Water motion and pH jointly impact the availability of dissolved inorganic carbon to macroalgae

The supply of dissolved inorganic carbon to seaweeds is a key factor regulating photosynthesis. Thinner diffusive boundary layers at the seaweed surface or greater seawater carbon dioxide (CO₂) concentrations increase CO₂ supply to the seaweed surface. This may benefit seaweeds by alleviating carbon limitation either via an increased supply of CO₂ that is taken up by passive diffusion, or via the down-regulation of active carbon concentrating mechanisms (CCMs) that enable the utilization of the abundant ion bicarbonate (HCO₃^-). Laboratory experiments showed that a 5 times increase in water motion increases DIC uptake efficiency in both a non-CCM (Hymenena palmata, Rhodophyta) and CCM (Xiphophora gladiata, Phaeophyceae) seaweed. In a field survey, brown and green seaweeds with active-CCMs maintained their CCM activity under diverse conditions of water motion. Whereas red seaweeds exhibited flexible photosynthetic rates depending on CO₂ availability, and species switched from a non-CCM strategy in wave-exposed sites to an active-CCM strategy in sheltered sites where mass transfer of CO₂ would be reduced. 97-99% of the seaweed assemblages at both wave-sheltered and exposed sites consisted of active-CCM species. Variable sensitivities to external CO₂ would drive different responses to increasing CO₂ availability, although dominance of the CCM-strategy suggests this will have minimal impact within shallow seaweed assemblages.