Unicellular algae, including Chlamydomonas reinhardtii, have the capacity to concentrate CO2 internally, allowing these organisms to grow photoautotrophically at CO2 concentrations lower than most higher plants can tolerate. This CO2 concentrating mechanism is inducible; it is present only in cells that have been grown with limiting CO2 conditions. An important aspect of this Ci (Ci = CO2 + HCO3- + CO3-2) accumulation system is located in the chloroplast. Intact chloroplasts isolated from low CO2- grown C. reinhardtii cells retain the ability to accumulate Ci to much higher levels than intact chloroplasts isolated from high Co2-grown cells. Five polypeptides that are specifically induced by growth on low CO2 and are missing in the high CO2-requiring strain CIA-5 have been identified. One of these proteins is a carbonic anhydrase located in the periplasmic space and another is a 36 kDa membrane associated polypeptide located in intact chloroplasts. The other three proteins are soluble and their function is unknown. The role of the chloroplast in Ci accumulation is being characterized by determining Ci uptake in intact chloroplasts from wild-type and high CO2-requiring C. reinhardtii strains. The role of the 36 kDa polypeptide in this uptake process is also being investigated. This protein is located on the chloroplast, possibly in the chloroplast envelope, and may be responsible, in part, for the increase in Ci uptake observed in chloroplasts from low CO2-grown cultures. The chloroplast carbonic anhydrase a low abundance, constitutively made protein that is required for efficient CO2 fixation is also being studied. The role of the three other low CO2-induced polypeptides is unknown. A cDNA library is being made to mRNA isolated from low CO2-grown cells and differentially screened with cDNA made from mRNA from low and high CO2-grown cells. This will identify many of the low CO2-inducible genes required for growth on low CO2. The eventual characterization of the proteins of this transport system will increase our understanding of the low CO2 adjustment in algae and the role of the chloroplast in this photosynthetic adaptation. Photosynthesis involves the photoassimilation of carbon dioxide (CO2) and the photoevolution of oxygen (O2). Partly due to this gas exchange activity of plants, earth's atmosphere changed in composition from one that was high in CO2 and low in O2 to one that is high in O2 and low in CO2. Under current conditions, the rate of CO2 fixation and thus plant growth, can be limited by CO2. Many unicellular algae can adapt to low CO2 conditions so that they become very efficient in their utiliation of CO2 for photosynthesis. This adaptation results from an enhancement of the cells' ability to transport and accumulate inorganic carbon. These studies characterize the protein components required for the efficient use of CO2 in the green alga Chlamydomonas reinhardtii. Emphasis is placed on the role of the chloroplast in inorganic carbon acquisition.
