Metabolic implications of non-photosynthetic carbon fixation in brown macroalgae

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Abstract
Photosynthetic carbon-fixation in the brown macroalgae Laminaria digitata (Huds.) Lamour., L. hyperborea (Gunn.) Foslie, L. saccharina (L.) Lamour. (Laminariales) as well as Fucus serratus L. and F. vesiculosus L. (Fucales; Phaeophyta) is accompanied by substantial non-photosynthetic CO2-fixation via β-carboxylation of phosphoenolpyruvate, due to appreciable activity of phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32). Analyses of short-term photosynthetic 14C-labelling of C4 dicarboxylic acids such as aspartate and malate indicate that β-carboxylation is also performed in the light. During photosynthesis the substrate of PEPCK is preferentially provided by 3-phosphoglycerate from the reductive pentose phosphate cycle. In the dark, phosphoenolpyruvate originates from dissimilation of reserve carbohydrate (mannitol) via Embden-Meyerhoff pathway. Differential determination of dark respiration rates, respiratory mannitol loss, photosyn-thetic 14C-labelling of mannitol, and activity of the mannitol converting enzyme, mannitol-1-phosphate dehydrogenase (EC 1.1.1.17) demonstrate that mannitol degradation is particularly intense in young, growing blade regions of Laminaria spp., where usually highest rates of non-photosynthetic (dark) CO2-assimilation are encountered. It is concluded from these findings and from experiments with monofluoracetate that mannitol metabolism and β-carboxylation of PEP in the dark are intimately correlated.