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.