How does rising atmospheric CO2 affect marine organisms?

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Biological Effects of "Ocean Acidification"
Volume 12, Number 5: 4 February 2009

As the atmosphere's CO2 content continues to rise, the pH of the world's oceans is expected to decline, driving a phenomenon described by climate alarmists as ocean acidification, to which they are already ascribing a host of imminent catastrophic consequences. Writing in the introduction to a special "theme section" of the journal Marine Ecology Progress Series, however, Vézina and Hoegh-Guldberg (2008) state that "without an understanding of how such a slow and continuous decline in pH is likely to affect ocean ecosystems, we may miss important aspects of this global ocean pH change," and that "to compound this uncertainty, recent research reveals counter-intuitive, positive/neutral effects of acidification on some organisms and processes." Hence, they "felt that it was worthwhile at this early stage to assemble articles that critically evaluate the current state of knowledge on this topic," and we felt it equally worthwhile to here review the major findings of those articles.

The first of the primary papers in the theme issue is by Pörtner (2008), who, as he describes it, "presents a set of hypotheses for a comprehensive mechanistic framework which brings the individual effects of the factors temperature, CO2 and hypoxia [a deficiency in the amount of oxygen reaching body tissues] together into an integrative picture of climate sensitivity at [the] organismal level." Hence, it is more of a how-to-proceed roadmap than a what-has-been-observed paper.

The second of the papers -- by Hofmann et al. (2008) -- is also of this nature, as it explores, in the words of its authors, "how the use of genomics-based tools that measure gene expression - DNA microarrays and quantitative PCR - can assist in this effort and reveal aspects of how calcifying marine organisms will respond to ocean acidification," while helping determine "whether organisms have sufficient physiological plasticity to adapt to the altered CO2 conditions." Its virtue, therefore, also lies in outlining a way to the future.

The next paper (Rost et al., 2008) assesses what its authors describe as "the possible responses [our italics] of different phytoplankton groups with regard to the expected [our italics] physico-chemical changes" that may result from ocean acidification, based on "current understanding of the underlying mechanisms that cause processes such as photosynthesis, calcification, and nitrogen fixation to be sensitive to ocean acidification." Hence, it too is more of a theoretical discussion of how the marine biosphere might operate in a high-CO2 world of the future, rather than an observation-derived view of how it may function.

Balch and Fabry (2008) "evaluate several approaches to discern the impact of ocean acidification on calcifying plankton, over basin scales," focusing "on estimates of the standing stock of particulate inorganic carbon (PIC) associated with calcifying plankton since it is thought that these organisms will be the most sensitive to ocean acidification." Thus, its intent is also to provide a guide to future research.

Atkinson and Cuete (2008) present "a short review of recent literature on how ocean acidification may influence [our italics] coral reef organisms and coral reef communities," concluding that "it is unclear [our italics] as to how, and to what extent, ocean acidification will influence calcium carbonate calcification and dissolution, and affect changes in community structure of present-day coral reefs," noting that "it is critical to evaluate the extent to which the metabolism of present-day reefs is influenced by mineral saturation states, and to determine a threshold saturation state at which coral communities cease to function as reefs," which things have also obviously not yet been done.

Some new and updated real-world assessments of coral growth are finally provided by Lough (2008), who reports that "average linear extension and calcification rates in Indo-Pacific Porites are linearly [and positively] related to average water temperatures through 23 to 30°C," based on data obtained from 49 different reefs. She also reports, however, that "coral growth characteristics at 2 of 3 reefs in the central Great Barrier Reef provide evidence of a recent decline," but she adds that "the exact causes of these declines cannot be identified at present nor can they, at present, be directly related to lower aragonite saturation state."

Noting that "the fates of tropical coral reefs and scleractinian corals have received most of the attention in the ongoing ocean acidification debate," Andersson et al. (2008) write that "much less attention has been given to marine calcifiers depositing calcium carbonate minerals containing significant proportions of magnesium ions ... and calcifying organisms living in high latitude and/or cold-water environments." Here again, however, they merely speculate on what will happen to these systems in a high-CO2 world of the future.

Kurihara (2008) focuses on "the effects of ocean acidification on early developmental and reproductive stages of calcifiers, both of which are believed to be the most vulnerable stages to environmental change within a life cycle." In doing so, he notes that certain laboratory experiments suggest that "ocean acidification has negative impacts on the fertilization, cleavage, larva, settlement and reproductive stages of several marine calcifiers, including echinoderm, bivalve, coral and crustacean species," and he concludes that "future changes in ocean acidity will potentially impact the population size and dynamics, as well as the community structure of calcifiers, and will therefore have negative impacts on marine ecosystems."

It should be pointed out, however, that most of the studies Kurihara cites did not observe statistically-significant negative effects of atmospheric CO2 enrichment until very large increases in the air's CO2 content were employed. In studies of sea urchins, for example, statistically-significant reductions in egg fertilization rates did not occur in Echinometra mathaei until the atmospheric CO2 concentration was raised a full 5,000 ppm above that of the ambient air; and in Hemicentrotus pulcherrimus, even a 10,000 ppm increase in the air's CO2 concentration was insufficient to elicit a statistically-significant decline in egg fertilization rate. In addition, Kurihara himself suggests that the great degree of scatter in the data may reflect "a degree of genetic variation for CO2 tolerance within populations," which may allow the species to readily adapt to a long-term upward trend in the air's CO2 content. And in the conclusion to his review, he acknowledges that "recent research has revealed that organisms could evolve within decades in response to strong pressures, which Stockwell et al. (2003) termed 'contemporary evolution'," citing the work of Collins and Bell (2004), to which we would add the studies of Collins and Bell (2006) and Collins et al. (2006).

Dupont et al. (2008) describe an actual experiment of theirs, where they placed cleaving embryos (two-cell stage) of the brittlestar Ophiothrix fragilis within three replicated sets of five-liter aquaria filled with filtered seawater of either control/natural pH of 8.1 or reduced pH of 7.9 or 7.7, which latter values correspond to water in equilibrium with air enriched to 500 and 1000 ppm CO2 above ambient air, respectively, as per the studies described by Kurihara. Under these conditions, they report that "after only eight days, all larvae at reduced pH (7.9 and 7.7) were dead, whereas control larvae (pH 8.1) showed only 30% mortality."

Although these results are about as negative as they could be, the five researchers note the possibility that their treatments "may have elevated the sensitivity of larvae (due to stress, suboptimal feeds, laboratory conditions, etc.)," and they too acknowledge that "if the pH continues to decrease as suggested by current models," we can expect "a strong selection for ... more tolerant species," and, we would add, more tolerant populations. Hence, we fully agree with them that it is vital to investigate, as they write in the final sentence of their paper, "effects of acidification on all aspects of the life cycle, and over several generations, to assess acclimation, adaptive potential and adaptation of key species," for we could well find that a little environmental-induced "fine-tuning" of the brittlestar's genetic makeup may have the capacity to overcome what currently appears to be a close-to-insurmountable problem.

Much the same thing is suggested by Ishimatsu et al. (2008), who write that "fish have been shown to maintain their oxygen consumption under elevated pCO2 conditions, in contrast to declines seen in several marine invertebrates," but that "impacts of prolonged CO2 exposure on reproduction, early development, growth, and behaviour of marine fish are important areas that need urgent investigation."

Ending on an extremely positive note is the final paper by Gutowska et al. (2008), who studied the cephalopod mollusc Sepia officinalis and found that it "is capable of not only maintaining calcification, but also growth rates and metabolism when exposed to elevated partial pressures of carbon dioxide." Over a six-week test period, for example, they found that "juvenile S. officinalis maintained calcification under ~4000 and ~6000 ppm CO2, and grew at the same rate with the same gross growth efficiency as did control animals," gaining approximately 4% body mass daily and increasing the mass of their calcified cuttlebone by over 500%. These findings thus led them to specifically conclude that "active cephalopods possess a certain level of pre-adaptation to long-term increments in carbon dioxide levels," and to generally conclude that our "understanding of the mechanistic processes that limit calcification must improve before we can begin to predict what effects future ocean acidification will have on calcifying marine invertebrates."

We agree. There have been more than enough speculative predictions of catastrophic negative impacts due to the ongoing rise in the air's CO2 content with regard to the ability of earth's oceans to sustain their many different lifeforms, as well as impassioned calls for immediate actions to reduce anthropogenic CO2 emissions, when for all we currently know, elevated atmospheric CO2 concentrations could well prove to be a net benefit to the marine biosphere, just as they are a huge blessing to earth's terrestrial lifeforms.

Sherwood, Keith and Craig Idso

Andersson, A.J., Mackenzie, .FT. and Bates, N.R. 2008. Life on the margin: implications of ocean acidification on Mg-calcite, high latitude and cold-water marine calcifiers. Marine Ecology Progress Series 373: 265-273.

Atkinson, M.J. and Cuet, P. 2008. Possible effects of ocean acidification on coral reef biogeochemistry: topics for research. Marine Ecology Progress Series 373: 249-256.

Balch, W.M. and Fabry, V.J. 2008. Ocean acidification: documenting its impact on calcifying phytoplankton at basin scales. Marine Ecology Progress Series 373: 239-247.

Collins, S. and Bell, G. 2004. Phenotypic consequences of 1,000 generations of selection at elevated CO2 in a green alga. Nature 431:566-569.

Collins, S. and Bell, G. 2006. Evolution of natural algal populations at elevated CO2. Ecology Letters 9: 129-135.

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.

Dupont, S., Havenhand, J., Thorndyke, W., Peck, L. and Thorndyke, M. 2008. Near-future level of CO2-driven ocean acidification radically affects larval survival and development in the brittlestar Ophiothrix fragilis. Marine Ecology Progress Series 373: 285-294.

Gutowska, M.A., Pörtner, H.O. and Melzner, F. 2008. Growth and calcification in the cephalopod Sepia officinalis under elevated seawater pCO2. Marine Ecology Progress Series 373: 303-309.

Hofmann, G.E., O'Donnell, M.J. and Todgham, A.E. 2008. Using functional genomics to explore the effects of ocean acidification on calcifying marine organisms. Marine Ecology Progress Series 373: 219-225.

Ishimatsu, A., Hayashi, M. and Kikkawa, T. 2008. Fishes in high-CO2, acidified oceans. Marine Ecology Progress Series 373: 295-302.

Kurihara, H. 2008. Effects of CO2-driven ocean acidification on the early developmental stages of invertebrates. Marine Ecology Progress Series 373: 275-284.

Lough, J.M. 2008. Coral calcification from skeletal records revisited. Marine Ecology Progress Series 373: 257-264.

Pörtner, H.O. 2008. Ecosystem effects of ocean acidification in times of ocean warming: a physiologist's view. Marine Ecology Progress Series 373: 203-217.

Rost, B., Zondervan, I. and Wolf-Gladrow, D. 2008. Sensitivity of phytoplankton to future changes in ocean carbonate chemistry: current knowledge, contradictions and research directions. Marine Ecology Progress Series 373: 227-337.

Vézina, A.F. and Hoegh-Guldberg, O. 2008. Effects of ocean acidification on marine ecosystems. Marine Ecology Progress Series 373: 199-201.