Role of C4 carbon fixation in Ulva prolifera, the macroalga responsible for the world's largest green tides

Most marine algae preferentially assimilate CO2 via the Calvin-Benson Cycle (C-3) and catalyze HCO3- dehydration via carbonic anhydrase (CA) as a CO2-compensatory mechanism, but certain species utilize the Hatch-Slack Cycle (C-4) to enhance photosynthesis. The occurrence and importance of the C-4 pathway remains uncertain, however. Here, we demonstrate that carbon fixation in Ulva prolifera, a species responsible for massive green tides, involves a combination of C-3 and C-4 pathways(,) and a CA-supported HCO3- mechanism. Analysis of CA and key C-3 and C-4 enzymes, and subsequent analysis of delta C-13 photosynthetic products showed that the species assimilates CO2 predominately via the C-3 pathway, uses HCO3- via the CA mechanism at low CO2 levels, and takes advantage of high irradiance using the C-4 pathway. This active and multi-faceted carbon acquisition strategy is advantageous for the formation of massive blooms, as thick floating mats are subject to intense surface irradiance and CO2 limitation. Liu et al. present evidence that carbon fixation in Ulva prolifera takes place via a combination of C-3 and C-4 pathways in combination with the enzyme carbonic anhydrase. The active and multi-faceted carbon acquisition strategy in U. prolifera is advantageous for the formation of massive blooms as the thick floating mats are subject to intense surface irradiance and CO2 limitation.