Most marine algae preferentially assimilate CO₂ via the Calvin-Benson Cycle (C₃) and catalyze HCO₃⁻ dehydration via carbonic anhydrase (CA) as a CO₂-compensatory mechanism, but certain species utilize the Hatch-Slack Cycle (C₄) to enhance photosynthesis. The occurrence and importance of the C₄ pathway remains uncertain, however. Here, we demonstrate that carbon fixation in Ulva prolifera, a species responsible for massive green tides, involves a combination of C₃ and C₄ pathways_, and a CA-supported HCO₃⁻ mechanism. Analysis of CA and key C₃ and C₄ enzymes, and subsequent analysis of δ¹³C photosynthetic products showed that the species assimilates CO₂ predominately via the C₃ pathway, uses HCO₃⁻ via the CA mechanism at low CO₂ levels, and takes advantage of high irradiance using the C₄ 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.

大多数海藻优先通过卡尔文-本森循环 (C ₃ )同化 CO ₂并通过碳酸酐酶 (CA)催化 HCO ₃ ⁻脱水作为 CO ₂补偿机制，但某些物种利用 Hatch-Slack 循环 (C ₄ )以增强光合作用。然而，C ₄途径的发生和重要性仍然不确定。在这里，我们证明了Ulva prolifera中的碳固定，一种负责大规模绿潮的物种，涉及 C ₃和 C ₄途径的组合_，以及 CA 支持的 HCO ₃ ^-机制。CA 和关键 C ₃和 C ₄酶的分析以及随后对 δ ¹³ C 光合产物的分析表明，该物种主要通过 C ₃途径同化 CO ₂，在低 CO ₂水平下通过 CA 机制使用 HCO ₃ ^-，并且利用 C ₄途径利用高辐照度。这种主动和多方面的碳采集策略有利于形成大量水华，因为厚浮垫会受到强烈的表面辐照度和 CO ₂限制。