Effect of Ocean Acidification on Iron Availability to Marine Phytoplankton

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Entry by Angelo Mao, AP 225, Fall 2010

Title: Effect of Ocean Acidification on Iron Availability to Marine Phytoplankton

Authors: Dalin Shi, Yan Xu, Brian M. Hopkinson, François M. M. Morel

Journal: Science

Volume: Vol 327

Pages: 676-679


Summary

Increased concentrations of carbon dioxide in the atmosphere lowers the ocean pH, which may have effects on phytoplankton species that form the basis of food chains in the ocean. The researchers determined that lower pH lowers iron uptake by phytoplankton. They determined that pH does not alter phytoplankton food uptake, and that it is the effect of acidification on the acid-base chemistry of release of iron from chelators that leads to lower iron uptake by phytoplankton species.

soft matter keywords: colloid, solubility, pH

Fe' concentration predicts uptake

Figure 1. Steady-state iron uptake in different species of phytoplankton. A, B, D, and E depict iron uptake as a function of total iron concentration at different pH. C and F depict iron uptake as a function of unchelated iron concentration, Fe'.

The researchers quantified uptake of iron in four different species of phytoplankton, "the coastal centric diatom Thalassiosira weissflogii, the open ocean–centric diatom Thalassiosira oceanica, the pennate diatom Phaeodactylum tricornutum, and the coccolithophore Emiliana huxley." Iron is present in the ocean in several forms. Most of the iron is chelated by organic compounds, whereas only a small fraction is directly dissolved in ocean water. In fact, acidification should increase the amount of iron directly dissolved in the ocean. However, the researchers will show that the amount of iron available for biological use and sequestered as chelated by organic compounds decreases with increased acidity.

Figure 1 shows an extreme case in which there is only one chelating agent at work, the tetracarboxylic acid EDTA. Although the uptake as a function of total iron concentration varied greatly with pH, the relations coalesced to one line when only unchelated iron was considered. The concentration of unchelated iron could be calculated by:

<math>Fe' = \sum_x [Fe(OH)_x^{(3-x)+}]</math>

Fe update does not depend on physiological response to pH

Figure 2. Uptake of iron by the species T. weissflogii from iron present in different forms: "(A) the aminocarboxylate EDTA, (B) the biscatecholate siderophore azotochelin, and (C) the trihydroxamate siderophore desferriferrioxamine B, and from iron in the forms of (D) freshly precipitated ferrihydrite and (E) ferrihydrite sequestered in the iron storage protein."

Although Figure 1 had demonstrated that unchelated concentrated correlated with iron uptake in phytoplankton, it was unclear whether this difference was due to the effect of pH on physiological processes, or the effect of pH on iron availability. The researchers chose forms of iron chelation that depended on pH to varying degrees. Figure 2A, for example, had iron chelated by EDTA, whose release of iron was strongly influenced by proton concentration.

<math>Fe(OH)_{0.6}Y + Ca^{2+} + 2.3H_2O = Fe(OH)_{2.9} + CaY + 2.3 H^+</math>

Increased pH would disfavor dissociation. On the other hand, figure 2B shows uptake when iron has been bound to bis-catecholate azotochelin.

<math>FeY + 2.9 H_2O = Fe(OH)_3 + H_{2.8}Y + 0.1 H^+</math>

The dissociation is not influenced greatly by pH. Figure 2B shows that uptake remains constant despite pH change, demonstrating that pH does not affect phytoplankton ability to uptake iron.

Fe concentration affects growth rate

Figure 3. Growth rate as a function of iron concentration.

Figure 3 translates the effects of iron uptake to growth rate as a function of cellular iron concentration. Figure 3D underscores that different growth rates are not a result of pH within the range of pH that would result from ocean acidification, but are rather a result of iron availability.

Conclusion

Ocean acidity affects iron uptake due to its effect on iron dissociation and not on phytoplankton physiology. This relates to the ideas of energetics, association and dissociation as discussed in class; the reason why dissociation of iron is unfavorable in acidic water is probably due to the decreased entropy of having more protons in solution. The topic of colloids is also related to this paper in that one of the chelators the researchers studied are oxyhydroxide colloids. However, the researchers were unable to quantify uptake of iron from this source because the oxyhydroxide colloids adsorbed to their experimental appratus.