6 ms·

Bacteria make nanomagnets for navigating the oceans

14 points by prat 17y ago

ars 17y ago
I would assume it's not actually magnets, but rather particles that can detect magnetic fields, because how would they magnetize it?
- moeffju 17y ago
  The text explains how they create magnetite (by oxidizing iron). Also: How would you detect magnetic fields without magnetic material - i.e. magnets?
  - ars 17y ago
    No, I mean something that is actually magnetic, i.e. has a magnetic field. Soft iron is affected by magnetic fields, but does not create a magnetic field of its own.
    - unwind 17y ago
      According to the article, they oxidize iron to create magnetite, which Wikipedia states (here: http://en.wikipedia.org/wiki/Magnetite http://en.wikipedia.org/wiki/Magnetite) "is the most magnetic of all the naturally occurring minerals on Earth". Sounds like the bacteria are well optimized. :)
    - mechanical_fish 17y ago
      Soft iron is affected by magnetic fields, but does not create a magnetic field of its own. Well... yes and no. Iron is ferromagnetic. If you look carefully at this word, and know a little Latin and/or a little chemistry, you will see that it was basically invented to describe iron. (Although nickel, cobalt, and other elements and compounds are also ferromagnetic.) Individual iron atoms have a magnetic moment, which is to say: they are tiny magnets. They both create a magnetic field and respond to other magnetic fields. And when iron crystallizes the atoms which are next to each other tend to align their magnetic fields. However, the typical iron-containing crystal has a large number of individual magnetic domains. Within a domain all the tiny magnetic iron atoms are aligned with each other, but each domain is randomly oriented with respect to the others. Thus, the tiny magnets tend to cancel out, and outside the iron we don't see it creating a net magnetic field. However, if you apply a (nonuniform) magnetic field to the iron it gets attracted, because the individual atoms do respond to magnetic fields even if they aren't all in line. [1] If you apply a magnetic field to the iron -- preferably a strong one -- possibly together with a bit of heat or some banging with a hammer, you can encourage its magnetic domains to realign themselves and produce a piece of iron that has a net magnetic moment. Now you have a magnet. This is what the bacteria are doing. Like most biological things, they are capable of taking iron apart to its component atoms and then putting those atoms together again, so this isn't especially astonishing behavior. The point of the oxidation, to create magnetite, is that a magnetite crystal bonds the iron particularly tightly, improving the odds that the individual atoms will retain their orientation. As you know, a rod of pure iron isn't very stable: It tends to turn into iron oxide all by itself, then flake apart. We call that rusting. -- [1] Of course, in the real world pieces of iron do tend to have a net magnetic moment... because they crystallize on the Earth, and the whole Earth has a magnetic field. But the Earth's field isn't very strong, so the orientation effect isn't very strong either.
      - ars 17y ago
        > If you apply a magnetic field to the iron -- preferably a strong one ..... produce .... net magnetic moment. > This is what the bacteria are doing. Like most biological things, they are capable of taking iron apart to its component atoms and then putting those atoms together again, so this isn't especially astonishing behavior. That's what I was saying - how are the bacteria doing this? They don't have access to a strong magnetic field (except the earth). Are you suggesting they are aligning the magnetic moments of the iron atom by atom? Is that really possible?
        mechanical_fish 17y ago
        They don't have access to a strong magnetic field (except the earth). When you assemble ferromagnetic atoms together one at a time they tend to line up with each other, even if the applied field is weak to nonexistent. This is why the orientation of the atoms in an iron bar aren't random from one atom to the next: instead they form "domains" where all the atoms point in one direction. If you are building a tiny enough iron bar it's probably pretty easy to have the whole thing come out as one domain. And bacteria aren't very big, so the bars of magnetite that they're building inside themselves are pretty small. Are you suggesting they are aligning the magnetic moments of the iron atom by atom? Is that really possible? Absolutely. This is how all crystal growth works. One atom at a time. You may not be used to thinking about chemistry in this way, but that is what chemistry is. Of course, you shouldn't think of the bacteria the way you think of human model-makers, consciously assembling tinkertoys. That kind of tedious fiddling is for people like us, who are long on understanding but short on design time. The well-designed bacterium uses chemistry instead: Metaphorically, you throw properly shaped tinkertoys into a bucket and shake it around and they assemble themselves. ;) In fact, if you take a liquid and chill it below the freezing point, what usually happens is that crystals start to form in more than one place, and then those crystals expand atom by atom until they bump into each other and fuse. For example, this is the reason why there is more than one magnetic domain in the typical iron bar. If you grew your iron bar under perfect temperature-controlled conditions from a single seed crystal, you might be able to grow an entire bar with one domain. That is what the bacteria are doing, but they get to cheat because the bars they are growing are so small, so they get to quit while they are ahead. :) If you try to grow a meter-long, centimeter-thick bar like this, you'll find that you've got trouble with temperature control, trouble with impurities, trouble with rust (as I said), and that because you're operating at a temperature above absolute zero there's always some randomness going on, trying to scramble your work. Having said that, it might be possible for humans to do this. We do similar things with silicon every day. The chips inside your computer were cut from a single crystal of silicon 25 cm in diameter and perhaps 1 meter long: http://en.wikipedia.org/wiki/Boule_(crystal) http://en.wikipedia.org/wiki/Boule_(crystal) http://en.wikipedia.org/wiki/Single_crystal http://en.wikipedia.org/wiki/Single_crystal But understanding the detailed atomic behavior of ferromagnetic materials is actually quite a hard problem. Lots of difficult thermodynamics. Amusing chaotic systems problems. I basically hit the wall in my graduate course on the subject. ;)

14 points by prat 17y ago

ars 17y ago

I would assume it's not actually magnets, but rather particles that can detect magnetic fields, because how would they magnetize it?

moeffju 17y ago
The text explains how they create magnetite (by oxidizing iron). Also: How would you detect magnetic fields without magnetic material - i.e. magnets?
- ars 17y ago
  No, I mean something that is actually magnetic, i.e. has a magnetic field. Soft iron is affected by magnetic fields, but does not create a magnetic field of its own.
  - unwind 17y ago
    According to the article, they oxidize iron to create magnetite, which Wikipedia states (here: http://en.wikipedia.org/wiki/Magnetite http://en.wikipedia.org/wiki/Magnetite) "is the most magnetic of all the naturally occurring minerals on Earth". Sounds like the bacteria are well optimized. :)
  - mechanical_fish 17y ago
    Soft iron is affected by magnetic fields, but does not create a magnetic field of its own. Well... yes and no. Iron is ferromagnetic. If you look carefully at this word, and know a little Latin and/or a little chemistry, you will see that it was basically invented to describe iron. (Although nickel, cobalt, and other elements and compounds are also ferromagnetic.) Individual iron atoms have a magnetic moment, which is to say: they are tiny magnets. They both create a magnetic field and respond to other magnetic fields. And when iron crystallizes the atoms which are next to each other tend to align their magnetic fields. However, the typical iron-containing crystal has a large number of individual magnetic domains. Within a domain all the tiny magnetic iron atoms are aligned with each other, but each domain is randomly oriented with respect to the others. Thus, the tiny magnets tend to cancel out, and outside the iron we don't see it creating a net magnetic field. However, if you apply a (nonuniform) magnetic field to the iron it gets attracted, because the individual atoms do respond to magnetic fields even if they aren't all in line. [1] If you apply a magnetic field to the iron -- preferably a strong one -- possibly together with a bit of heat or some banging with a hammer, you can encourage its magnetic domains to realign themselves and produce a piece of iron that has a net magnetic moment. Now you have a magnet. This is what the bacteria are doing. Like most biological things, they are capable of taking iron apart to its component atoms and then putting those atoms together again, so this isn't especially astonishing behavior. The point of the oxidation, to create magnetite, is that a magnetite crystal bonds the iron particularly tightly, improving the odds that the individual atoms will retain their orientation. As you know, a rod of pure iron isn't very stable: It tends to turn into iron oxide all by itself, then flake apart. We call that rusting. -- [1] Of course, in the real world pieces of iron do tend to have a net magnetic moment... because they crystallize on the Earth, and the whole Earth has a magnetic field. But the Earth's field isn't very strong, so the orientation effect isn't very strong either.
    - ars 17y ago
      > If you apply a magnetic field to the iron -- preferably a strong one ..... produce .... net magnetic moment. > This is what the bacteria are doing. Like most biological things, they are capable of taking iron apart to its component atoms and then putting those atoms together again, so this isn't especially astonishing behavior. That's what I was saying - how are the bacteria doing this? They don't have access to a strong magnetic field (except the earth). Are you suggesting they are aligning the magnetic moments of the iron atom by atom? Is that really possible?
      - mechanical_fish 17y ago
        They don't have access to a strong magnetic field (except the earth). When you assemble ferromagnetic atoms together one at a time they tend to line up with each other, even if the applied field is weak to nonexistent. This is why the orientation of the atoms in an iron bar aren't random from one atom to the next: instead they form "domains" where all the atoms point in one direction. If you are building a tiny enough iron bar it's probably pretty easy to have the whole thing come out as one domain. And bacteria aren't very big, so the bars of magnetite that they're building inside themselves are pretty small. Are you suggesting they are aligning the magnetic moments of the iron atom by atom? Is that really possible? Absolutely. This is how all crystal growth works. One atom at a time. You may not be used to thinking about chemistry in this way, but that is what chemistry is. Of course, you shouldn't think of the bacteria the way you think of human model-makers, consciously assembling tinkertoys. That kind of tedious fiddling is for people like us, who are long on understanding but short on design time. The well-designed bacterium uses chemistry instead: Metaphorically, you throw properly shaped tinkertoys into a bucket and shake it around and they assemble themselves. ;) In fact, if you take a liquid and chill it below the freezing point, what usually happens is that crystals start to form in more than one place, and then those crystals expand atom by atom until they bump into each other and fuse. For example, this is the reason why there is more than one magnetic domain in the typical iron bar. If you grew your iron bar under perfect temperature-controlled conditions from a single seed crystal, you might be able to grow an entire bar with one domain. That is what the bacteria are doing, but they get to cheat because the bars they are growing are so small, so they get to quit while they are ahead. :) If you try to grow a meter-long, centimeter-thick bar like this, you'll find that you've got trouble with temperature control, trouble with impurities, trouble with rust (as I said), and that because you're operating at a temperature above absolute zero there's always some randomness going on, trying to scramble your work. Having said that, it might be possible for humans to do this. We do similar things with silicon every day. The chips inside your computer were cut from a single crystal of silicon 25 cm in diameter and perhaps 1 meter long: http://en.wikipedia.org/wiki/Boule_(crystal) http://en.wikipedia.org/wiki/Boule_(crystal) http://en.wikipedia.org/wiki/Single_crystal http://en.wikipedia.org/wiki/Single_crystal But understanding the detailed atomic behavior of ferromagnetic materials is actually quite a hard problem. Lots of difficult thermodynamics. Amusing chaotic systems problems. I basically hit the wall in my graduate course on the subject. ;)