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Skeletal trade-offs in coralline algae in response to ocean acidification


Ocean acidification is changing the marine environment, with potentially serious consequences for many organisms. Much of our understanding of ocean acidification effects comes from laboratory experiments, which demonstrate physiological responses over relatively short timescales1,2,3,4,5,6,7,8,9,10. Observational studies and, more recently, experimental studies in natural systems suggest that ocean acidification will alter the structure of seaweed communities11,12,13. Here, we provide a mechanistic understanding of altered competitive dynamics among a group of seaweeds, the crustose coralline algae (CCA). We compare CCA from historical experiments (1981–1997) with specimens from recent, identical experiments (2012) to describe morphological changes over this time period, which coincides with acidification of seawater in the Northeastern Pacific14,15,16. Traditionally thick species decreased in thickness by a factor of 2.0–2.3, but did not experience a change in internal skeletal metrics. In contrast, traditionally thin species remained approximately the same thickness but reduced their total carbonate tissue by making thinner inter-filament cell walls. These changes represent alternative mechanisms for the reduction of calcium carbonate production in CCA and suggest energetic trade-offs related to the cost of building and maintaining a calcium carbonate skeleton as pH declines. Our classification of stress response by morphological type may be generalizable to CCA at other sites, as well as to other calcifying organisms with species-specific differences in morphological types.

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Figure 1: Thallus thickness profiles.
Figure 2: Filament wall thicknesses.
Figure 3: Environmental context at Tatoosh Island.

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We would like to thank R. T. Paine for making his archival coralline algal specimens available for tissue analysis, Q. Guo for assistance on the SEM at the University of Chicago Materials Research Science and Engineering Center, F. B. McCoy for clerical assistance, and the Makah Tribe for access to Tatoosh Island and permission to conduct field research. R. T. Paine, C. A. Pfister, T. Price and J. T. Wootton provided helpful comments. S.J.M. would like to acknowledge research funding provided by the United States National Science Foundation Doctoral Dissertation Improvement Grant DEB-1110412 (C. A. Pfister and S.J.M.), the Achievement Rewards for College Scientists Foundation, and United States Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate Fellowship, 32 CFR 168a, and by the United States National Science Foundation Graduate Research Fellowship, Grant No. 1144082. The United States National Science Foundation OCE-09-28232 (C. A. Pfister) and DEB-09-19420 (J. T. Wootton) funded Tatoosh research. F.R. would like to acknowledge the support of the Leverhulme Trust, RJ5540.

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S.J.M. collected field specimens and designed the experiment. F.R. took SEM photos. S.J.M. and F.R. took measurements, analysed and discussed the data. S.J.M. wrote the manuscript with contributions from F.R.

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Correspondence to S. J. McCoy.

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McCoy, S., Ragazzola, F. Skeletal trade-offs in coralline algae in response to ocean acidification. Nature Clim Change 4, 719–723 (2014).

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