Early reproductive stages in the crustose coralline alga Phymatolithon lenormandii are strongly affected by mild ocean acidification

Coralline algae (Corallinales, Rhodophyta) are predicted to be negatively impacted by near-future ocean acidification. The effect of low pH/high pCO2 on early life stages of Phymatolithon lenormandii (Areschoug) Adey was studied in a perturbation experiment. Several parameters including mortality, calcification (calcein staining) and development (growth and abnormalities) have been monitored for a month under experimental conditions ranging from pHT = 8.00 (pCO2 = 398 μatm) and pHT = 7.55 (pCO2 = 1,261 μatm). Our results demonstrate that survival and development of P. lenormandii early life stages can be impacted by small pH changes (ΔpH < −0.1 pH unit). A negative impact of decreasing pH was observed including an increased mortality and a higher rate of abnormalities. Growth and calcification were still observed at the lowest pH (ΔpH = −0.45). Growth rate was similar at all tested pH, but the maintenance of the skeleton under low pH was only possible through a persistent dynamic dissolution/calcification process, an energetically costly mechanism potentially draining resources from other vital processes.