Can sealife cope with increasing ocean acidification?

CARBON DIOXIDE IN THE OCEAN

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Projected carbon dioxide emissions leading to stabilization of atmospheric carbon dioxide concentrations at different levels from Knutti et al. 2005.  Graphic: IPCC 2007

Carbon dioxide is among the greenhouse gasses released by many human activities, including driving cars, heating and cooling homes, and producing some types of materials in factories, among others. Each year, about 36 billion metric tons of carbon dioxide are released into the atmosphere (IPCC 2007a). Within the last 200 years, the ocean has absorbed approximately 30% of the carbon dioxide emissions produced by humans (Feely et al., 2004; Sabine et al. 2004). As more and more carbon dioxide accumulates in the ocean, the water is decreasing pH and carbonate ion concentration. Currently, the ocean has about 387 parts per million (ppm) of carbon dioxide and a pH of about 8.1. The Intergovernmental Panel on Climate Change (IPCC) predicted an increase in carbon dioxide by the year 2100 due to continued human activities that release greenhouse gasses (IPCC 2007b). The IPCC predicted a moderate increase in carbon dioxide in the ocean to 540 ppm and a pH of about 7.96, assuming some changes to human activities to reduce production of greenhouse gasses. The IPCC also considered the potential increase in carbon dioxide concentration, assuming “business as usual,” leading to 1020 ppm carbon dioxide concentration in the ocean and a pH of 7.88. These predicted changes in pH have important ecological consequences for marine ecosystems.

Elevated carbon dioxide in the ocean is known to affect the ability of marine animals, such as corals, clams, snails, abalones, sea urchins, sea stars, and other calcifying organisms, to make their shells (Fabry et al. 2008). The vulnerability of particular species depends on the animal’s ability to cope with the changes in the pH of ocean water. Even if an animal can compensate for increased acidification, there may be a physiological cost, which may jeopardize the animal’s survival or reproduction. Scientific research already has demonstrated that increased ocean acidification can affect calcification, growth, metabolism, reproduction, development, survival and photosynthesis of some animals and plants. PISCO scientist Dr. Gretchen Hofmann and her research team at the University of California, Santa Barbara are studying how ocean acidification affects the physiology of marine animals.

PISCO SCIENTISTS INVESTIGATE IMPACTS OF OCEAN ACIDIFICATION

Together, PISCO scientists, Drs. Anne Todgham of San Francisco State and Gretchen Hofmann of the University of California, Santa Barbara, studied how moderate and large changes in carbon dioxide concentration in the ocean affect the physiology of purple sea urchin (Strongylocentrotus purpuratus) larvae (Todgham and Hofmann 2009). Using a new technology, DNA oligonucleotide microarrays to profile gene expression, the scientists investigated how the urchin larvae might react to future increases in ocean acidification. The scientists selected purple sea urchins for this experiment because urchins are ecologically critical species in coastal rocky intertidal environments. In addition, what is known about urchin biology readily supported the exploration of how ocean acidification would influence the urchins. Namely, sea urchins and other echinoderms can be quite plastic (or flexible) in their development under different environmental conditions, they make a calcareous shell (which likely would be affected by reduced pH of ocean water) and their genome has been sequenced and documented


Adult  purple sea urchins. Photo: Gretchen Hofmann

The scientists gathered adult urchins from the around the Goleta Pier (Goleta, CA) using SCUBA gear. Eggs were collected from female urchins and spawning was induced in male urchins so the scientists could fertilize the eggs and produce larvae. Some larvae were raised in buckets of aerated seawater at current levels of carbon dioxide (380 ppm) and a pH of about 8.01, while others were raised in aerated buckets of seawater with moderately increased carbon dioxide (540 ppm) and a pH of 7.96. A third group of larvae were raised in ocean water with high concentrations of carbon dioxide (1020 ppm) and a pH of 7.88, which was the “business as usual” scenario predicted for 2100 by the IPCC. Development of the urchin larvae was assessed at 29 hours, 40 hours, and 70 hours after fertilization. RNA was extracted from samples of each of the three treatments. The scientists made oligonucleotide microarrays to track activity of different genes involved in calcification, cellular stress, development, metabolism, and other physiological processes.

ocean acidification causes decreased gene function

At moderate levels of carbon dioxide (540 ppm), sea urchin larvae exhibited statistically significant changes in about 8% of the genes, particularly those involved with biomineralization, cellular stress and metabolism. At high carbon dioxide (1020 ppm), about 17% of the genes showed decreased response, particularly for the process of cleaning out dead cell tissues (apoptosis), cellular stress responses, and metabolism. The experiment showed that urchin larvae have decreased capacity to transport ions within and across the cell walls under conditions of increased ocean acidification. The scientists also found that the genes required for skeletogensis (or creation of the calcareous exoskeleton) and the calcification process decreased for urchins raised at moderate and high carbon dioxide levels.

ocean acidification reduces cellular stress responses

Urchin larvae that developed under both moderate and high levels of carbon dioxide had significantly lower production of genes involved in cellular stress response, particularly genes used for maintaining protein integrity and defending against oxidative stress and toxins. The reduction in ability of the urchin larvae to adjust to these stressors makes them vulnerable to other stressors associated with ocean change, which are, not only increased ocean acidification but, also increased temperature (of 2-4oC) and increased incidence of low-oxygen (or “hypoxic”) zones. When the larvae already are compromised because of their exposure to low pH, they may not be able to grow and reproduce normally in the face of all three stressors combined (acidification, increased temperature and low oxygen).
 

ocean acidification supresses metabolism

Suppression of metabolism (or energy production) is an adaptive strategy to survive during harsh environmental conditions. Urchin larvae exposed to moderate and high levels of carbon dioxide had lower production of metabolic genes (those involved with production of ATP). Also, under the conditions of increased ocean acidification, the urchin larvae reduced costly physiological processes, such as protein synthesis. Decreased expression of genes that regulate energy supply indicates that larvae exposed to higher carbon dioxide are likely suppressing their metabolism. Previous studies showed suppression of metabolism with exposure to very high levels of carbon dioxide (5,000 – 10,000 ppm) but this study shows that the same physiological suppression occurs with exposure to moderate levels of carbon dioxide (540 ppm), which is predicted to be inevitable in the next 90 years. Animals may be able to suppress their energy production in the short-term to withstand harsh conditions, but long-term exposure to those conditions is likely to lead to decreased growth and potential disruption of development.

Embryonic development in purple sea urchin larvae progressing from the 2-cell, 4-cell, unhatched blastula, gastrula, prism, and early pluteus stages (left to right). Photos: Tim Crombie

ocean acidification decreases cellular renewal

Another consequence of exposure to high levels of carbon dioxide (1020 ppm) is sea urchin larvae decrease production of genes that control cell death and clean out dead cells. Reduced capacity to regulate cell death could have consequences for development of new cell structures and tissues. For example, larvae exposed to high levels of carbon dioxide may not be able to remodel their cells appropriately during metamorphosis from a bilaterally symmetrical larva to a radially symmetrical juvenile.

summary


Recently metamorphosed purple sea urchin. Photo: Gerardo Amador

In this experiment, Dr. Hofmann and her team determined that, after only 40 hours of exposure to moderate and high levels of carbon dioxide, purple sea urchin larvae suffered negative effects on development, growth, energy production, and stress tolerance. The typical larval developmental period for sea urchins is a month or more, which means, if ocean acidification increases as predicted, exposure of these animals to moderate or high carbon dioxide concentrations would be prolonged, potentially leading to additional negative consequences for these physiological processes. Combined with the effects of ocean warming and reduced oxygen, ocean acidification likely will have detrimental effects on sealife, especially those shell-forming animals, including urchins, clams, mussels, snails, abalone and corals. In the future, Dr. Hofmann and her team will continue to investigate the costs and consequences for sealife of inhabiting in a changing ocean.

references

  • Fabry, V.J., B.A. Seibel, R.A. Feely, and J.C. Orr. 2008. Impacts of ocean acidification on marine fauna and ecosystem processes. ICES Journal of Marine Science 65: 414-432.

  •  Feely, R.A., C.L. Sabine, K. Lee, W. Berelson, J. Kleypas, V.J. Fabry, and F.J. Millero. 2004. Impact of anthropogenic CO2 on the CaCO3 system in the oceans. Science, 305(5682): 362–366.

  • IPCC. 2007a. Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.)]. IPCC, Geneva, Switzerland, 104 pp. 

  • Sabine, C.L., R.L. Feely, N. Gruber, R.M. Key, K. Lee, J.L. Bullister, R. Wanninkhof, C.S. Wong, D.W.R. Wallace, and B. Tibrook, et al. 2004. The oceanic sink for anthropogenic CO2. Science 305: 367-371.

  • Todgham, A.E., and G.E. Hofmann. 2009. Transcriptomic response of sea urchin larvae Strongylocentrotus purpuratus to CO2-driven seawater acidification. The Journal of Experimental Biology 212: 2579-2594.    

figure reference

 

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