"In roughly one-third of the ocean, iron is a limiting nutrient for phytoplankton, the photosynthetic organisms responsible for about half of the contemporary global primary production," resesarcher Dalin Shi told environmentalresearchweb. "As the chemistry of iron, the biologically important element, is most sensitive to pH, the ongoing acidification of seawater will alter its availability to phytoplankton. Thus, through changes in iron availability, ocean acidification may affect primary production and the ecology of phytoplankton."
Shi and colleagues found that acidification decreased the iron uptake rate of diatoms and coccolithophores in the laboratory, as predicted by changes in iron chemistry. And an increased amount of carbon dioxide did not affect the algae's iron requirements. The iron uptake rate of the diatom Thalassiosira weissflogii in samples of surface Atlantic water, meanwhile, decreased by about 10–20% when the pH was changed from around 8.4 to 7.8.
"It seems likely that unless iron supply to surface seawater increases as a result of global change, the overall effect of ocean acidification should be an increase in the iron-stress of phytoplankton populations in some areas of the ocean, for example the HNLC (high nutrient, low chlorophyll) regions," said Shi.
HNLC regions occur in the equatorial and sub-arctic Pacific Ocean, the Southern Ocean, and in some areas with strong upwelling, for example off the coasts of central and northern California and Peru.
Around one-third of the carbon dioxide released into the atmosphere by man's activities each year is absorbed by the oceans, leading to acidification. Some have predicted that ocean pH will decrease by 0.2–0.3 by the end of the century.
"[These results] could help us understand how the ongoing increase in atmospheric carbon dioxide will affect marine ecosystems, in this case, through changes in the availability of key nutrients," said Shi. "However, to better predict changes in primary production in iron-limited regions caused by ocean acidification, it will also require knowledge of changes in iron inputs to the ocean and in iron requirements by the organisms due to rising carbon dioxide."
Shi says that the next step is to conduct field work in HNLC waters. "The ultimate goal is to examine the effect of ocean acidification on the uptake of iron bound to natural ligands by natural phytoplankton assemblages and on the use of iron in their physiology," he added.
The researchers reported their work in Science.