Biophysicists have long wondered how swimming micro-organisms co-ordinate the motions of their arms – or flagella – as they propel themselves through water. Now, researchers at the Delft University of Technology in the Netherlands have shown that the single-cell alga Chlamydomonas reinhardtii synchronizes its two flagella via internal fibres within the cell, rather than via interactions with the surrounding fluid – as had been previously thought.

Many bacteria and algae, and other tiny cells, possess whip-like appendages called flagella, which they use to propel themselves. Similar, shorter-moving filaments, known as cilia, are also found on the surface of many cells, which perform functions such as moving fluids and other particles over the cell. Flagella and cilia tend to synchronize their beating motions with one another. This co-ordinated beating is often vital to the role of these filaments, but the physical mechanism behind the synchronization in not well understood.

Synchronized breaststroke

Most research on synchronization has focused on Chlamydomonas reinhardtii, a single-cell green alga that swims with two flagella that beat in opposite directions and perform a breaststroke-like motion. Previous studies have suggested that the synchronization is driven by hydrodynamic coupling in the liquid surrounding the cell. The idea is that the flow generated by one flagellum acts on the other flagellum and vice versa, causing them to move in synchronicity. However, these studies are far from conclusive, and there is also some evidence that other mechanisms are involved.

To study the extent to which flagella respond to hydrodynamic forces, Daniel Tam and colleagues held individual C. reinhardtii at the tip of a pipette inside a flow chamber. The team then sloshed fluid back and forth across the alga and recorded the motion of the flagella.

The researchers found that while the breaststroke-like movement of the flagella can be controlled by external flow, the hydrodynamic forces needed for synchronization are greater than those produced by the beating flagella. The flagella did synchronize with the external flow, but only if the forcing frequency of the flow was very close to the natural beating frequency of the flagella. The intrinsic beating frequencies – the rates at which single flagellum beat in isolation – of the flagella of C. reinhardtii individuals, however, differ from each other by as much as 30%. The researchers calculate that to generate hydrodynamic forces large enough to drive one another at such frequency differences, the flagella would need to produce flows 30 times the natural swimming speed of the algae.

Internal connection

From these observations, the team was able to conclude that synchronization is caused by fibres within the cell. "Chlamydomonas doesn't react as strongly to flows as we would have expected," Tam explains, which means it is "very likely that hydrodynamic interaction does not play a role in the synchronization". He cautions that this does not rule out the idea that hydrodynamic flows drive synchronization in other micro-organisms.

As for how C. reinhardtii is able to perform the breaststrokes, Tam and colleagues suspect that synchronization occurs via a contractile fibre – the distal striated fibre – that connects the two flagella. This had been suggested previously but never tested. Tam and his team investigated the significance of this intracellular coupling using a mutant strain of C. reinhardtii that has defects in the contractile fibre – the vfl3 mutant. They found that the two flagella in mutant algae always beat in an asynchronous fashion.

The study is described in Physical Review Letters. In a review that accompanies the letter, Marco Polin of the University of Warwick in the UK and Idan Tuval of the Mediterranean Institute for Advanced Studies in Spain write that "The results, which the authors unfortunately only describe qualitatively, are clear: without the fibres, the flagella fail to synchronize. These experiments are promising, and unequivocally point to the importance of intracellular mechanical coupling."

Raymond Goldstein and colleagues at the University of Cambridge in the UK have completed a comprehensive study of the nature of synchrony and the significance of intracellular connections in C. reinhardtii and other related organisms, with a preprint available on the arXiv server. Goldstein told that they found "the vfl mutants can display synchrony if the flagella are close enough together – it varies how close they are cell by cell – exactly as you would expect from hydrodynamic coupling", but this involves a different front-crawl-like swimming motion. This, he explains, "is evidence that the two processes, hydrodynamics and internal coupling, compete" to determine the form of flagella synchronization that is observed.