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 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?