Ultrastructural and biochemical adaptation of algal cells to limiting CO2 concentrations
The pyrenoid is a prominent proteinaceous structure found in the stroma of the chloroplast in unicellular eukaryotic algae, most multicellular algae, and some hornworts. The pyrenoid contains the enzyme ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) and is sometimes surrounded by a carbohydrate sheath. In Chlamydomonas reinhardtii the pyrenoid starch sheath is formed rapidly in response to a decrease in the CO2 concentration in the environment. In this report we show that the unicellular green algae Chlamydomonas acidophila and Chlorella spK also form a starch sheath when adapted to low CO2 conditions. We also report that in some high CO2-requiring C. reinhardtii strains, pyrenoid starch sheath formation is partly inhibited or absent. The inhibition of the pyrenoid starch sheath is also seen when the carbonic anhydrase inhibitor ethoxyzolamide is added to cells placed in an environment low in CO2. These observations support the idea that the ultrastructural reorganization of the pyrenoid starch sheath under low CO2 conditions plays a role in the CO2 concentrating mechanism in C. reinhardtii as well as in other eukaryotic algae.