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

Blue light stimulates photosynthesis in brown algae by activating a mechani sm for carbon acquisition. As this is connected with liberation of CO2 from an internal pool, it had previously been suggested that a C-4 pathway migh t operate, with CO2 mobilization h om an intermediate mediated by the actio n of blue light. The possibility that such a C-4 mechanism functions was te sted in Ectocarpus siliculosus by investigating metabolite pools and carbon fixation products. Extracts were analysed by HPLC with an ion exchange col umn. With the exception of citrate/isocitrate all metabolite levels were fa r too low to serve as the intermediate pool. No changes in the concentratio ns of any of the metabolites were found after activation by blue light foll owing red light pre-irradiation. During short-term incubations with C-14 in organic carbon, the major label was found in phosphoglycerate. There was al so clear labelling of peaks containing malate plus pyruvate and in citrate/ isocitrate. In addition, glycerate contained high levels of radioactivity i n some culture strains of Ectocarpus siliculosus. In chase experiments, the radioactivity of all metabolites continued to increase, indicating the pre sence of considerable amounts of intracellular inorganic carbon. Transient labelling was not observed for any of the fixation products, When Ectocarpu s was pre-irradiated with red light and then stimulated by blue light, fixa tion rates increased with a time course that correlated with the activation of oxygen evolution. Photosynthetic oxygen evolution was investigated in v ivo after the addition of possible intermediates to carbon-free seawater. M ost of the exogenous applied substrates did not enhance photosynthesis, eit her 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 d uring the breakdown of the metabolite in the medium. Taken together, the da ta argue strongly against the operation of a C-4 pathway in Ectocarpus. It is suggested that the internal pool mobilized by blue light contains inorga nic carbon.