Bacterial compasses - can creatures detect magnetic fields?

It’s not just birds that are sensitive to the Earth’s magnetic field. There are lots of animals and other organisms that are emerging now as being sensitive to magnetism. Chris Smith put this to Richard Harrison from Cambridge University...

Richard - Yes, that’s right. And one of the best known examples of an organism that uses the magnetic field to navigate is in a type of bacteria we call magnetotactic bacteria. And these are amazing little creatures that have learnt to build chains of magnetite nanoparticles inside their cell, which essentially turns that bacterium into a compass and they swim along the Earth’s magnetic field lines so that they can find, very efficiently, the correct level in the water column for them to live.

Chris - I was going to say, what would be the advantage to a bacterium to be able to sense the magnetic field?

Richard - Well, they live under very specific conditions. They need: too much oxygen would be poisonous to them, too little is again not ideal so they live at the redox boundary between the two. If you swim around randomly trying to find that level it can be a very inefficient way, but if you orient yourself in the field and swim in an exactly straight line then you get there very efficiently.

Chris - I read a story in the journal eLife. About three years ago researchers published a very interesting paper on tiny microscopic worms, and they found that these worms would swim in a certain direction through the jelly they grow them in when they’re in England. But if you send them to Australia, to Adelaide where they did these experiments, the worms will swim in the opposite direction.

And studying the worms, it turns out that there is a set of nerve cells at the back of the worm which are very long in one direction but very narrow in the other axis. And they think they behave a bit like an antennae, which means they might be sensitive to the inclination of the Earth’s magnetic field. Because, if you also do the experiment and grow these worms in an applied magnetic field, you can change this behaviour. And if you abolish those nerve cells that are in the worms they lose the ability to detect magnetic fields. So it looks like they have evolved a way independently to have their own magnetic antennae inside the worm, again, to find the right level for where they need to feed.

Richard - Yep, that’s very interesting. And the idea there is in the northern hemisphere the inclination of the Earth’s magnetic field is pointing down towards the ground, but if go into the southern hemisphere the Earth’s magnetic field is pointing up into the sky. So if they’re sensitive to that change in orientation then that might affect the way that they swim.

Chris - Be tricky for them if they went to Mars then. As we were hearing from Kathy earlier that Mars has lost its magnetic field, things like that just wouldn’t work would they?

理查德,火星确实有磁场的耳朵ly on in its history. And there are the infamous claims of evidence for these magnetotactic bacteria in the Allan Hills ALH84001 meteorite where people have found the magnetite nanocrystals which are remarkably similar to those that we find in Earth-based bacteria. But I should say, not everyone believes that theory.

Chris - And talking of people believing it or not, there are people also working on whether humans have these abilities, and we just ignore them because we have more dominant skills like our eyes and a GPS so we tend to suppress these other potentially latent magnetic-sensitive skills.

Richard - Yes. I know people who are working on that and they’re finding some interesting results. Unfortunately, I’m not allowed to talk about it because it’s top secret research, but we can say that there is some evidence pointing towards the fact that large primates, should we say, might be able to sense the magnetic field.