Effects of Tidally Driven Variation on the Response of Coralline Algae to Ocean Acidification

As atmospheric CO₂ levels continue to rise, our oceans are becoming more acidic, making it difficult for calcareous organisms like coralline algae to calcify. Coralline algae are early colonizers after disturbances and foundational species that initiate succession by inducing larval settlement of many invertebrate species. However, coralline algae tend to be more susceptible to experimentally elevated pCO₂ than other calcifiers, likely due to the higher magnesium content in their calcite skeleton, which can render them more soluble. Magnesium content varies between individuals and is context dependent, thus could be a mechanism of acclimation for algae recruiting to harsh environments. To test this hypothesis, we collected Corallina officinalis from tide pools that experience extreme daily variation and from a well-flushed site that experiences lower daily variation in seawater pH. Samples were placed for 22 days in 1L microcosms bubbled with air enriched with pCO₂, with values ranging from preindustrial lows (280 uatm) to predicted highs over the next century (1120 uatm) over 6 treatment levels. C. officinalis collected in the isolated tide pools showed decreased growth (~50%) both in net calcification (measured via buoyant weight method) and linear extension (visualized with fluorescent stain) in low and high pCO₂ levels, with growth peaking at an optimal pCO₂ value of approximatly 300 uatm similar to present-day conditions. In contrast C. officinalis collected from the flushed site had no response to pCO₂ treatments but had significantly lower growth overall. Tide pool two showed higher inclusion of magnesium in its carbonate skeleton which could explain its more pronounced response to the pCO₂  treatments. While living in harsh environments can acclimate coralline algae to high pCO₂, overall growth rates are substantially lower and will likely be insufficient to alleviate effects of ocean acidification.