Affinity, capacity and oxygen sensitivity of two different mechanisms for bicarbonate utilization in Ulva lactuca L. (Chlorophyta)

Ulva lactuca, collected on the west coast of Sweden at the end of May, was able to utilize the HCO3− pool of seawater only through extracellular dehydration via carbonic anhydrase, followed by uptake of the CO2 formed. A decrease in the CO2 supply via this mechanism resulted in the gradual development of an additional method of HCO3− utilization, namely a direct uptake of HCO3−. Photosynthesis could then be supported by either a ‘HCO3− dehydration mechanism’ or a ‘HCO3− uptake mechanism’. Through selective inhibition of either of these mechanisms, the physiological properties of the other could be assessed. These properties suggest that the HCO3− uptake mechanism of U. lactuca is important under conditions when low concentrations of inorganic C, high pH and high external O2 concentrations would limit photosynthesis supported by the HCO3− dehydration mechanism. Such conditions may occur during intense irradiation of the alga in rockpools or in shallow bays with low rates of water exchange. The results are discussed in relation to a possible coupling between mechanisms for inorganic C acquisition and cell structure (or even morphology) of green macroalgae. They also illustrate some necessary precautions when using Michaelis–Menten kinetics for estimations of Vmax and K1/2 values.