Activation by blue light of inorganic carbon acquisition for photosynthesis in Ectocarpus siliculosus: organic acid pools and short-term carbon fixation

Abstract

Blue light stimulates photosynthesis in brown algae by activating a mechanism for carbon acquisition. As this is connected with liberation of CO₂ from an internal pool, it had previously been suggested that a C₄ pathway might operate, with CO₂ mobilization from an intermediate mediated by the action of blue light. The possibility that such a C₄ mechanism functions was tested in Ectocarpus siliculosus by investigating metabolite pools and carbon fixation products. Extracts were analysed by HPLC with an ion exchange column. With the exception of citrate/isocitrate all metabolite levels were far too low to serve as the intermediate pool. No changes in the concentrations of any of the metabolites were found after activation by blue light following red light pre-irradiation. During short-term incubations with ¹⁴C inorganic carbon, the major label was found in phosphoglycerate. There was also clear labelling of peaks containing malate plus pyruvate and in citrate/isocitrate. In addition, glycerate contained high levels of radioactivity in some culture strains of Ectocarpus siliculosus. In chase experiments, the radioactivity of all metabolites continued to increase, indicating the presence of considerable amounts of intracellular inorganic carbon. Transient labelling was not observed for any of the fixation products. When Ectocarpus was pre-irradiated with red light and then stimulated by blue light, fixation rates increased with a time course that correlated with the activation of oxygen evolution. Photosynthetic oxygen evolution was investigated in vivo after the addition of possible intermediates to carbon-free seawater. Most of the exogenous applied substrates did not enhance photosynthesis, either in red light or after blue light stimulation. A strong enhancement seen after the addition of oxaloacetate was most likely due to release of CO₂ during the breakdown of the metabolite in the medium. Taken together, the data argue strongly against the operation of a C₄ pathway in Ectocarpus. It is suggested that the internal pool mobilized by blue light contains inorganic carbon.