Ocean acidification detrimentally affects mineralization and photosynthetic efficiency of the brown alga Padina pavonica at a CO₂ vent

Increasing atmospheric CO₂ concentrations are causing ocean acidification potentially leading to significant consequences for marine organisms, particularly those that build calcium carbonate structures. In the present study, we examined the effects of decreasing pH on the photosynthetic efficiency and spectral reflectance of the calcifying alga Padina pavonica naturally occurring along a natural pH gradient generated by a CO₂ vent near the Island of Panarea (Italy). Based on the photoacoustic determination of photosynthetic light energy utilization efficiency, we found that the photosynthetic efficiency of P. pavonica decreased with decreasing pH. In the most acidified sites, calcium carbonate across the algae decreased while the changes in absorption light at 483 nm (carotenoids) in relation to green light (562 nm) ratio indicate an increase in the contribution of carotenoids to algal pigments towards the more acidic sites. Here we show, for the first time, the effects of seawater acidification on the optical properties of a Mediterranean calcifying phototroph under low pH conditions at CO₂ vents.