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Effect of Ocean Acidification ‘turned on its head’ – Science 17 Apr 08

Ocean acidification doesn’t necessarily mean less calcification, or at least for coccolithophorids, according to a new study by Iglesias-Rodriguez et al. (Science, 17 April 2008).

As atmospheric concentrations of carbon dioxide rise due to human activities, the oceans will continue to become more acidic as they absorb the extra gas, but the effects on calcifying phytoplankton may not be simple to predict. In fact, over the industrial age, while CO2 emissions are steadily rising, coccolithophorids seemed to have increased their calcification. Specially for EUROCEANS’ Research Highlights, Scott Doney offers a reaction on the recent article:

‘The Iglesias-Rodriguez et al. paper puts a new biological twist on the rapidly emerging issue of ocean acidification. Previous laboratory studies found that the resulting acidification hinders the formation of calcium carbonate shells in a wide host marine organisms, including the abundant photosynthetic plankton group of coccolithophores. Iglesias-Rodriguez et al. turn this paradigm on its head; in their culture experiments, coccolithophore cells grown under high CO2 conditions grew larger, taking up both more organic carbon and inorganic carbon. In addition, they observed an increase in the size of the individual calcium carbonate plates or coccoliths surrounding the cells.  

Examining a high-resolution sediment core from the North Atlantic, the authors found apparent confirmatory evidence in the historical record, a sharp and distinct increase in coccolith size after about 1960 that follows the rise in atmospheric CO2.’

Why the U-turn?

‘There are a number of potential causes for the disagreement with earlier studies that found decreasing calcification with acidification including differences in analytical methods (whether one changes just pH or pH and CO2 simultaneously) and the possibility that biological responses to acidification may vary across species and growth conditions. The new results here echo other recently reported studies showing elevated photosynthesis and nitrogen fixation rates by non-calcifying planktonic cyanobacteria.’- says Scott Doney.

The article’s lead author, Debora Iglesias-Rodriguez from National Oceanography Centre in Southampton, explains: ‘Our experimental design uses air containing elevated CO2 partial pressures which increase CO2 (the source of carbon for photosynthesis) and bicarbonate (the source of carbon for calcification in coccolithophores) in seawater. Some of the previous experiments used acid to decrease the pH of the medium, and this causes an increase in CO2 but a negligible effect on bicarbonate. Contrary to previous results of decreased calcification under high CO2 levels, which could potentially act as a negative feedback on atmospheric CO2 levels, the observed increase in both calcification (CO2 outgassing) and photosynthesis (CO2 ingassing) suggest that future coccolithophore populations will neither greatly ameliorate nor exacerbate atmospheric CO2 rise.’ 

The results highlight the ‘urgent need to agree on methodological protocols to investigate ocean acidification’ says the article’s lead author.

Ulf Riebesell, lead author of results pointing to detrimental effects of ocean acidification on calcifying phytoplankton agrees with the need to standardize methods, but also adds that ‘to explain these seemingly contradictory results we have to better understand the mechanisms underlying biogenic calcification’.  

Bottom line!

The contradictory results do not diminish the urgency to address ocean acidification caused by rising anthropogenic CO2 emissions, as the new results concern just a single species and need to be confirmed across taxa before conclusions for the ecosystem can be drawn.

‘The field of ocean acidification research is fast moving, and a growing body of evidence suggests that the impact of rising CO2 on upper ocean ecology will be more nuanced than previously thought, with both winners and losers and the possibility for significant reorganizations at the base of the pelagic marine foodweb.’ – concludes Scott Doney.

Artwork, coccolithophores in a changing ocean. [Image courtesy of Rosa Maria Rodriguez-Seoane]

RELEVANT LINKS:
- Response to the article above, Riebesell et al., 2008, Science 322, 1466
- Bleak future for acid oceans - Nature 6 June 2008
- Ocean Acidification – an EUROCEANS film in French and English with the participation of James Orr and Jean-Pierre Gattuso
- Ocean Acidification Network - http://www.ocean-acidification.net/
- Ocean Acidification – An information outlet on ocean acidification moderated by Jean-Pierre Gattuso http://oceanacidification.wordpress.com/



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