Ongoing ocean acidification due to increasing atmospheric CO2 may reduce the bioavailability of dissolved iron (Fe), which is a limiting nutrient in large oceanic regions, according to a study by researchers at Princeton University. A paper on their work was published online 14 January in the journal Science.
Unrelated to global warming, the dissolution of additional atmospheric CO2 in the ocean is leading to predictable changes in the chemistry of the water, including an increase in pCO2, a decrease in pH, and a decrease in the carbonate ion concentration, [CO3 2-]. While the effects of increasing pCO2 and decreasing [CO3 2-] on phytoplankton have received some attention, the authors note, the potential effects of the decrease in pH, nearly 0.3 pH units for a doubling of pCO2, has not.
Iron is the biologically important element whose chemistry is most sensitive to pH. The bulk of Fe(III) in the ocean is known to be chelated by organic compounds, and the fraction that is not chelated is present as hydrolyzed species, Fe(OH)x(3-x)+, with the neutral tri-hydroxy species, Fe(OH)3, being very insoluble. As ocean waters acidify, decreasing the hydroxide ion concentration, iron’s speciation and solubility will be altered.
—Shi et al.
The study by Shi et al. showed that the bioavailability of dissolved Fe may decline due to ocean acidification. Acidification of media containing various Fe compounds decreases the Fe uptake rate of diatoms and coccolithophores to an extent predicted by the changes in Fe chemistry. A slower Fe uptake by a model diatom with decreasing pH was also seen in experiments with Atlantic surface water.
In areas where particulate Fe inputs are important, this effect may be partially compensated by the increased effectiveness of some chelators in dissolving Fe from oxyhydroxides and/or by enhancing the photo-induced redox cycle of Fe. We have found so far no evidence that an increase in pCO2 above present day values results in a lower Fe requirement. It thus seems likely that, unless Fe inputs to surface seawater increase as a result of global change, the net result of seawater acidification should be an increase in the Fe-stress of the phytoplankton in many areas of the oceans.
—Shi et al.
Dalin Shi, Yan Xu, Brian M. Hopkinson, François M. M. Morel (2010) Effect of Ocean Acidification on Iron Availability to Marine Phytoplankton. Science Express. doi: 10.1126/science.1183517