Both Point Down

In 1975, Richard Blakemore noticed something unusual in a sample of pond sediment: bacteria were swimming toward the north end of the microscope slide. When he flipped the magnet, they reversed. Under electron microscopy, he found chains of magnetic crystals inside the cells — each crystal a single magnetic domain, the whole chain acting as a bar magnet, aligning the bacterium with Earth's field.

He named them magnetotactic bacteria. The name described exactly what he saw.

What took longer to understand was what they were actually doing.

The bacteria live in the zone where oxygenated water meets anoxic sediment — a thin layer where oxygen concentration is neither too high (damaging to their chemistry) nor too low (useless for respiration). The challenge is finding that layer and staying in it. Too far up and the oxygen kills them. Too far down and they can't breathe. The layer is thin, and the search, without any shortcut, is three-dimensional.

The shortcut: Earth's magnetic field isn't horizontal. It's inclined — the field lines angle downward as you move toward the poles. A bacterium aligned with the field and swimming along it is, as a consequence, also swimming at an angle toward the sediment. What could be a 3D search collapses to 1D: just follow the field, and let the inclination do the rest.

Northern hemisphere bacteria are north-seeking. Southern hemisphere bacteria are south-seeking. The populations have opposite magnetic polarity — not because north and south mean something different biologically, but because the field lines angle downward in different compass directions depending on hemisphere. Both populations are, in functional terms, seeking the same thing: gravitational down, toward the sediment, toward the thin oxygen-optimal layer. "North-seeking" and "south-seeking" describe the compass direction. The actual target is vertical.

The experiment that reveals this: take northern hemisphere bacteria south of the equator. The same magnetosomes, the same polarity, now pull them upward — away from sediment, toward lethal oxygen. They would die in the wrong hemisphere. Evolution in the southern hemisphere selected against that polarity, not because "south-seeking" is inherently correct but because south happens to point down there. Both populations are doing identical things. The hemisphere difference is what makes the distinction visible.

From inside a single hemisphere, you can't tell whether the bacteria have a compass or a depth gauge. The two hypotheses make the same predictions about everything you can observe locally. Only the hemisphere experiment breaks the symmetry — and only if you know to ask not "which direction do they swim" but "which direction, relative to gravity, does that correspond to."

The name "magnetotaxis" is still accurate: they do orient to magnetic fields. But it describes the mechanism at the point of observation, before the function was known. The function turned out to be aerotaxis — navigating oxygen gradients — with a magnetic geometric shortcut. Both words are in use now: "magneto-aerotaxis" appears in the literature alongside the older "magnetotaxis." But the shorter name stuck, and names shape questions.

"Magnetotaxis" points you toward other compass-users: migratory birds, fish, turtles. "Aerotaxis with a gravity proxy" points you toward other organisms solving the same gradient-finding problem through different shortcuts. These are different comparisons, and they surface different questions. I don't know how much the framing has mattered. The underlying phenomenon is the same either way.

What I keep thinking about: the hemisphere difference is a natural experiment, but only interpretable in retrospect. Blakemore discovered north-seeking bacteria in Massachusetts. Southern hemisphere populations were found later and described as anomalous — reversed, opposite — until the question shifted from "why are these backwards" to "what are both of them actually tracking." The reversal was the clue. Identifying it as a clue required already suspecting that north and south weren't the right categories.

The bacteria don't know they're north-seeking or south-seeking. That distinction is only visible from outside, using the hemisphere as a variable. From inside — from the bacterium's actual operational situation — there is only the field and the gradient and the thin layer where the chemistry works. The compass heading is incidental. Both populations point toward the same thing.