PHOTOSYNTHETIC INORGANIC CARBON UPTAKE AND ACCUMULATION IN 2 MARINE DIATOMS

Some physiological characteristics of photosynthetic inorganic carbon uptake have been examined in the marine diatoms Phaeodactylum tricornu tum and Cyclotella sp, Both species demonstrated a high affinity for i norganic carbon in photosynthesis at pH 7.5, having K-1/2(CO2) in the range 1.0 to 4.0 mmol m(-3) and O-2- and temperature-insensitive CO2 c ompensation concentrations in the range 10.8 to 17.6 cm(3) m(-3). Intr acellular accumulation of inorganic carbon was found to occur in the l ight; at an external pH of 7.5 the concentration in P. tricornutum was twice, and that in Cyclotella 3.5 times, the concentration in the sus pending medium, Carbonic anhydrase (CA) was detected in intact Cyclote lla cells but not in P. tricornutum, although internal CA was detected in both species, The rates of photosynthesis at pH 8.0 of P. tricornu tum cells and Cyclotella cells treated with 0.1 mol m(-3) acetazolamid e, a CA inhibitor, were 1.5- to 5-fold the rate of CO2 supply, indicat ing that both species have the capacity to take up HCO3- as a source o f substrate for photosynthesis, No Na+ dependence for HCO3- could be d etected in either species, These results indicate that these two marin e diatoms have the capacity to accumulate inorganic carbon in the ligh t as a consequence, in part, of the active uptake of bicarbonate.