m−2s−1. The red form appeared more resistant to high irradiance and no inhibition in O2evolution was observed up 2000 µmol m−2s−1. However, when these results are expressed on a cell number basis the rate of oxygen evolution was significantly higher in the green form.