Role of C 4 carbon fixation in Ulva prolifera , the macroalga responsible for the world’s largest green tides

Most marine algae preferentially assimilate CO 2 via the Calvin-Benson Cycle (C 3 ) and catalyze HCO 3 − dehydration via carbonic anhydrase (CA) as a CO 2 -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 HCO 3 − mechanism. Analysis of CA and key C 3 and C 4 enzymes, and subsequent analysis of δ 13 C photosynthetic products showed that the species assimilates CO 2 predominately via the C 3 pathway, uses HCO 3 − via the CA mechanism at low CO 2 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 CO 2 limitation.