Volume 15, Number 5: 1 February 2012
Many are the studies that claim that increasing atmospheric CO2 concentrations will lead to a condition described as ocean acidification, where the pH of seawater declines and it becomes ever more difficult for calcifying marine organisms to produce skeletal structures. However, in a culture experiment with two algal symbiont-bearing, reef-dwelling foraminifers (Amphisorus kudakajimensis and Calcarina gaudichaudii), which was conducted in seawater under five different pCO2 conditions - 245, 375, 588, 763 and 907 ppm, maintained with a precise pCO2-controlling technique - Hikami et al. (2011) found that net calcification of A. kudakajimensis was indeed reduced under higher pCO2, but that calcification of C. gaudichaudii did just the opposite and actually increased with increased pCO2.
This latter result, although seemingly strange, is anything but unusual; for the nine researchers report that various taxa of coccolithophores and sea urchins "show enhanced calcification in environments with higher pCO2," citing the work of Iglesias-Rodriguez et al. (2008), Doney et al. (2009) and Ries et al. (2009). And they say that "different populations of Emiliania huxleyi have shown decreased, increased, or unchanged calcification in response to higher pCO2," citing Fabry (2008).
In discussing the findings of their experiment, Hikami et al. say that the upward trend in the calcification of C. gaudichaudii in response to ocean acidification "can probably be attributed to the increase in CO2, possibly through enhancement of symbiont photosynthesis, a phenomenon known as the CO2-fertilizing effect," citing Ries et al. (2009), although the concept was first described several years earlier by Idso et al. (2000). And in discussing possible causes of the two contrasting types of calcification response to atmospheric CO2 enrichment (positive and negative), they speculate that "the type of symbiont influences the strength of the CO2-fertilizing effect."
Hikami et al. note, for example, that "Calcarina hosts diatoms as its symbiotic algae, whereas Amphisorus hosts dinoflagellates," and they state that "both a single-species culture experiment (Wu et al., 2010) and a mesocosm bloom experiment (Engel et al., 2008) have shown that high-CO2 seawater is favorable to diatom growth," and that "Badger et al. (1998) pointed out that a rise in CO2 may lead to enhanced phytoplankton growth owing to the low affinity of the carboxylating enzyme (Rubisco) for CO2." Hence, they suggest "it is possible that Calcarina acquires an increased amount of energy from its symbiotic diatoms under high pCO2 conditions, leading to enhanced calcification," while noting Rost et al. (2006) report that "dinoflagellates use HCO3- as their carbon source, so their rate of carbon fixation may remain unaffected by fluctuating CO2 levels."
In concluding the report of their study, Hikami et al. speculate that the vastly different impacts of seawater chemistry that they observed "may be attributable to the different types of symbiotic algae hosted by Amphisorus and Calcarina." And we further speculate that the well-known phenomenon of symbiont shuffling may therefore enable many other calcifying organisms, which currently are negatively influenced by ocean acidification, to ultimately move into the positively-influenced category. See Coral Reefs (Bleaching - Responses: Symbiont Shuffling) in our Subject Index for more information on this intriguing subject.
Sherwood, Keith and Craig Idso
References
Badger, M.R., Andrews, T.J., Whitney, S.M., Ludwig, M., Yellowlees, D.C., Leggat, W. and Price, G.D. 1998. The diversity and coevolution of Rubisco, plastids, pyrenoids, and chloroplast-based CO2-concentrating mechanisms in algae. Canadian Journal of Botany 76: 1052-1071.
Doney, S.C., Fabry, V.J., Feely, R.A. and Kleypas, J.A. 2009. Ocean acidification: The other CO2 problem. Annual Review of Marine Science 1: 169-192.
Engel, A., Schulz, K.G., Riebesell, U., Bellerby, R., Delille, B. and Schartau, M. 2008. Effects of CO2 on particle size distribution and phytoplankton abundance during a mesocosm bloom experiment (PcECE II). Biogeosciences 5: 509-521.
Fabry, V.J. 2008. Marine calcifiers in a high-CO2 ocean. Science 320: 1020-1022.
Hikami, M., Ushie, H., Irie, T., Fujita, K., Kuroyanagi, A., Sakai, K., Nojiri, Y., Suzuki, A. and Kawahata, H. 2011. Contrasting calcification responses to ocean acidification between two reef foraminifers harboring different algal symbionts. Geophysical Research Letters 38: 10.1029/2011GL048501.
Idso, S.B., Idso, C.D. and Idso, K.E. 2000. CO2, global warming and coral reefs: Prospects for the future. Technology 7S: 71-94.
Iglesias-Rodriguez, M.D., Halloran, P.R., Rickaby, R.E.M., Hall, I.R., Colmenero-Hidalgo, E., Gittins, J.R., Green, D.R.H., Tyrrell, T., Gibbs, S.J., von Dassow, P., Rehm, E., Armbrust, E.V. and Boessenkool, K.P. 2008. Phytoplankton calcification in a high-CO2 world. Science 320: 336-340.
Ries, J.B., Cohen, A.L. and McCorkle, D.C. 2009. Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. Geology 37: 1131-1134.
Rost, B., Richter, K.-U., Riebesell, U. and Hansen, P.J. 2006. Inorganic carbon acquisition in red-tide dinoflagellates. Plant, Cell and Environment 29: 810-822.
Wu, Y., Gao, K. and Riebesell, U. 2010. CO2-induced seawater acidification affects physiological performance of the marine diatom Phaeodactylum tricornutum. Biogeosciences 7: 2915-2923.