The world’s smallest compasses
An amazing discovery of how humble bacteria can sense direction
There is nothing, many outdoor enthusiasts agree, like the odours from mud at the bottom of ponds and marshes. To sniff these characteristic scents is to think of happy adventures like the time the canoe got stuck in the marsh. The mud, with its bad smell and consistency like glue, is merely incidental to the happy occasion. Most of us would rather not disturb these sediments, nor release the contained gases.
Some people, however, make careers out of studying organisms in smelly marsh mud. Richard P. Blakemore was such an individual. As a graduate student at the University of Massachusetts, his job in the summer of 1975 was to collect bacteria from the mud of brackish marshes along the Atlantic coast. After each expedition, having collected odoriferous samples of mud, he would return to the laboratory to grow cultures of the organisms in his samples.
One strain of bacteria reacted in a most unexpected way. No matter what the environmental conditions, these organisms persisted in swimming northward in the water so that they always accumulated at the northern edge of the droplet on the microscope slide. Mr Blakemore wondered if these organisms (about two-thousandths of a millimetre long and about a quarter as wide) were sensing magnetic north.
A bar magnet was placed on the microscope, thereby overriding the earth’s magnetic field. The bacteria did indeed change direction, swimming to the north end of the magnet. With this simple study, Mr Blakemore opened the door to a new field in biology: navigation of organisms by magnetic cues.1 There is evidence that not only a wide variety of bacteria, but also some algae, insects, slugs and pigeons may use sophisticated magnetic compasses.
Evolutionists generally do not like the idea that living creatures illustrate good ‘design’—because design suggests God, the omniscient Creator. They prefer to believe that organisms came about on their own, through processes which had no intelligent direction. They generally say organisms are ‘adapted’ rather than designed. Nevertheless, famous evolutionist Stephen Jay Gould proclaimed the new magnetic bacterium to be a ‘striking example of good design: an organism that builds an exquisite machine within its own body. The machine is a magnet; the organism a “lowly”? bacterium.’2
Cultures from more than a dozen types of bacteria have, since 1975, been found to display magnetic responses. Muds from both freshwater and marine environments, as well as sewage treatment ponds, have included such talented microorganisms.
What amazing machines magnetic bacteria have been shown to contain. The magnet in these tiny cells is made up of 20 or so roughly cubic particles arranged in a line along the long axis of the cell. Each particle is about 50 nm (nanometres) on each side (a nm is 10-9 m or one millionth of a millimetre). These particles are not only the appropriate size for their host cells, but in physical terms they represent the only design that would work as a magnet on this small scale.
The particles are made of a type of iron oxide called magnetite or lodestone. In the laboratory it is difficult to make magnetite particles of this small size. Biologists are unsure how the bacteria achieve such precision.
Anyone who has looked through the microscope at tiny objects suspended in water cannot fail to notice how they are all in constant vibrating motion, even in living cells. This ‘Brownian’ motion results from random jostling of molecules, and the warmer they are, the faster they move.
The magnetite particles, however, must not be jostled by Brownian motion, otherwise there would be no magnetic orientation. Thus each particle must be able to orient itself firmly in the direction of the earth’s magnetic field.
Other than as interesting curiosities, what benefit could bacteria receive from the ability to swim in a northerly direction? Biologists believe that this ability may provide a mechanism to enable bacteria to orient themselves in space. Large organisms know where they are relative to earth’s surface because gravity provides the cue. This is much harder for bacteria, however, because of their small size—gravity is very weak in comparison to other forces like surface tension for example.
In the northern hemisphere, not only do magnets point in the direction of the north magnetic pole (on a horizontal axis) but the needle also dips down. The further north one is located, the steeper is the incline of the magnet’s needle. At the north pole the needle points straight down. In the southern hemisphere the needle points more steeply up, the further one moves to the south.
Scientists suspect that magnets in the northern hemisphere may indicate to these tiny cells which direction is down. In the southern hemisphere such magnets would indicate the opposite direction—up. At the equator, however, magnets are no help.
These bacteria are apparently using compasses to swim downward in the sediments. Most of these organisms can tolerate little or no oxygen in their environment. Generally there is a trend to lower oxygen levels as one moves down in the mud. The magnetic cue enables these organisms to move down, away from higher oxygen concentrations. This phenomenon represents an unusual and sophisticated solution to a common problem for microorganisms.
This is one of those wonders in nature which, only a few years ago, we could scarcely imagine.
The gifts and talents conferred on each creature, even the tiniest, fit their role in nature and their ecological requirements. How wonderful is their Creator!
Scientific American 245(6):42–49, December 1981. Return to text.
Natural History, 88:25, November 1979. Return to text.