Photosynthesis of Ectocarpus siliculosus in red light and after pulses of blue light at high pH – evidence for bicarbonate uptake

Abstract

Pulses of blue light cause stimulation of red light saturated photosynthesis in Ectocarpus siliculosus, because blue light activates the operation of a pathway for inorganic carbon (Ci) acquisition by inducing the mobilization of CO2 from an intermediate metabolite. In the absence of exogenous Ci, photosynthetic rates roughly equal those of CO2 release by respiration. In seawater of pH 9·5 (2·3 mol m–3 total Ci, but concentrations of free CO2 below 0·2 mmol m–3), photosynthesis was clearly above these rates, although they were only ≈ 30% of those in normal seawater (≈ pH 8). The degree and the time course of the stimulations of photosynthesis by pulses of blue light were unaltered at high pH. Essentially the same characteristics were found after buffering or in the presence of acetazolamide, an inhibitor of extracellular carbonic anhydrase activity. Therefore, it is concluded that Ectocarpus is able to directly take up HCO3– in addition to CO2 (uptake of CO32– cannot be excluded). The dependence of photosynthesis on Ci at pH 9·5 was biphasic, with Ci below 0·2 mol m–3 having no effect at all. In Ci‐free seawater, the shapes of the stimulations after blue light pulses differed for pH 6, pH 8 and pH 9·5. At low pH, only the fast peak (maximum ≈ 5 min after blue light) was detected, whereas at high pH mainly the slow peak (maximum ≈ 20 min after blue light) was observed. At the intermediate pH 8, both peaks were present. As inhibition of total carbonic anhydrase by ethoxyzolamide brought out the fast peak of the stimulations at pH 9·5 it is concluded that the fast component was due to a transient disequilibrium of an intracellular pool of Ci which, after blue light, was fed by CO2 released from the postulated storage intermediate.