THE ACTIVE UPTAKE OF CARBON-DIOXIDE BY THE MARINE DIATOMS PHAEODACTYLUM-TRICORNUTUM AND CYCLOTELLA SP

Mass spectrometry has been used to investigate the uptake of CO2 by two marine diatoms, Phaeodactylum tricornutum and Cyclotella sp, The time course of CO2 formation in the dark after addition of 100 mmol m(-3) dissolved inorganic carbon (DIC) to cell suspensions showed that external carbonic anhydrase (CA) was not present in cells of P. tricornutum but was present in Cyclotella sp, In the absence of external CA, or when it was inhibited by 5 mmol m(-3) acetazolamide, cells of both species pre-incubated with 100 mmol m(-3) DIC rapidly depleted almost all of the free CO2 (3.2 mmol m(-3) at pH 7.5) from the suspending medium within seconds of illumination and prior to the onset of steady-state photosynthesis, Addition of bovine CA quickly restored the HCO3--CO2 equilibrium in the medium, indicating that the initial depletion of CO2 resulted from the selective uptake of CO2 rather than uptake of all DIC species, Transfer of cells to the dark caused a rapid increase in the CO2 concentration in the medium, largely as a result of the efflux of unfixed inorganic carbon from the cells, The measured CO2 uptake rates for both species accounted for 50% of the total DIC uptake at HCO3--CO2 equilibrium, indicating that HCO3- was also being taken up. These results indicate that both Phaeodactylum tricornutum and Cyclotella sp, have the capacity to transport CO2 actively against concentration and pH gradients.