|Iron Availability in the Southern Ocean|
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.
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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