ULTRASTRUCTURAL AND BIOCHEMICAL ADAPTATION OF ALGAL CELLS TO LIMITINGCO2 CONCENTRATIONS

The pyrenoid is a prominent proteinaceous structure found in the strom a of the chloroplast in unicellular eukaryotic algae, most multicellul ar algae, and some hornworts. The pyrenoid contains the enzyme ribulos e 1,5 bisphosphate carboxylase/oxygenase (Rubisco) and is sometimes su rrounded by a carbohydrate sheath. In Chlamydomonas reinhardtii the py renoid starch sheath is formed rapidly in response to a decrease in th e CO2 concentration in the environment. In this report we show that th e unicellular green algae Chlamydomonas acidophila and Chlorella spK a lso form a starch sheath when adapted to low CO2 conditions. We also r eport 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 anhydras e inhibitor ethoxyzolamide is added to cells placed in an environment low in CO2. These observations support the idea that the ultrastructur al reorganization of the pyrenoid starch sheath under low CO2 conditio ns plays a role in the CO2 concentrating mechanism in C. reinhardtii a s well as in other eukaryotic algae.