Inorganic carbon acquisition by the chrysophyte algaMallomonas papillosa

Photosynthetic characteristics of the chrysophyte alga Mallomonas papillosa Harris et Bradley were investigated to determine whether this species has some form of CO2-concentrating mechanism. The effect of external pH on the photosynthetic oxygen evolution rate of air-grown cells demonstrated an optimum in the pH range 5.0–7.0. This species lacked external carbonic anhydrase, and the cells had no capacity for direct bicarbonate uptake and had a low affinity for dissolved inorganic carbon. Measurement of the fluxes of CO2and O2in photosynthesizing cells at pH 7.0, using mass spectrometry, displayed no rapid uptake but only a slow depletion of CO2from the medium upon illumination. Furthermore, CO2uptake and O2evolution by M. papillosa was greatly reduced by iodoacetamide, an inhibitor of CO2fixation. The overall internal pH of M. papillosa was determined by distribution of14C-benzoic acid over the pH range 5.5–6.0 and [2-14C]-5,5-dimethyloxazolidine-2,4-dione over the pH range 6.5–7.0 between the cells and medium. As the external pH was lowered from 7.0 to 5.5, there was a decrease in the internal pH of M. papillosa cells from 8.31 to 7.75. The ΔpH was great enough to allow the intracellular accumulation of inorganic carbon by the diffusive uptake of CO2.