Tikalon Blog by Dev Gualtieri

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Bacterial Iron Isotopes

July 22, 2013

Our knowledge of the existence of atoms is not that old. Democritus named the atom (α-τομ, "not cutable") about 450 BC, but his arguments about its existence were philosophical , not scientific . The scientific basis of atomism only occurred in 1805, when the chemist , John Dalton , presented experiments with a matching atomic theory .

Dalton's work didn't address the size of atoms . That was left to the Austrian physical chemist , Johann Josef Loschmidt . Loschmidt determined the probable size of "air molecules ," and he obtained a constant , now called Loschmidt's number , which is the number of molecules of an ideal gas in a given volume ; that is, its number density .

This huge number has an established modern value of 2.6868 x10²⁵ per cubic meter at standard temperature and pressure (STP, 273.15 K (0 °C) and 100 kPa (0.986 atm )). Considering how large that large number is, imagine the audacity of trying to get funding for an experiment in which you intend to count individual atoms. That's somewhat more practical in this age of nanotechnology , but physicist , Raymond Davis, Jr., proposed this in the 1960s . Not only that, but his experiment was operated almost a mile underground in a gold mine .[1]

His Homestake experiment , located in the Homestake Mine in Lead, South Dakota , was designed to detect solar neutrinos .[1-2] In 1939, Hans Bethe published the supposed nuclear reactions that power the Sun , and these produce an abundance of electron neutrinos . Since these neutrinos had never been detected, Davis decided to give it a try.

One trouble with neutrinos is that they don't interact very often with other particles . The neutrino detector devised by Davis was a tank filled with 100,000 gallons of the common dry-cleaning chemical , tetrachloroethylene . This was a convenient source of chlorine , and a chlorine nucleus (atomic number 17) will transmute to an argon nucleus (atomic number 18) when hit by a neutrino. An elaborate system was devised to extract these argon atoms from the tank.[1-2]

John Bahcall had calculated that there would only be about five argon atoms generated per day. Less than two argon atoms were found each day.[2] Statistics showed that the measured neutrino flux was one-third of what was calculated . Neither the measurement nor the calculation was wrong. Davis had discovered neutrino "flavor" oscillations , since his detector responded to just one of three types of neutrinos. For this work, Davis shared the 2002 Nobel Prize in Physics .

Figure caption

Raymond Davis, Jr. (October 14, 1914 - May 31, 2006)

Davis was lucky to have lived to a reasonable old age, since his neutrino experiment needed extensive corroboration .

After that, the 2000 Wolf Prize in Physics , the 2001 National Medal of Science , and the 2002 Nobel Prize in Physics came in quick succession.

(2001 National Science Foundation photograph, via Wikimedia Commons.)

Preliminary data on a similar atom counting feat has just been shared by Shawn Bishop , a physicist from the Technische Universität München , who reported on the work of a German -Austrian team looking for traces of the iron isotope , Fe-60 , on the Earth .[4-8] This isotope has a half-life of 2.62 million years , so none of it would remain from the formation of the Earth . This iron isotope, however, is created in supernovas , so a nearby supernova could have deposited some of the isotope on Earth in the recent past. The problem, however, is how to find it.

The tactic used by this research team is to mine iron that was concentrated by ancient oceanic bacteria and deposited onto the seabed at their demise. As I wrote in a previous article (Magnetic Yeast, March 7, 2012), some animals , including some bacteria,[9-10] possess receptors for magnetic field , a property known as magnetoception . These receptors contain the iron-bearing mineral, magnetite , Fe₃O₄, as crystals about 80 nanometers in diameter .

Magnetic bacterium Magnetospirillum gryphiswaldense (MSR-1)

TEM image of Magnetospirillum gryphiswaldense(MSR-1), a freshwater Gram-negative microaerophilic bacterium.

The bacteria synthesize magnetite (Fe₃O₄) nanoparticles that are arranged in a chain inside the cell allowing the organism to orient itself in Earth's geomagnetic field.

(Photograph by Zachery Oestreicher and Brian H. Lower . Used with permission.)

The first detection of environmental Fe-60 came in 2004 with its discovery in a ferromanganese crust from the floor of the equatorial Pacific Ocean . The specimen was dated to be about 2.2 million years old, so the originating supernova would have been near that time, which is about the time that modern humans appeared.[5,7] The originating supernova was likely the one discovered as a remnant in the Scorpius–Centaurus star cluster .[6] In general, supernovas are bad for your health, since they also emit copious gamma radiation . Bishop, as quoted in APS News , said, "That we're here talking about it indicates that the supernova wasn't too close."[6]

As the Earth passed through the supernova's debris cloud, bacteria would have incorporated that iron isotope, dissolved into ocean water from atmospheric dust, into their magnetite crystals.[5-6] The German-Austrian team analyzed a portion of a core specimen obtained during the Pacific Ocean Drilling Program . They took samples at 100,000 year intervals in its span of 1.7-3.3 million years ago.[5,7]

Cassiopeia-A False Color Image

A false color image of supernova remnant , Cassiopeia A , formed from Hubble telescope , Spitzer telescope and Chandra X-ray Observatory images.

(NASA/JPL-Caltech image, via Wikimedia Commons.)

The iron was extracted by a chemical technique that selected just the iron from biological sources. Forty grams of sediment produced just three milligrams of iron, of which just one part in 10¹⁵ is Fe-60. The team analyzed for Fe-60 by radiating it with a cesium ion beam in an accelerator at the Maier-Leibnitz-Laboratory near Munich . The cesium, bound to iron, was counted at a particle counter .[6] Said Bishop in APS News, "We're literally counting individual atoms of iron-60 that come out of the sample material."[6]

Cautioning that the data are preliminary and their technique has not passed peer review , Bishop reported at the Denver, Colorado , meeting of the American Physical Society that the Fe-60 peaks at about 2.2 million years ago, at the purported time of the Scorpius–Centaurus supernova about 424 light years from the Sun.[4,7] The research team is now analyzing a second core specimen to verify the Fe-60 signal.[6-7]

It's interesting to note that I once grew a crystal , isotopically-enriched with another isotope of iron, iron-57 , for a nuclear physics experiment.[11]

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