Plankton, bacteria, algae, protozoa: many microorganisms inhabit the seas, lakes or even our intestines. Everyone, moving – usually with one or more flagella – creates currents that influence the movements of its congeners, sometimes to the point of synchronization. Far from trivial, collective motions thus generated play an important role in the survival and growth of settlements, facilitating access to nutrients and oxygen in the oceans, including the mixing produced by plankton and microalgae promote the absorption of carbon dioxide by marine phytoplankton production and oxygen that results.

If physics describes propel a microorganism in a fluid, little is known about how the movements of each micro-organism generate collective behavior. By measuring the currents produced by videomicroscopy by microalgae in motion, two independent teams have found that the influence of a microorganism on its environment was more complex than previously thought.

The first team of the University of Cambridge, England, measured the average speed of micrometric beads suspended in a liquid containing two micro-algae very different: either Volvox carteri, Small algae spherical diameter of 400 micrometers, which moves through a thousand small cells form flagella distributed over its surface, or Chlamydomonas reinhardtii, A unicellular green alga 10 micrometers in diameter swimming breaststroke with two flagella. We thought that swimming, a micro-organism gave rise to a small local deformation of the current that faded quickly with distance. It turns out that not only this model is not appropriate, but the currents vary with the micro-organism. Volvox carteri firstly modifies the fluid velocity in a larger radius and on the other hand, has a stronger effect than expected at very short distance; Chlamydomonas reinhardtii, In turn, creates vortices on the sides and a stream in front of her.

The second team, Haverford College, United States, studied using a high-speed camera speed ball around micrometer Chlamydomonas reinhardtii during a single movement of breaststroke, in a space flat enough to force the algae to move in two dimensions: each movement of breaststroke, not only the algae produces eddies and current described by the Cambridge team but, by reducing its flagella along its walls, these vortices and the reverse flow direction.

If we are still far from understanding the collective behavior of microorganisms, the first brick is laid: computer modeling, the two teams showed that the fluid mechanics correctly describes the observed behavior. The next step is to use these results to model the disturbances of fluid due to several micro-organisms. And expand research to other structures, such as bacteria or plankton …