Ocean acidification increases phytobenthic carbon fixation and export in a warm-temperate system

Abstract The response of photosynthetic organisms to rising CO2 levels is a key topic in ocean acidification research. Most of the work in this field has focused on physiological responses in laboratory conditions which lack ecological realism. Studies using seeps as natural analogues for ocean acidification have demonstrated shifts in algal community composition, but the effect of CO2 on carbon fixation and export remains unclear. Here, we deployed artificial substrata in a warm-temperate region of Japan to collect algal communities using a CO2 seep off Shikine Island. Diatoms became dominant on settlement substrata in areas with elevated CO2 levels, whereas macroalgae dominated at present-day levels of CO2 (reference site). This was supported by pigment composition; fucoxanthin content, characteristic of diatoms, was higher at the high CO2 site, while more Chlorophyll b, which is characteristic of Chlorophyta, was found in the reference site. Algal communities that recruited in water with high levels of CO2 had elevated rates of photosynthesis. Algal biomass was similar on all settlement panels, regardless of CO2 concentration. Much of the carbon that was fixed by algae in the high CO2 conditions was exported, likely due to detachment from the substratum. Diatoms that dominated under high CO2 conditions are more easily transported away as they have no holdfast, whereas newly settled macroalgae became firmly attached at present-day levels of CO2. These results show that ocean acidification may fundamentally alter coastal carbon cycling, increasing photosynthesis and carbon export from coastal ecosystems in warm-temperate biogeographic regions due to a shift in community composition from perennial to ephemeral algae.