Photosynthetic inorganic carbon uptake and accumulation in two marine diatoms

Some physiological characteristics of photosynthetic inorganic carbon uptake have been examined in the marine diatoms Phaeodactylum tricornutum and Cyclotella sp. Both species demonstrated a high affinity for inorganic carbon in photosynthesis at pH7.5, having K 1/2 (CO 2 ) in the range 1.0 to 4.0 mmol m -3 and O 2- and temperature-insensitive CO 2 compensation concentrations in the range 10.8 to 17.6 cm 3 m -3 . Intracellular accumulation of inorganic carbon was found to occur in the light ; 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 suspending medium. Carbonic anhydrase (CA) was detected in intact Cyclotella cells but not in P. tricornutum, although internal CA was detected in both species. The rates of photosynthesis at pH 8.0 of P. tricornutum cells and Cyclotella cells treated with 0.1 mol m -3 acetazolamide, a CA inhibitor, were 1.5- to 5-fold the rate of CO 2 supply, indicating that both species have the capacity to take up HCO 3 - as a source of substrate for photosynthesis. No Na + dependence for HCO 3 - could be detected in either species. These results indicate that these two marine diatoms have the capacity to accumulate inorganic carbon in the light as a consequence, in part, of the active uptake of bicarbonate.

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