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The Resilience of a Coralline Red Algae to Ocean Acidification

Paper Reviewed
Donald, H.K., Ries, J.B., Stewart, J.A., Fowell, S.E. and Foster, G.L. 2017. Boron isotope sensitivity to seawater pH change in a species of Neogoniolithon coralline red alga. Geochimica et Cosmochimica Acta 217: 240-253.

Coralline red algae play an important role in coral reef ecology. They provide a vital food source for marine grazers and their presence helps to stabilize the reef crest and reduce coastal erosion.

Previous work exploring the effects of ocean acidification on these calcareous organisms has shown an inherent resilience in their ability to calcify under moderate-to-extreme changes in seawater pH (pHsw), which resilience may result from biologically-mediated processes that increase the pH of the seawater at the site of calcification (pHcf). Seeking to provide further insight on this topic, Donald et al. (2017) recently examined the boron isotopic composition (δ11B) of a coralline red algae (Neogoniolithon sp.) to investigate how its pHcf was impacted by ocean acidification.

To accomplish their work, the five researchers estimated pHcf from δ11B measurements of Neogoniolithon specimens that were cultured under four pCO2 conditions (409, 606, 903 and 2856 µatm, corresponding to pHsw values of 8.19, 8.05, 7.91 and 7.49, respectively) over a period of 60 days. Results indicated that the coralline red algae elevated the pHcf relative to the pHsw in each pCO2 treatment by an average of 1.20 pH units, with the ΔpH (pHcf minus pHsw) being lowest in the ambient (8.05) pHsw treatment and highest (1.26) in the second most acidic treatment (7.91 pHsw). Thus, the algal taxon was indeed able to biologically-mediate its pHcf in response to changes in pHsw.

The influence of pHsw on both pHcf and the calcification rate of Neogoniolithon is plotted in Figure 1 below. As indicated there, this coralline algal species is able to elevate its pHcf so as to increase its rate of calcification under moderate levels of ocean acidification (pHsw of 7.91 and 8.05), which increase the authors say is "most likely due to CO2-fertilization of [algal] photosynthesis" that is limited in Neogoniolithon at these lower pCO2 conditions. What is more, as illustrated by the red shading in the figure, which area represents the confidence interval of the calcification rate observed by Neogoniolithon at ambient pHsw (8.19), calcification rates do not fall outside this bound until pHsw drops below ~7.6, which value is unlikely to occur in even the most pessimistic of future ocean acidification scenarios. Thus, we can expect that this coralline red algae will maintain or improve its rate of calcification in the years, decades and even centuries to come.

Figure 1. The influence of pHsw on pHcf (black circles) and net calcification (red squares) of Neogoniolithon sp. The red shaded area represents the confidence interval of the calcification rate observed at ambient seawater pH (8.19), illustrating that this coralline algae species is able to elevate its pHcf in order to maintain similar calcification rates to that observed at present under future scenarios of ocean acidification where the pHsw falls to levels perhaps as low as 7.6.

Posted 31 January 2018