At first glance, you might think that a well-stirred vat of Chlamydomonas augustae algae was simply a suspension of inert green goo; but you'd be wrong. Swimming away from bright light, toward dim illumination and against gravity, the algae eventually set up bioconvection flows. They form intricate swirling patterns that allow light to penetrate deep in the suspension and promote nutrient and gas mixing to keep the algae well supplied. But no one had systematically investigated the effect that different light intensities and illumination orientation had on the algal distributions. Rosie Williams and Martin Bees from the University of Glasgow explain that finding optimal algal growth conditions is essential for industries that hope to exploit algae for biofuels and vegetable oil production. So, they decided to illuminate algae to find out how light influences the distribution of algal suspensions (p. 2398).

Varying the concentration of algae exposed to white light and analysing the mottled distributions, the team saw that the more concentrated algal suspensions formed patterns more quickly than the dilute samples. Also, the pattern formed by the concentrated algae was much finer and more tightly packed than the relatively diffuse pattern formed by the dilute algae. In addition, the duo found that the algae did not respond at all to red light, producing patterns on the same scale regardless of the light's intensity. ‘This lack of response implies that illumination by red light is equivalent to practically no illumination,’ say Williams and Bees.

Focusing on the effect of white light intensity on the algal distribution, the duo illuminated algal suspensions from below with light intensities ranging from a dim 645 lx to a bright 4780 lx. They saw that the patterns became tighter as the intensity increased to 2020 lx. Then, as the intensity increased to 2710 lx, the pattern's distribution became more sparse and, although each pattern was distinct, they all reformed on the same scale when the experiments were repeated at the strongest light intensities.

However, when the duo illuminated the algae from above, the algae behaved differently. This time the pattern became more diffuse as the intensity rose from 645 lx to 1330 lx, before tightening again as the illumination became brighter.

‘To explain these results, we recall that there is a competition between bottom-heavy induced upswimming (gravitaxis), gyrotaxis due to viscous and gravitational torques and phototaxis towards/away from weak/bright light, distinguished by the critical light intensity,’ the duo says. They also add that their observations agree qualitatively with theoretical predictions, although they admit that making comparisons between theoretical and experimental observations is difficult. Ultimately, Williams and Bees hope to be able to refine their measurements to obtain better estimates of the critical light intensitys at which the algae are no longer attracted to or repelled by light.

C. R.
M. A.
A tale of three taxes: photo-gyro-gravitactic bioconvection
J. Exp. Biol.