Coralline algae gain tolerance to ocean acidification over multiple generations of exposure

Crustose coralline algae (CCA) play crucial reef-building roles in the photic zones of nearshore ecosystems globally and are highly susceptible to ocean acidification^1-3. Yet the extent to which CCA can gain tolerance to ocean acidification over multiple generations of exposure is unknown. We show that juvenile CCA calcification is highly sensitive to ocean acidification initially, but the effects disappear after 6 generations of exposure. A reciprocal transplant experiment in the 7^th generation where half of all replicates were swapped between treatments of the opposite mean pH confirmed that they had acquired tolerance to low pH  and not simply to laboratory conditions. Neither exposure to greater pH variability, nor chemical conditions within the micro-scale calcifying fluid internally, played a role in fostering this capacity. Our results demonstrate that reef-accreting taxa can gain tolerance to ocean acidification over multiple generations of exposure, allowing the possibility that this cosmopolitan species could maintain their critical ecological role in reef-formation.