Effects of ocean acidification on iron availability to marine phytoplankton

Dalin Shi is currently a professor at the State Key Laboratory of Marine Environmental Science, Xiamen University, China. His research focuses on the biogeochemical cycling of trace metals in the ocean and their roles in the global carbon and nitrogen cycles.

About one-third of the anthropogenic carbon dioxide (CO2) released into the atmosphere dissolves in the ocean, increasing the partial pressure of CO2 (pCO2) and lowering the pH in surface water. These changes in seawater chemistry, commonly referred to as ocean acidification, will likely have significant effects on marine phytoplankton, which are responsible for about half of the contemporary global primary production (Field et al., 1998) and form the basis of all marine food webs.

In vast areas of the oceans, the vanishingly low concentration of iron (Fe) often limits the growth of marine phytoplankton (Martin and Fitzwater, 1988). It is known that the bulk of Fe in the ocean is chelated by organic compounds (Rue and Bruland, 1995), and that the availability of Fe to phytoplankton depends largely on its chemistry in seawater, which is highly sensitive to changes in pH (Shaked and Lis, 2012). We therefore examined the effect of ocean acidification on the complexation of Fe by organic ligands, and, hence, its bioavailablility to phytoplankton. In the presence of organic complexing agents with various chemical functionalities, the bioavailability of dissolved Fe to model species of diatoms and coccolithophores was observed to decline at low pH. This effect is quantitatively explained by the decrease in the free Fe concentration, Fe’, in seawater with decreasing pH and is thus not a physiological response of the organisms.

The extent to which changes in pH affect a ligand’s ability to bind Fe depends on the number of protons (H+) released upon dissociation of Fe from the ligand (Y). The dissociation reactions can be written in a simplified but general form as:

FeY + 3H2O = Fe(OH)3 + HxY + (3–x)H+

The number of protons released varies between 3 (e.g., for the tetracarboxylate EDTA) and 0 (e.g., for the bis-catecholate azotochelin), and depends on the acidity of the binding moieties, the affinity of the ligand for other metals such as Ca2+ and Mg2+. Consequently, the effective binding strength of ligands that release more protons is more sensitive to changes in pH. In field manipulation experiments, a slower rate of Fe uptake by Thalassiosira weissflogii, with decreasing pH, was observed in both coastal and oceanic Atlantic surface water samples where Fe was bound to natural Fe-chelating ligands (Figure). This result is in agreement with the laboratory data, though the magnitude of the pH effect on the uptake of Fe was modest, suggesting that little of the Fe was bound to carboxylic acid moieties or other ligands that release protons upon dissociation in the field samples (Shi et al., 2010).

We note that the decrease in Fe bioavailability caused by the change in Fe chelation at low pH is only one of several potential effects that ocean acidification may have on Fe limitation of phytoplankton in the oceans. For example the rate of dissolution and precipitation of Fe oxides particles, through thermal or photochemical mechanisms, may also change and, perhaps, compensate for the increase in Fe complexation.

References

Field, C., et al. (1998). Primary production of the biosphere: Integrating terrestrial and oceanic components. Science 281:237-240.
Martin, J. and Fitzwater, S. (1988). Iron-deficiency limits phytoplankton growth in the Northeast Pacific subarctic.
Nature 331:341-343.
Rue, E. and Bruland, K. (1995). Complexation of iron(iii) by natural organic-ligands in the central North Pacific as determined by a new competitive ligand equilibration adsorptive cathodic stripping voltammetric method. Mar Chem. 50:117-138.
Shaked, Y. and Lis, H. (2012). Disassembling iron availability to phytoplankton. Front Microbiol. Chem. 3:123.
Shi, D., et al. (2010). Effects of ocean acidification on iron availability to marine phytoplankton. Science 327:676-679.

Shi D., Xu Y., Hopkinson B. M. & Morel F. M. M., 2013. Effects of ocean acidification on iron availability to marine phytoplankton. SOLAS NEWS 15, Summer 2013: 8-9. Article.


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