Acquisition of inorganic carbon by the marine diatom Thalassiosira weissflogii

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 CO₂ by 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?