How does rising atmospheric CO2 affect marine organisms?

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The Capacity of a Damselfish to Adapt to Ocean Acidification

Paper Reviewed
Schunter, C., Welch, M.J., Nilsson, G.E., Rummer, J.L., Munday, P.L. and Ravasi, T. 2018. An interplay between plasticity and parental phenotype determines impacts of ocean acidification on a reef fish. Nature Ecology & Evolution 2: 334-342.

In providing the background for their study, Schunter et al. (2018) say that "most observations regarding impacts of ocean acidification come from short-term experiments that do not account for population heterogeneity and individual variation in tolerance potentially important to adaptive processes," adding that "acutely exposing animals to elevated CO2 for days to weeks cannot predict the potential for long-term acclimation and adaptation." Indeed, they correctly note that "long-term developmental studies and multigenerational experiments that incorporate individual variation in tolerance are needed to better understand and predict the effects of elevated CO2 on populations and their capacity to adapt." And so it was that the team of six researchers set out to conduct just such an experiment on the spiny damselfish (Acanthochromis polyacanthus).

To accomplish their objective, Schunter et al. first exposed adult damselfish to a projected future CO2 level of 750 ľatm, subjecting them to an acute 7-day exposure period of ocean acidification. At the end of the 7 days, the fish were examined for their behavioral response to a chemical alarm cue. Based on their response, the adult fish were matched into either behaviorally tolerant or sensitive breeding pairs and kept in seawater of either current or elevated CO2 levels for three months until the breeding season.

Following successful fertilization and hatching, offspring of the tolerant and sensitive breeding pairs were immediately placed into separate tanks from their parents, where they were reared under control or elevated CO2 conditions for five months. The experimental design produced four treatments, including "(1) control conditions, (2) acute elevated CO2 treatment, in which offspring developed in control conditions but were acutely exposed to elevated CO2 for the last 4 days [of the experiment], (3) developmental elevated CO2 treatment, in which [control] offspring were immediately placed into elevated CO2 after hatching, and (4) transgenerational elevated CO2 treatment where parents and offspring were exposed to elevated CO2." In an effort to "tease apart the acute response to elevated CO2 from the responses to longer-term development under elevated CO2 and differences that occur due to parental exposure to elevated CO2," Schunter et al. examined differences in the genome-wide gene expression in the brains of the fish in each treatment at the end of their five-month exposure period.

In discussing their findings, the authors report there were significant differences in the brain transcriptome among offspring of behaviorally tolerant and sensitive parents, revealing "a clear influence of the parental phenotype of the offspring's response to elevated CO2." However, they also report that "the acute and developmental CO2 treatments had larger overall effects on the transcriptome than did the parental phenotype." And in this regard, Schunter et al. note that the four-day acute elevated CO2 treatment resulted in the largest treatment-specific response of differentially expressed genes (i.e., this was the treatment most severely affected by elevated CO2).

A less-severe response to ocean acidification was observed for fish reared from hatching under elevated CO2 in the developmental treatment, where "fewer treatment-specific [gene] responses were observed." And in the transgenerational treatment, things got even better, with the authors reporting that the transcriptional changes they observed in the acute and developmental CO2 treatments "returned to baseline levels in fish that were transgenerationally exposed to elevated CO2 levels."

In commenting on their several findings, Schunter et al. say their work "emphasizes the influence of environmental exposure of the parents as well as the parental phenotype in the response of fish to future ocean acidification," adding that "both parental variation in tolerance and cross-generation exposure to elevated CO2 are crucial factors in determining the response of reef fish to changing ocean chemistry."

We find such findings to be both revealing and encouraging.

Consider, for example, the fact that the vast majority of all ocean acidification studies conducted to date have employed conditions similar to the acute treatment of this study, where the responses of marine organisms to elevated CO2 are examined following a relatively short exposure period. Such experimental conditions do not allow for adaptation and thus exaggerate and skew the potential impacts of ocean acidification toward the negative. However, when scientists do allow for adaptation and incorporate real-world examples of such into their ocean acidification studies (e.g., developmental plasticity and/or transgenerational acclimation), the negative outcomes observed under acute treatments are lessened or absent, as observed in the present study. Recognizing these facts, and the much longer period of time that species will have to adapt (and even evolve) to rising CO2 levels in the future, it should be abundantly clear that the presently-espoused alarmist view of ocean acidification harming marine life and driving species toward extinction is premature and unrealistically negative. And it will remain in this sorry state until there is an adequate amount of research conducted to incorporate the adaptive and evolutionary responses of marine organisms to elevated CO2 across multiple generations.

Posted 5 July 2018