Friday, June 7, 2013

Resource Letter BSSMF-1: Biological Sensing of Static Magnetic Fields

In the October 2012 issue of the American Journal of Physics, physicist Leonard Finegold published Resource Letter BSSMF-1: Biological Sensing of Static Magnetic Fields (Volume 80, Pages 851-861). Finegold recommends that a good starting point for mastering the topic of magnetoreception is Kenneth Lohmann’s News and Views article in Nature.
35. “Magnetic-field perception: News and Views Q and A,” K. J. Lohmann, Nature 464, 1140–1142 (2010). (E) 
I looked it up, and it does indeed provide a well-written summary of the field in a reader-friendly question-and-answer format.

In the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss magnetotactic bacteria. We write that
“Bacteria in the northern hemisphere have been shown to seek the north pole. Because of the tilt of the earth’s field, they burrow deeper into the environment in which they live. Similar bacteria in the southern hemisphere burrow down by seeking the south pole.” 
Finegold also reviews this topic. The excerpt reproduced below serves both as an up-date to IPMB and as a sample of the style of an American Journal of Physics resource letter.
“Certain bacteria move in response to the earth’s magnetic field (Ref. 35), swimming along the field lines, and have been excellently reviewed (Ref. 36). The ‘sensing’ element is magnetite (an iron oxide) or greigite (an iron sulfide) (Ref. 37). The bacteria would swim toward the boundary between oxygenated and oxygen-poor regions. Until recently, there was the comforting idea that there are two groups of bacteria with opposite sensors, depending on which of the earth’s hemispheres they reside. Alas, both groups have now been found in the same place; it appears that their polarity is correlated with the local redox potential (Ref. 38 and 39). In addition, some bacteria use only the axial property of the field (i.e., they swim both with or against the field direction), whereas others use the vector property (i.e., they swim either with or against the field direction). Details of the behavior have been elucidated by applying magnetic fields to bacteria in a spectrophotometer cuvette, with genetic analysis (Ref. 39).

35. “South-seeking magnetotactic bacteria in the Southern Hemisphere,” R. P. Blakemore, R. B. Frankel, and Ad. J. Kalmijn, Nature 286, 384–385 (1980). (A)

36. “Bacteria that synthesize nano-sized compasses to navigate using Earth’s geomagnetic field,” L. Chen, D. A. Bazylinski, and B. H. Lower, Nature Education Knowledge 1(10), 14 (2010). (I)

37. “The identification and biogeochemical interpretation of fossil magnetotactic bacteria,” R. E. Kopp and J. L. Kirschvink, Earth-Sci. Rev. 86, 42–61 (2008). (A)

38. “South-seeking magnetotactic bacteria in the northern hemisphere,” S. L. Simmons, D. A. Bazylinski, and K. J. Edwards, Science 311, 371–374 (2006). (A)

39. “Characterization of bacterial magnetotactic behaviors by using a magnetospectrophotometry assay,” C. T. Lefevre, T. Song, J. P. Yonnet, and L. F. Wu, Appl. Environ. Microbiol. 75, 3835–3841 (2009). (A)”
Magnetoreception is a field that often stirs debate. Russ and I outline one such debate in IPMB
Kirschvink (1992) proposed a model whereby a magnetosome in a field of 10−4–10−3 T could rotate to open a membrane channel. As an example of the debate that continues in this area, Adair (1991, 1992, 1993, 1994) argued that a magnetic interaction cannot overcome thermal noise in a 60-Hz field of 5 × 10−6 T. However, Polk (1994) argues that more biologically realistic parameters, including a large number of magnetosomes in a cell, could allow an interaction at 2 × 10−6 T.”
The key citations in the debate are
Adair, R. (1991). Constraints on biological effects of weak extremely-low-frequency electromagnetic fields. Phys. Rev. A 43: 1039–1048.
Kirschvink, J. L. (1992). Comment on “Constraints on biological effects of weak extremely-low-frequency electromagnetic fields.” Phys. Rev. A 46: 2178–2184.
Adair, R. (1992). Reply to “Comment on ‘Constraints on biological effects of weak extremely-low-frequency electromagnetic fields.’” Phys. Rev. A 46: 2185–2187.
For those of you who like this sort of thing, here is another example from Finegold’s resource letter. The debate is about, of all things, if cows align themselves in magnetic fields!
“A surprising finding is that cattle and deer seem to align themselves in an approximate north-south (geomagnetic) direction. The evidence is from world-wide satellite photographs from Google Earth, supported by ground observations of more than 10,000 animals, and is hard to rebut. The satellite photographs do not have enough resolution to show the direction (north or south) in which the animals face.
72. “Magnetic alignment in grazing and resting cattle and deer,” S. Begall, J. Cerveny, J. Neef, O. Vojtech, and H. Burda, Proc. Natl. Acad. Sci. U.S.A. 105, 13453–13455 (2008). (I)
As Usherwood asks, why on Earth should cattle and deer prefer this alignment? Possible interpretations are that the satellite photographs are made close to noon, so there may be physiological reasons (heating, cooling) for animals to align or to view predators better.
73. “Cattle and deer align north (-north-east),” J. Usherwood, J. Exp. Biol. 212, iv (2009). (E)
Partly to rule out sun compass effects, Burda et al. investigated ruminant alignment under high-voltage (and hence high-current, low-frequency) power lines and found that the geomagnetic north-south alignment was disturbed; the disturbance was correlated with the alternating fields. Such disturbance might instead be because the animals felt protected by (or preferring) the overhead lines or pylons or because of the audible (to humans at least) corona discharge. A good control for this would be to look at ruminants under power lines being repaired, carrying no current; this is difficult to do. The authors ingeniously compared the nonalignment under N-S and E-W trending power lines and found that the nonalignment followed the resultant total magnetic field. Their conclusions have been challenged (Ref. 75), and they have a lively rebuttal (Ref. 76), to which the challengers have replied (Ref. 77). Hence, the initially persuasive evidence, that cattle and deer detect magnetic fields, may need re-examination.

74. “Extremely low-frequency electromagnetic fields disrupt magnetic alignment of ruminants,” H. Burda, S. Begall, J. Cerven, J. Neef, and P. Nemec, Proc. Natl. Acad. Sci. U.S.A. 106, 5708–5713 (2009). (I)
75. “No alignment of cattle along geomagnetic field lines found,” J. Hert, L. Jelinek, L. Pekarek, and A. Pavlicek, J. Comp. Physiol., A 197, 677–682 (2011). (I)
76. “Further support for the alignment of cattle along magnetic field lines: Reply to Hert et al.,” S. Begall, H. Burda, J. Cerveny, O. Gerter, J. Neef-Weisse, and P. Nemec, J. Comp. Physiol. [A] 197, 1127–1133 (2011). (I)
77. “Authors’ Response,” J. Hert, L. Jelinek, L. Pekarek, and A. Pavlicek, J. Comp. Physiol. [A] 197(12), 1135– 1136 (2011). (I) ”
Finegold also discusses magnet therapy, a topic I am extremely skeptical about, and that I have discussed before in this blog. He cites his own editorial with Flamm
Magnet therapy,” L. Finegold and B. L. Flamm, Br. Med. J. 332, 4 (2006) (E) 
which concludes
“Extraordinary claims demand extraordinary evidence. If there is any healing effect of magnets, it is apparently small since published research, both theoretical and experimental, is weighted heavily against any therapeutic benefit. Patients should be advised that magnet therapy has no proved benefits. If they insist on using a magnetic device they could be advised to buy the cheapest—this will at least alleviate the pain in their wallet.”

No comments:

Post a Comment