Warming and CO2 enrichment drive contrasting CO2-concentrating mechanism responses in two Mediterranean marine macrophytes

Mixed meadows of the seagrass Cymodocea nodosa and the green macroalga Caulerpa prolifera are commonly found in Mediterranean coastlines. These two marine macrophytes are increasingly exposed to ocean warming and increased dissolved CO 2 conditions, but how the interactive effects of these stressors remodel their photosynthetic physiology remains poorly resolved. We conducted a 15-day full factorial flow-through microcosm experiment crossing two temperatures (25 and 30 °C) with two CO 2 concentrations (400 and 1000 ppm), to analyse the responses in growth, photophysiology and operation of their biophysical CO 2 -concentrating mechanisms (CCMs). The two species operated fundamentally different CCMs, with proton extrusion coupled to extracellular carbonic anhydrases as the major CCM component in C. nodosa , and anion-exchange HCO 3 - transport in C. prolifera . Warming was the main driver of the reorganization of CCM components in both species, whereas elevated CO 2 had comparatively minor effects on the individual components. Notably, combined warming and elevated CO 2 interactively restored several CCM components of the seagrass to control levels, suggesting that CO 2 enrichment did not deactivate CCMs. Warming stimulated net photosynthesis and leaf elongation in C. nodosa whereas in C. prolifera neither warming nor elevated CO 2 significantly modified net photosynthesis or rhizoid elongation, and the additional fronds formed under warming did not translate into biomass gain. These divergent physiological trajectories, observed under microcosm conditions after a 15-day incubation, suggest that near-future climate conditions may favour the ecophysiological performance of the seagrass over that of the macroalga in co-occurring Mediterranean meadows.