How does rising atmospheric CO2 affect marine organisms?

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Direct Biological Effects of Elevated CO2 on Coral Reefs
Langdon, C., Broecker, W.S., Hammond, D.E., Glenn, E., Fitzsimmons, K., Nelson, S.G., Peng, T.-S., Hajdas, I. and Bonani, G.  2003.  Effect of elevated CO2 on the community metabolism of an experimental coral reef.  Global Biogeochemical Cycles 17: 10.1029/2002GB001941.

The authors begin their paper by noting "several recent studies have shown that an increase in the partial pressure of CO2 in the atmosphere can have a negative effect on coral and reef community calcification as a result of a decrease in the aragonite saturation state," citing the studies of Gattuso et al. (1998), Kleypas et al. (1999), Langdon et al. (2000), Leclercq et al. (2000) and Marubini et al. (2001).  Specifically, they report that a doubling of the air's CO2 concentration has "resulted in an 11-40% decline in calcification of corals and coralline algae measured over time periods ranging from 3 hours to 2 years," concluding that this phenomenon could contribute to "a loss of coral coverage followed by loss of framework and habitat for fish and invertebrates."

We have discussed this concept at length and found it wanting in terms of real-world verification [see Coral Reefs (Calcification) in our Subject Index].  In their current paper, however, Langdon et al. take a somewhat different approach to the subject - one that we heartily endorse, in fact - noting that the studies they cited in support of CO2-induced decreases in coral calcification "did not consider the impact that rising CO2 might have on the rate of production of organic matter by macroalgae," which they note "are not conspicuous on healthy reefs, but due to various anthropogenic pressures ? are becoming increasingly abundant."  Hence, in their new study, they seek to determine if the ongoing rise in the air's CO2 content might give these coral competitors an unwanted advantage in the never-ending struggle for dominance on coral reefs, a concept that is not without experimental support, as they report that "laboratory studies have found that the photosynthesis of many macroalgae is limited by inorganic carbon supply in natural seawater [Brorwitzka and Larkum, 1976; Borowitzka, 1981; Surif and Raven, 1989; Maberly, 1990; Levavasseur et al., 1991; Gao et al., 1993a, 1993b]."

What was done
The authors manipulated the carbonate chemistry of the water in the coral reef mesocosm at the Biosphere-2 facility near Oracle, Arizona, USA, to simulate present-day and doubled atmospheric CO2 concentrations, while they studied gross primary production and calcification of the macrophyte-dominated ecosystem that has a coral cover of 3%.

What was learned
The authors report that "predictions that elevated CO2 would stimulate the primary production of macroalgae and possibly counteract a decrease in their calcification due to the declining saturation state of the water are not borne out by the results of this study."  Apparently, as they continue, "gross primary production and calcification in algal-dominated communities can become uncoupled."

What it means
In the words of the authors, "these results are good news for undisturbed coral reefs in as much as rising atmospheric CO2 will not give macroalgal growth a boost and hasten the transformation of reef community structure from coral to algal dominance."

Borowitzka, M.A.  1981.  Photosynthesis and calcification in the articulated coralline alga Amphiroa anceps and A. foliaceaeMarine Biology 62: 17-23.

Borowitzka, M.A. and Larkum, A.  1976.  Calcification in the green algal Halimeda, III, The sources of inorganic carbon for photosynthesis and calcification and a model of the mechanisms of calcification.  Journal of Experimental Botany 27: 879-893.

Gao, K., Aruga, Y., Asada, K., Ishihara, T., Akano, T. and Kiyohara, M.  1993a.  Calcification in the articulated coralline alga Corallina pilulifera, with special reference to the effect of elevated CO2 concentration.  Marine Biology 117: 129-132.

Gao, K., Aruga, Y., Asada, K. and Kiyohara, M.  1993b.  Influence of enhanced CO2 on growth and photosynthesis of the red algae Gracilaria sp. and G. chilensisJournal of Applied Phycology 5: 563-571.

Gattuso, J.-P., Frankignoulle, M., Bourge, I., Romaine, S. and Buddemeier, R.W.  1998.  Effect of calcium carbonate saturation of seawater on coral calcification.  Global and Planetary Change 18: 37-46.

Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J.-P., Langdon, C. and Opdyke, B.N.  1999.  Geochemical consequences of increased atmospheric CO2 on corals and coral reefs.  Science 284: 118-120.

Langdon, C., Takahashi, T., Marubini, F., Atkinson, M., Sweeney, C., Aceves, H., Barnett, H., Chipman, D. and Goddard, J.  2000.  Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef.  Global Biogeochemical Cycles 14: 639-654.

Leclercq, N., Gattuso, J.-P. and Jaubert, J.  2000.  CO2 partial pressure controls the calcification rate of a coral community.  Global Change Biology 6: 329-334.

Levavasseur, G., Edwards, G.E., Osmond, C.B. and Ramus, J.  1991.  Inorganic carbon limitation of photosynthesis in Ulva rotundata (Chlorophyta).  Journal of Phycology 27: 667-672.

Maberly S.C.  1990.  Exogenous sources of inorganic carbon for photosynthesis by marine macroalgae.  Journal of Phycology 26: 439-449.

Marubini, F., Barnett, H., Langdon, C. and Atkinson, M.J.  2001.  Dependence of calcification on light and carbonate ion concentration for the hermatypic coral Porites compressaMarine Ecology Progress Series 220: 153-162.

Surif, M.B. and Raven, J.A.  1989.  Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): Ecological and taxonomic implications.  Oecologia 78: 97-105.

Reviewed 8 October 2003