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Iron Availability in the Southern Ocean Print
Friday, 21 June 2013 10:08

The Southern Ocean, circling the Earth between Antarctica and the southernmost regions of Africa, South America, and Australia, is notorious for its high-nutrient, low-chlorophyll areas, which are rich in nutrients—but poor in essential iron. Sea life is less abundant in these regions because the growth of phytoplankton—the marine plants that form the base of the food chain—is suppressed. A study by scientists from South Africa’s Stellenbosch University, Princeton University, and the Advanced Light Source (ALS) suggests that it is not just a lack of iron, but a lack of iron in an easy-to-use form, that is affecting the ecosystems. The researchers sampled two north-south corridors across the Southern Ocean, traveling an easterly transect between the base of the South African National Antarctic Expeditions (SANAE IV) in Queen Maud Land and Cape Town, and a westerly transect between SANAE IV and South Georgia Island. Along the way they collected particles containing solid iron from a series of ocean systems with different characteristics.

Animal, Vegetable or Mineral?

Iron is a limiting nutrient in many parts of the oceans, nowhere more so than in the Southern Ocean’s photic zone, which receives enough sunlight for photosynthesis to occur, but whose biological diversity is limited due to a lack of bioavailable iron (Fe). Distinct variations in the oxidation state and composition of iron particles exist between the coasts of South Africa and Antarctica, with different iron pools occurring in different frontal zones. Both the solubility and the bioavailability of particulate-bound iron vary according to differences in Fe oxidation state, mineralogy, crystallinity, structural impurities (like aluminum), and the structure and concentration of dissolved organic ligands. These variations can result in solubility differences that may affect the production of bioavailable dissolved iron, impacting primary productivity in the ocean—in this case, the growth of phytoplankton, the primary plant food source for bigger marine life—and the larger marine life it supports.

At bottom left, the kinds of iron species found in two transects of the Southern Ocean are shown in descending order from most soluble (yellow and red) to least soluble (purple and blue). The pie charts at right show the proportions of each species sampled at points between the SANAE base and Cape Town, and the pie charts at left show the samples between SANAE and South Georgia Island. (ACC stands for Antarctic Circumpolar Current.) The map shows chlorophyll concentrations in milligrams per square meter, per the scale at bottom right. There is a rough tendency for soluble iron regions to show greater chlorophyll concentrations.

Metal oxides are key components in both in technological and biological processes that are often governed by careful control over the physical and chemical properties of metal–oxygen bonds. For example, knowledge of the exact nature of highly covalent metal oxo bonds involving heavy metals such as uranium, neptunium, and plutonium is important to efficient remediation of radioactively contaminated sites. Drug metabolism and cholesterol synthesis (by the P450 family of enzymes) and the "splitting" of water (by photosystem II, a light-dependent protein complex) are examples of metal oxide functions found in nature. Techniques that can improve our models of electronic structure for metal–oxygen interactions are important to the development of artificial systems that can mimic the selectivity and efficiency of such systems.

At ALS Molecular Environmental Sciences Beamline 11.0.2, scientists from Princeton University and ALS Beamline Scientist Tolek Tyliszczak analyzed the samples using scanning transmission x-ray microscopy (STXM). STXM combines microscopy with spectroscopy, allowing the researchers to distinguish a number of different iron species and compounds in particles of different shapes and sizes. Though the dominant fraction of the marine iron pool is known to occur in the form of solid-phase particles, its chemical speciation and mineralogy have been challenging to characterize on a regional scale. The oxidation state and coordination environment of Fe in particles were analyzed by collecting Fe L3-edge x-ray absorption near-edge structure (XANES) spectra, recorded using STXM at a resolution of 12nm under ambient conditions. This enabled the research team to determine the solubility of the iron particles and how easily those particles could be taken up by plankton or other organisms.

The most common forms of iron are Fe(II) and Fe(III). Sea water contains plenty of highly oxidizing Fe(III), but it’s insoluble, and life has had to develop special mechanisms to absorb it. Fe(II) is readily soluble and easily taken up by plants and bacteria but is sparse in sea water, except when freshly dumped there by continental run-off or wind-blown dust. However, most dust carries a compound of both Fe(II) and Fe(III) called magnetite.

The researchers tested their particles for pure Fe(II), pure Fe(III), and magnetite. They also measured the association of iron with other materials, including aluminum, which is a solubility modifier and source indicator. In the array of particles analyzed, ranging from 20 to 700 nm in diameter, researchers found that the iron solubility of their samples ranged over three orders of magnitude. Then they compared the predominant kinds of iron in a region with the pattern of summertime chlorophyll, that is, phytoplankton growth.

In the eastern transect, iron in the soluble Fe(II) form, either pure or dominant in a mixture, occurred near the coasts of Africa and Antarctica. The regions of open ocean in between were dominated by Fe(III) or magnetite. The soluble Fe(II) regions clearly showed more phytoplankton growth.

The westward transect yielded less soluble iron, even though the entire area was within the circumpolar current and close to the continental shelf of Antarctica. Nevertheless, the team concluded that the abundance and distribution of more soluble forms of iron “reveals trends that allude to the effect of Fe speciation on biology, and vice versa.”


 

Research conducted by: B.P. von der Heyden (Stellenbosch University and Princeton University), A.N. Roychoundhury (Stellenbosch University), T.N. Mtshali (Stellenbosch University and the Council for Scientific and Industrial Research, South Africa), T. Tyliszczak (ALS, Berkeley Lab), and S.C.B. Myneni (Princeton University).

Research funding: Operation of ALS, SSRL, and APS are supported by U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES).

Publication about this research: B.P. von der Heyden, A.N. Roychoundhury, T.N. Mtshali, T. Tyliszczak, and S.C.B. Myneni, "Chemically and Geographically Distinct Solid-Phase Iron Pools in the Southern Ocean," Science 338, 1199 (2012).

ALS Science Highlight #271

 

ALSNews Vol. 343