How does rising atmospheric CO2 affect marine organisms?

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Responses of Marine Benthic Microalgae to Declining pH Values
Johnson, V.R., Brownlee, C., Rickaby, R.E.M., Graziano, M., Milazzo, M. and Hall-Spencer, J.M. 2013. Responses of marine benthic microalgae to elevated CO2. Marine Biology 160: 1813-1824.

The authors write that "in response to low ambient CO2 concentrations, most marine microalgae have evolved a carbon concentrating mechanism (CCM) to elevate concentrations at the site of carbon fixation (Beardall and Giordano, 2002; Raven and Beardall, 2003; Raven et al., 2011)." And they note, in this regard, that since "increases in dissolved CO2 are predicted to cause down-regulation of microalgal CCM capacity (Giordano et al., 2005; Hopkinson et al., 2011)," which should reduce the energetic costs of CCMs (Raven, 1991), this phenomenon "will potentially allow more energy for other growth processes." Therefore, they also state that "as the carbon acquisition mechanisms and efficiencies of CCMs differ between algae, it is thought that rising CO2 will benefit different species to varying degrees (Hein and Sand-Jensen, 1997; Tortell et al., 2000; Rost et al., 2003; Beardall and Raven, 2004; Riebesell, 2004; Fu et al., 2008) and may result in dramatic community shifts with profound consequences for marine biogeochemistry (Hutchins et al., 2009)."

What was done
To further explore the potentialities of this situation, Johnson et al. compared periphyton assemblages on artificial substrata installed along a coastal CO2 gradient - which ranged from a median value of 419 to 592 to 1,611 ppm - at a shallow-water cold-vent system off the island of Vulcano, NE Sicily, with the aim of (1) testing the hypothesis that periphyton assemblages respond to CO2 gradients and (2) characterizing any changes in diatom and cyanobacteria populations to better understand the ecological effects of real-world ocean acidification.

What was learned
The six scientists report that periphyton communities were indeed "altered significantly as CO2 concentrations increased," and that "CO2 enrichment caused significant increases in chlorophyll a concentrations and in diatom abundance." Furthermore, "by using chl a as an index of the photosynthetic standing crop (Underwood, 1984)," they indicate that "periphyton biomass was found to increase substantially (fivefold) at the CO2-enriched stations," indicative of the fact that "elevations in CO2 stimulate primary productivity in these benthic assemblages."

What it means
Johnson et al. conclude that their findings are "likely to have wide-ranging consequences from local-scale influences on the structure of overlying benthic communities to effects on food web structure and larger-scale biogeochemical cycles." And all of these ecosystem-scale effects appear to be positive.

Beardall, J. and Giordano, M. 2002. Ecological implications of microalgal and cyanobacterial CO2 concentrating mechanisms and their regulation. Functional Plant Biology 29: 335-347.

Beardall, J. and Raven, J.A. 2004. The potential effects of global climate change in microalgal photosynthesis, growth and ecology. Phycologia 43: 31-45.

Fu, F.-X., Zhang, Y., Warner, M.E., Feng, Y., Sun, J. and Hutchins, D.A. 2008. A comparison of future increased CO2 and temperature effects on sympatric Heterosigma akashiwo and Prorocentrum minimum. Harmful Algae 7: 76-90.

Giordano, M., Beardall, J. and Raven, J.A. 2005. CO2 concentrating mechanisms in algae: mechanism, environmental modulation, and evolution. Annual Review of Plant Biology 56: 99-131.

Hein, M. and Sand-Jensen, K. 1997. CO2 increases oceanic primary production. Nature 388: 526-527.

Hopkinson, B.M., Dupont, C.L., Allen, A.E. and Morel, F.M.M. 2011. Efficiency of the CO2-concentrating mechanism of diatoms. Proceedings of the National Academy of Sciences USA 108: 3830-3837.

Hutchins, D.A., Mulholland, M.R. and Fu, F.-X. 2009. Nutrient cycles and marine microbes in a CO2-enriched ocean. Oceanography 22: 128-145.

Raven, J.A. 1991. Physiology of inorganic C acquisition and implications for resource use efficiency by marine phytoplankton: Relation to increased CO2 and temperature. Plant, Cell and Environment 14: 779-794.

Raven, J.A. and Beardall, J. 2003. CO2 acquisition mechanisms in algae: carbon dioxide diffusion and carbon dioxide concentrating mechanisms. In: Larkum, A.W.W., Raven, J.A. and Douglas, S. (Eds.). Advances in Photosynthesis and Respiration.

Raven, J.A., Giordano, M., Beardall, J. and Maberly, S. 2011. Algal and aquatic plant carbon concentrating mechanisms in relation to environmental change. Photosynthesis Research 109: 281-296.

Riebesell, U. 2004. Effects of CO2 enrichment on marine phytoplankton. Journal of Oceanography 60: 281-296.

Rost, B., Riebesell, U. and Burkhardt, S. 2003. Carbon acquisition of bloom-forming marine phytoplankton. Limnology and Oceanography 48: 55-67.

Tortell, P.D., Rau, G.H. and Morel, F.M.M. 2000. Inorganic carbon acquisition in coastal Pacific phytoplankton communities. Limnology and Oceanography 45: 1485-1500.

Underwood, A.J. 1984. The vertical distribution and seasonal abundance of intertidal microalgae on a rocky shore in New South Wales. Journal of Experimental and Marine Biology and Ecology 78: 199-220.

Reviewed 20 November 2013