Photosynthesis might be limited by light, not inorganic carbon availability, in three intertidal Gelidiales species

• The interaction between incident photon fluence density (PFD) and dissolved inorganic carbon (DIC) availability on photosynthesis is reported for three species of Gelidiales (Rhodophyta) from the Canary Islands. • Photosynthetic O₂ evolution in response to both DIC concentration and PFD, and external carbonic anhydrase activity were measured in thalli of Gelidium canariensis, Gelidium arbuscula and Pterocladiella capillacea maintained in a controlled-environment room. • No detectable external carbonic anhydrase activity, high sensitivity to alkaline pH and moderate values of photosynthetic conductance for DIC indicated that the three species had a low capacity for using the external pool of HCO₃ ^- ; therefore photosynthetic rates were dependent on CO₂ availability. The seawater concentration of DIC was insufficient to saturate photosynthesis at high PFD; photosynthesis vs PFD was not affected by DIC concentration at 0.07-0.04 mol O₂ mol^-1 photon. ANOVA revealed that incident PFD had a greater effect than DIC availability on photosynthesis rates in G. canariensis. • Photosynthesis is probably limited by incident light rather than DIC concentration despite the apparent low affinity for HCO₃ ^- in G. canariensis, G. arbuscula and P. capillacea in their natural habitats.