In one of our first-year biology labs the students spend a bit of time looking down the microscope at various algae & protozoa. Some of their samples come from a container of interestingly weedy water from my fishpond. Not only is the pond covered with duckweed & Elodea, but it turns out to have a wide range of tiny unicellular plants & animals, & some not quite so tiny, such as Volvox.
Actually that pond has a whole thriving ecosystem. We must be doing something right, because the goldfish keep breeding like, well, goldfish, & every year we see the split husks of mayfly & dragonfly nymphs, adhering to the reeds where the animals climbed to make the final moult into their adult forms. And I suspect that either the nymphs, or the bigger goldfish, eat a lot of newly-hatched little fish, because at first we see large numbers of them - little bigger than animated eyelashes - but then each year we end up with just 1-2 new additions to the goldfish family. But I digress...
Volvox is a colonial green alga. Someone in the class will almost always spot one, bumbling relatively slowly across their slide in the company of Paramoecium, Euglena, Spirogyra, & other members of that microcosmic world. However, Volvox can grow up to 2mm across & dwarfs the other organisms swimming with it.
An individual Volvox is a hollow ball of cells, interconnected by strands of cytoplasm. (Apparently you can sometimes find things like rotifers going along for the ride, living within the ball.) The individual cells are often described as 'Chlamydomonas-like' (Ueki et al. 2010), as they are very similar in appearance to the unicellular alga Chlamydomonas, including the presence of a light-sensitive 'eyespot' & a pair of flagella.
Now, the presence of the flagella leads to an interesting question. Like Chlamydomonas, Volvox is motile, moving around as a result of the beating of all those whip-like flagella. Which makes a lot of sense, as the ability to move towards a light source would give a considerable adaptive advantage to a green alga, which needs light in order to photosynthesise. But for this to happen the beating of all those flagella (several thousand of them, in the bigger organisms) must be coordinated. How is this achieved, in an organism that's basically a ball of cells?
Ueki et al. (2010) studied Volvox rousseletti in an attempt to answer this question, by exposing the organisms to light stimuli & looking to see what happened in terms of flagellar action & the way in which individual cells responded to light. It seems that Volvox, despite the appearance of a homogeneous ball of cells, actually has a recognisable anterior & posterior end, or 'pole'. The researchers found that the 'beat frequency' of flagella changed when a Volvox was exposed to light, & in addition the 'effective stroke' - that is, the stroke causing movement in a particular direction - was reversed,
What's more, they found that the front (anterior) half of the organism was more responsive to light than the posterior half, such that only the anterior cells responded to light in a way that changed its pattern of movement. This could be related to the size of those light-sensitive eyespots: bigger on the anterior half, grading to either tiny, or absent altogether, at the posterior pole.
How does all this work in a way that sees Volvox show positive phototaxis, consistently moving towards a light source? Well, there's a tendency for these balls of cells (the authors call them 'spheroids') to rotate gently as they move through the water, especially when they're not exposed to a directional light source. This is because, on any given cell, both flagella beat in the same direction, towards the posterior pole, & addition they 'beat in parallel planes pushing the [Vollvox] in the posterior-anterior direction' (Ueki et al. 2010). Overall, the effect is to propel the organism along in a generally forward direction while at the same time rotating gently on its axis - it actually looks as if it's rolling along, which is where the Latin name comes from.. However, if this rotation turns the anterior half towards a point source of light, the flagella on those illuminated anterior cells reverse the direction of their beat. The result: the direction of rotation is reversed. This brings other anterior cells into the light & their flagella in turn reverse their direction of beat, Meanwhile the posterior cells just keep right on beating, & the overall effect of this is a slightly erratic movement towards the light. (Just think of the complicated lighting, camera, & microscopy setup needed to capture all this!)
Reading this paper has made me view those little green pond-wanderers in quite a different light, a view I'll have to share with next year's algal-lab class :-)
Ueki, N., Matsunaga, S., Inouye, I., & Hallmann, A. (2010). How 5000 independent rowers coordinate their strokes in order to row into the sunlight: Phototaxis in the multicellular green alga Volvox BMC Biology, 8 (1) DOI: 10.1186/1741-7007-8-103