Variability in the benefits of ocean acidification to photosynthetic rates of macroalgae without CO₂-concentrating mechanisms

Increasing concentrations of surface-seawater carbon dioxide (CO₂) (ocean acidification) could favour seaweed species that currently are limited for dissolved inorganic carbon (DIC). Among them, those that are unable to use CO₂-concentrating mechanisms (CCMs) to actively uptake bicarbonate (HCO₃^–) across the plasmalemma are most likely to benefit. Here, we assess how the DIC uptake and photosynthetic rates of three rhodophytes without CCMs respond to four seawater CO₂ concentrations representing pre-industrial (280 μatm), present-day (400 μatm), representative concentration pathway (RCP) emissions scenario 8.5 2050 (650 μatm) and RCP 8.5 2100 (1000 μatm). We demonstrated that the photosynthetic rates of only one species increase between the preindustrial and end-of-century scenarios, but because of differing photosynthetic quotients (DIC taken up relative to O₂ evolved), all three increase their DIC uptake rates from pre-industrial or present-day scenarios to the end-of-century scenario. These variable, but generally beneficial, responses highlight that not all species without CCMs will respond to ocean acidification uniformly. This supports past assessments that, on average, this group will likely benefit from the impacts of ocean acidification. However, more concerted efforts are now required to assess whether similar benefits to photosynthetic rates and DIC uptake are also observed in chlorophytes and ochrophytes without CCMs.