Photosynthetic traits of the ubiquitous and prolific macroalga Ulva (Chlorophyta): a review

Ulva is an opportunistically growing green macroalgal genus, the worldwide distribution of which can partly be explained by its ability to employ two alternative modes of photosynthetic inorganic carbon acquisition. In temperate areas, the less efficient (in terms of inorganic carbon utilization), externally acting, carbonic anhydrase-catalysed HCO₃^– to CO₂ conversion mechanism prevails. However, when growing in warmer and higher irradiance regions, or if transferred to high-pH seawater, Ulva features a highly efficient HCO₃⁻ uptake system unique among macroalgae. In addition, the light reactions acclimate effectively to various irradiances, including full sunlight in the intertidal. The following topics are discussed in this review, often in a historical perspective, in the context of two questions: (1) Is there a need for a CO₂ concentrating mechanism? (Yes!) and (2) if so, is Ulva a C₄ alga? (No!). How Ulva utilizes external HCO₃⁻ for its photosynthetic needs is discussed, considering the ability of Ulva to increase pH values to >10 in enclosed areas such as rockpools, and its ecological consequences. The ability of intertidal Ulva to photosynthesize when emergent is addressed, and mechanisms protecting the light reactions from high irradiances are reviewed. Finally, Ulva is viewed in the context of future environments of increased CO₂ and ocean acidification. HIGHLIGHTS Photosystems II and I of Ulva are well protected from high irradiances. Ulva is unique among macroalgae in using a plasma membrane HCO₃^– transport system. Ulva’s efficient photosynthetic traits allow it to thrive worldwide.