Genome-wide identification of genes involved in carbon fixation in Saccharina japonica and responses of putative C-related genes to bicarbonate concentration and light intensity.

Brown algae play a dominant role in the primary productivity of coastal ecosystems and may have an efficient carbon fixation. In this work, 56 genes involved in inorganic carbon fixation were identified from the Saccharina japonica genome. Sequence structure analysis of these genes showed the existence of corresponding function domains and active amino acid sites highly conserved with other stramenopile species. The predicted subcellular localizations showed that Calvin cycle-related enzymes predominantly reside in the plastid and that putative C-related enzymes are mainly distributed in the mitochondrion. We determined the transcriptional profiles and enzymatic activities of these C-related enzymes in response to the KHCO concentrations and light intensities. Pyruvate orthophosphate dikinase (PPDK) presented the greatest response to low HCO concentrations and high light intensity. Phosphoenolpyruvate carboxykinase (PEPCK) was up-regulated at low HCO concentrations to compensate for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and might be the crucial decarboxylase in this kelp. We propose that S. japonica might possess a PPDK- and PEPCK-dependent C-like pathway that enables its rapid growth in natural coastal environments.