A microscopic predator-prey chase

In terms of behaviour, animals have plants beat – though some would argue that plants have their own brand of intelligence.

Not all photosynthesizing beasts are firmly planted, though, and many that live in the water can move. Aquatic algae, for instance, often have whip-like structures (called cilia and flagella) that they can use to propel themselves along in the water. Some land plants also produce flagellated sperm that can move on their own volition.

H. akashiwo

A single-celled marine algae with flagella for getting around. From Wikimedia.

In the ocean, the ability to move can be beneficial, allowing algal cells to find food or move to a suitable environment. Motile cells can also avoid their predators by swimming away – something land plants definitely cannot do. Swimming algae incredibly slow, topping out at about half a centimetre per minute – but a new study suggests that the slow race between algae and their predators might be responsible for a far bigger, more dangerous phenomenon.

Elizabeth Harvey and Susanne Menden-Deuer – two researchers at the University of Rhode Island – studied the movement patterns of Heterosigma akashiwo, a marine algae with a worldwide distribution1. The “akashiwo” part of the name comes from the Japanese word for red tide, or the colloquial name for dramatic events where blooming red algae populations colour the sea blood-red. When these algae are toxic, the red tide events can wreak havoc on coastal ecosystems, killing large numbers of birds, fish and sometimes even humans. Blooms of H. akashiwo algae have been linked to several major fish kills in the past 20 years, including the loss of thousands of farmed salmon2.

Harvey and Menden-Deuer watched H. akashiwo on the run in the lab. They introduced the algal cells into a narrow vertical tank designed to mimic the salinity and flow of a natural river estuary. Then, after a few minutes, they let zooplankton predators in after them. In a separate experiment, the researchers also tested how the algal cells responded to water from tanks where the predators were held, since this water would contain traces of chemicals that the algae could use to senseĀ approaching danger.

In both experiments, the H. akashiwo cells took off, moving nearly 40% faster than their regular speed, and fleeing to a low-salinity zone near the water surface. The algae also kept up the pace long after the predators had been removed – something the researchers can’t yet explain.

This increase in pace also led to a staggering 3-fold increase in the algal population growth rate in the experimental tanks. Harvey and Menden-Deuer used a mathematical model to extend their results to a larger scale, and they found that this population boom could easily lead to a massive red tide event. What’s more, these events only occurred when they incorporated the ability to flee from predators into the mathematical model.

By linking individual and population dynamics, Harvey and Menden-Deuer show that individual movements might be responsible for large-scale fluctuations in algal populations. These results are important if we want to prevent toxic algal blooms – a major source of ecological and economic damage in coastal regions. But they also remind us that complex phenomena can arise from the collective behaviour of a large number of individuals – and understanding how this happens could help us solve an important environmental problem.

Further Reading

  1. Harvey and Menden-Deuer. 2012. PLoS ONE. e46438.
  2. Hard et al. 2000. J of Fish Biology 56: 1387-1397.