How does rising atmospheric CO2 affect marine organisms?

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Evolution (Aquatic Life) -- Summary
In a paper published in the Annual Review of Ecology, Evolution, and Systematics, Willis et al. (2006) report that "on tropical coral reefs, the simultaneous mass spawning of many species of stony corals represents a unique breeding strategy among animals and suggests that hybridization might have played a role in the evolution of this functionally important group," which they characterize as "the cornerstone of the coral reef ecosystem," and which they describe as being "increasingly threatened by human and climate-related impacts." They also note that "an upsurge of studies on the reproduction of scleractinian corals has shown that synchronized spawning among more than two species occurs in the majority of reef regions," and that "in highly synchronized events, up to 35 species in sympatry may spawn within two hours of each other."

In light of these several observations from the world of nature, we have to ask ourselves if it might be possible that the hybridization of corals may represent a viable strategy for their successfully adapting to periodic intervals of global warming and cooling. After analyzing reams of materials found in a host of peer-reviewed scientific papers reporting information pertinent to this vital question, the five biologists who reviewed the subject arrived at the following conclusions.

(1) "Hybridization has contributed to the evolution of many ecologically dominant and structurally important corals in diverse and significant ways." (2) "Greater growth and survival of juvenile hybrids in environmentally variable and extreme habitats suggest a role for hybrids in adaptation to new environments." (3) "The capacity of the hybrid Acropora prolifera to colonize marginal habitats distinct from its parent species and evidence of hybridization at geographical boundaries of the Caribbean Montastraea support an evolutionary role for hybridization in range expansion and adaptation to changing environments." (4) "Outcomes of hybridization are likely to be significant for the future resilience of reef corals ... by providing options for rapid response to changing environments and climatologies as well as increasing resilience to novel disease challenges." And (5) "Given the implications of ocean warming for increasing frequency and severity of mass bleaching events and emerging coral diseases, it is important to realize that hybridization has contributed to the evolutionary diversification of corals and that it has the potential to contribute significantly to the resilience of coral species and the coral reef ecosystem into the future."

In light of these several real-world-based and data-driven conclusions, it would appear that global warming -- even "unprecedented" global warming, as climate alarmists are fond of predicting -- need not spell the demise of earth's coral reef ecosystems. We must remember that the tenacious scleractinian corals have been around since the Triassic (Veron, 1995), or some 200 to 250 million years ago; and considering the findings of Willis et al., they will probably be around for a good long time to come.

In an earlier study that investigated the potential for evolutionary change in corals, Kumaraguru et al. (2003) assessed the degree of damage inflicted on a number of coral reefs within Palk Bay -- which is located on the southeast coast of India just north of the Gulf of Mannar -- by a major warming event that produced monthly mean sea surface temperatures of 29.8 to 32.1C from April through June of 2002, after which they assessed the degree of recovery of the reefs. This work revealed that "a minimum of at least 50% and a maximum of 60% bleaching were noticed among the six different sites monitored." However, as they continue, "the corals started to recover quickly in August 2002 and as much as 52% recovery could be noticed."

In comparison, the three researchers note that the "recovery of corals after the 1998 bleaching phenomenon in the Gulf of Mannar was very slow, taking as much as one year to achieve similar recovery," i.e., to achieve what was experienced in one month in 2002. Consequently, in words descriptive of the phenomenon known as symbiont shuffling -- whereby successive episodes of high-temperature-induced bleaching of corals lead to their being repopulated with species or subspecies of zooxanthellae that are ever more tolerant of high temperatures, which makes successive incarnations of coral reefs ever more resistant to new occurrences of warming-induced bleaching and more resilient in terms of their rate of recovery -- the Indian scientists say that "the process of natural selection is in operation, with the growth of new coral colonies [following coral bleaching], and any disturbance in the system is only temporary," which leads them to conclude in the final sentence of their paper that "the corals will resurge under the sea."

Most recently, and citing evidence that suggests that "populations may persist locally through phenotypic plasticity and microevolution of life history traits to deal with increasing temperature, which may buffer against changes in community structure," Van Doorslaer et al. (2007) declare that as far as they know, "there are no studies using experimental evolution experiments simulating global warming to rigorously evaluate the potential of populations to genetically adapt to global warming." Hence, they proceeded to fill this important gap, working with a zooplankter as opposed to the dinoflagellate algae that primarily comprise the zooxanthellae or intracellular endosymbionts that inhabit the soft tissues of corals.

As the four researchers describe it, they "combined the realism and rigid, replicated experimental design of a large-scale mesocosm study where populations of the zooplankter Simocephalus vetulus were exposed for 1 year to different global warming scenarios" -- (1) unheated control, (2) IPCC scenario A2, and (3) A2+50% downscaled to the regional level, where A2 refers to predicted temperatures for the period 2071-2100 -- "with a life table experiment under laboratory conditions at three temperatures [18, 22 and 26C] that eliminated confounding, nongenetic factors." In doing so, they say they "were able to demonstrate a rapid microevolutionary response (within 1 year) in survival, age at reproduction and offspring number to elevated temperatures in S. vetulus populations inoculated in large mesocosms," and they state that "these responses may allow the species to maintain itself under the forecasted global warming scenarios, as evidenced by the persistence of S. vetulus populations in the mesocosms under all three climate scenarios." In addition, they report observing an approximate "five times higher survival at 26C of clones exposed for 1 year to global warming scenario A2+50% compared with clones exposed to the unheated control temperature regime," which observation, in their words, "strongly indicates rapid microevolution of the ability to cope with higher temperatures."

In discussing their findings, the four researchers say their results "provide solid proof for a rapid microevolutionary response to global warming in both survival and the subcomponents of individual performance (age at reproduction and number of offspring), which may allow populations of S. vetulus to persist locally under predicted scenarios of global warming." In addition, they state that "such microevolutionary responses may buffer changes in community structure under global warming and help explain the outcome of previous mesocosm studies finding only marginal effects of global warming at the community level."

In light of the results of the three research papers reviewed above, it seems highly likely that many aquatic plants and animals may possess the ability to rapidly evolve in ways that should enable them to successfully cope with the degree of warming that is predicted to result from projected business-as-usual anthropogenic greenhouse-gas emissions. But what about potential effects of the ongoing rise in the air's carbon dioxide concentration?

Perhaps the biggest worry here is the negative influence this phenomenon has been claimed to exert on calcification; but as described in the many papers we have reviewed on the topic that are archived in our Subject Index under the two sub-headings of Corals and Other Marine Organisms, this theoretical climate-alarmist contention is not supported by real-world observations. In addition, many studies of both micro and macro marine and freshwater Aquatic Plants indicate that they typically become more productive when the air above the water in which they reside is enriched with CO2 to concentrations greater than that of today's atmosphere.

Interestingly, this response also appears to be subject to positive evolutionary change, as found by Collins et al. (2006), who propagated ten replicate lines from each of two clones of the freshwater microalga Chlamydomonas reinhardtii within a phytotron by batch-culturing them in flasks through which either air of 430 ppm CO2 was continuously bubbled or air of gradually increasing CO2 concentration was bubbled over the course of development of 600 generations of the microalga, at which point in time a concentration of 1050 ppm was reached and maintained throughout the development of 400 more algal generations. This work revealed a 50% increase in steady-state CO2 uptake rate in response to the ultimate 620-ppm increase in atmospheric CO2 concentration, which roughly equates to a 25% increase in growth for the 300-ppm increase in the air's CO2 concentration that is typically employed in the majority of CO2 enrichment studies of land plants.

If the results obtained by Collins et al. for the freshwater Chlamydomonas reinhardtii are typical of what to expect of marine microalgae -- which Field et al. (1998) suggest may provide nearly half of the primary production of the planet -- the totality of earth's plant life may well provide a significant brake upon the rate at which the air's CO2 content may increase in the future, as well as the ultimate level to which it may rise. A rough idea of just how powerful this phenomenon might prove to be is provided by Collins et al., when they note that mathematical simulations have indicated that pre-industrial levels of CO2 may well have been as high as 460 ppm without the help of the biological pump (Sarmiento and Toggweiler, 1984) provided by dying phytoplankton that carry carbon into deep ocean sediments, whereas actual pre-industrial atmospheric CO2 levels were around 280 ppm (Etheridge et al., 1996), or 180 ppm lower.

Altogether, it would appear that earth's aquatic life has the capacity to rapidly evolve the ability to: (1) overcome any negative influences global warming might have on them, (2) capitalize upon the productivity-enhancing effects of atmospheric CO2 enrichment, and thereby (3) significantly temper the rate and ultimate level of CO2-induced global warming.

References
Collins, S., Sultemeyer, D. and Bell, G. 2006. Changes in C uptake in populations of Chlamydomonas reinhardtii selected at high CO2. Plant, Cell and Environment 29: 1812-1819.

Etheridge, D.M., Steele, L.P., Langerfelds, R.L., Francey, R.J., Barnola, J.-M. and Morgan, V.I. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn CO2. Journal of Geophysical Research 101: 4115-4128.

Field, C.B., Behrenfeld, M.J., Randerson, J.T. and Falkowski, P. 1998. Primary production of the biosphere: integrating terrestrial and oceanic components. Science 281: 237-240.

Kumaraguru, A.K., Jayakumar, K. and Ramakritinan, C.M. 2003. Coral bleaching 2002 in the Palk Bay, southeast coast of India. Current Science 85: 1787-1793.

Sarmiento, J.L. and Toggweiler, J.R. 1984. A new model for the oceans in determining atmospheric pCO2. Nature 308: 621-624.

Van Doorslaer, W., Stoks, R., Jeppesen, E. and De Meester, L. 2007. Adaptive microevolutionary responses to simulated global warming in Simocephalus vetulus: a mesocosm study. Global Change Biology 13: 878-886.

Veron, J.E.N. 1995. Corals in Space and Time. Cornell University Press, Ithaca, New York, USA.

Willis, B.L., van Oppen, M.J.H., Miller, D.J., Vollmer, S.V. and Ayre, D.J. 2006. The role of hybridization in the evolution of reef corals. Annual Review of Ecology, Evolution, and Systematics 37: 489-517.

Last updated 2 January 2008