Abstract

Recent data on the physiology of inorganic carbon acquisition by the model marine diatom Thalassiosira weissflogii (Grunow) demonstrate the importance of the catalytic equilibration of HCO₃^-and CO₂by carbonic anhydrases located in the periplasm and in the cytoplasm. These enzymes can use Zn, Co or Cd as their metal centre, and their activity increases at low ambient CO₂. The silica frustule provides buffering for extracellular CA activity, The transmembrane transport of CO₂ may occur by passive diffusion. Under CO₂ limitation, the cytoplasmic HCO₃^–is used to form malate and oxaloacetic acid via phosphoenolpyruvate carboxylase. It appears that subsequent decarboxylation of these compounds in the chloroplast regenerates CO₂ near the site of Rubisco, and thus provides the organism with an effective unicellular C₄ photosynthetic pathway. These results, together with other published data, bring up two major questions regarding inorganic carbon acquisition in diatoms: What is the major species of inorganic carbon (CO₂ or HCO₃^–) transported across the membrane under natural conditions? And what is the form of carbon (inorganic or organic) accumulated by the cells?