Activities of principal photosynthetic enzymes in green macroalga Ulva linza: functional implication of C4 pathway in CO2 assimilation

The green-tide-forming macroalga Ulva linza was profiled by transcriptome sequencing to ascertain whether the alga carries both C-3 and C-4 photosynthesis genes. The key enzymes involved in C-4 metabolism including pyruvate orthophosphate dikinase (PPDK), phosphoenolpyruvate carboxylase (PEPC), and phosphoenolpyruvate carboxykinase (PCK) were found. When measured under normal and different stress conditions, expression of rbcL was higher under normal conditions and lower under the adverse conditions, whereas that of PPDK was higher under some adverse conditions, namely desiccation, high salinity, and low salinity. Both ribulose-1, 5-biphosphate carboxylase (RuBPCase) and PPDK were found to play a role in carbon fixation, with significantly higher PPDK activity across the stress conditions. These results suggest that elevated PPDK activity alters carbon metabolism in U. linza leading to partial operation of the C-4 carbon metabolism, a pathway that, under stress conditions, probably contributes to the hardy character of U. linza and thus to its wide distribution.