A microscopic speck of green algae can
trot like a horse. Or gallop. Biophysicist Kirsty Wan  compares the gaits of creatures large and

Moving diagonally opposite limbs, or flagella
in this case, in unison —
that’s a trot, Wan says. Her lab, at the University of Exeter in England, is
working on the conundrum of how single-celled creatures, with no nervous system
or brain, coordinate “limbs” to create various gaits. Some of those movements
get far trickier than trots and gallops.

Her work echoes that of 19th century
photographer Eadweard Muybridge, who used a then-novel imaging technique to
reveal hoof positions obscured in the blur of a horse galloping. Wan now creates
Muybridge moments for microalgae.
Using a range of microscopy analytics on what she calls “my private collection
of weird algae,” Wan and colleagues have documented microalgae that coordinate from
four to 16 flagella.

In some four-limbed cells, flagella can move
in neighborly pairs, pulling back in a sort of double-vision breaststroke. To
these microscopic critters, water feels thicker than the splashy stuff that giant
humans easily swish aside. So the algal breaststroke has little glide. It’s more
like a slog through molasses.

This single-celled alga activates its four flagella in the pattern used by a galloping horse. Then something not obvious to a human startles the alga (Carteria crucifera) into retreat. The cell pulls itself together though, and presses on.

Wan looked hard for microalgae with
eight flagella and found three species. One, Pyramimonus octopus, has a gait unlike any Muybridge ever saw. Wan
calls it rotary breaststroke. Flagella across from each other in the array of eight
will curl in for the stroke as their neighbors are uncurling a few beats behind.

is a twitchy microbe that
goes through “shocks,” Wan says. An alga swims along, then “like a knee-jerk
reaction,” it changes direction, though she can’t see what spooks it. In
comparison, when she watches a two-flagella Chlamydomonas
species, “sometimes it twirls; sometimes it spins,” but there’s nothing so dramatic
as the abrupt pullback.

The single-celled Arctic alga Pyramimonus octopus coordinates eight flagella when it swims. Even when tethered in place, the cell’s opposite pairs curl and uncurl in what biophysicist Kirsty Wan, of the University of Exeter in England, calls a rotary breaststroke. 

The trickiest example she’s seen may have been lost to science. Wan once grew the Arctic’s P. cyrtoptera, the only microalgal species she knows of with an array of 16 flagella. Sometimes opposite pairs of flagella stroke in unison as the motion ripples around and around the array in a gait she calls a “wave.” Her colony died, however, and so did her supplier’s. “I hope it still exists somewhere in the world,” she says. “Otherwise, I might have … taken the last footage.”