Inorganic carbon accumulation and photosynthesis by Chlorella pyrenoidosa

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Abstract
Photosynthesis and intracellular inorganic C were measured in air-grown and high CO2 grown C. pyrenoidosa under various environmental and chemical treatments, using a silicone-oil centrifugation technique. At alkaline pH, both cell types accumulated inorganic carbon, presumably HCO3-, in a light-dependent and saturable process. The formation of the internal pool was kinetically observed and was used as a substrate for photosynthesis. Rates of C influx always exceeded the rates of photosynthesis in air-grown cells, whereas in high CO2 grown cells influx rates closely paralleled photosynthesis. At acid pH inorganic C was also accumulated, but the characteristics of this accumulation differed from that seen at alkaline pHs. It was more rapid and was not affected by any of the treatments that affected accumulation at higher pHs. However, the internal inorganic carbon pool provided CO2 for photosynthesis. This accumulation can be explained partly by CO2 diffusion according to the pH gradient existing across the plasmalemma and partly by nonspecific binding of CO2 to proteins and amino acids. The results are consistent with the view that air-grown C. pyrenoidosa is capable of utilizing their CO2 of HCO3- as a substrate for the internal inorganic C pool.