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Under future ocean acidification (OA), increased availability of dissolved inorganic carbon (DIC) in seawater may enhance seagrass productivity. However, the ability to utilise additional DIC could be regulated by light availability, often reduced through land runoff. To test this, two tropical seagrass species,  Cymodocea serrulata  and  Halodule uninervis  were exposed to two DIC concentrations (447 μatm and 1077 μatm  p CO 2 ), and three light treatments (35, 100, 380 μmol m -2  s -1 ) for two weeks. DIC uptake mechanisms were separately examined by measuring net photosynthetic rates while subjecting  C .  serrulata  and  H .  uninervis  to changes in light and addition of bicarbonate (HCO 3  - ) use inhibitors (carbonic anhydrase inhibitor, acetazolamide) and TRIS buffer (pH 8.0). We observed a strong dependence on energy driven H + -HCO 3  -  co-transport (TRIS, which disrupts H +  extrusion) in  C .  serrulata  under all light levels, indicating greater CO 2  dependence in low light. This was confirmed when, after two weeks exposure, DIC enrichment stimulated maximum photosynthetic rates (P max ) and efficiency (α) more in  C .  serrulata  grown under lower light levels (36–60% increase) than for those in high light (4% increase). However,  C .  serrulata  growth increased with both DIC enrichment and light levels. Growth, NPP and photosynthetic responses in  H .  uninervis  increased with higher light treatments and were independent of DIC availability. Furthermore,  H .  uninervis  was found to be more flexible in HCO 3  -  uptake pathways. Here, light availability influenced productivity responses to DIC enrichment, via both carbon fixation and acquisition processes, highlighting the role of water quality in future responses to OA.

Light Levels Affect Carbon Utilisation in Tropical Seagrass under Ocean Acidification