Tumbled around in the surging surf, many types of seaweed have opted for a flexible approach that allows them to deform and go with the flow. However, some species have armoured themselves with rigid calcium-impregnated cells for protection; ‘Which would seem to pose a real problem’, says Mark Denny from the Hopkins Marine Station of Stanford University, USA. He explains that the otherwise inflexible algae have overcome their paradoxical rigidity by evolving joints – known as genicula – between the calcified segments, which are built from a remarkably resilient material that allows them to bend and sway. Explaining that the joint material is stronger, more extensible and more fatigue resistant than the tissue of other algal fronds and that the joints comprise long hexagonal cells that are anchored solely at each end to the adjoining segments – in much the same way as steel strands in suspension bridge cables – Denny and his colleague Felicia King set about reconciling what they knew about the structure of the joints at the cellular level with the molecular structure of the joints and their movements as they are wrenched around by the water.
Collecting samples of the pink Calliarthron cheilosporioides seaweed was simply a matter of scrambling down onto the rocky seashore beneath Denny's lab when the tide was low armed with a kitchen knife to prise the seaweed free from the shoreline. Once back in the lab, Denny and King embarked on a series of laborious experiments where they tested the joint material's responses to the kinds of extreme stresses and strains that they experience routinely as the waves tug at them repeatedly.
Explaining that most biological materials are viscoelastic – that is they are partially elastic, allowing them to snap back into shape after bending and deforming, and partially viscous, allowing them to stretch and permanently deform – Denny and King were surprised to find that instead of becoming stiffer as the joints deformed (as most viscoelastic materials would), the seaweed's joints remained flexible and their breaking strain increased: ‘That is unusual’, says Denny. King also found that the joints were able to dissipate much of their energy, in addition to deforming and extending, as she continually tugged at them. And when the duo calculated the amount that the joint at the base of the algae deformed over the course of its 6 year life, they found that it extended by only 36%, which is well within its limits – they only break when extended to twice their original length – ‘So it's safe’, says Denny.
Having discovered how resilient the joints are, Denny and King set about building a mathematical model in a bid to understand their performance. ‘Springs and dashpots [shock absorbers] are the basic building blocks when constructing mathematical models of materials’, says Denny, who admits that assembling the components in a way that reproduced the joint material's performance was challenging. However, the duo eventually came up with a relatively simple arrangement – a side-by-side spring and shock absorber, attached to a second spring inline with a shock absorber. Estimating the stiffness of the two springs and the viscosity of the two model shock absorbers from their earlier measurements, the duo was impressed when their model effectively mimicked the behaviour of the joints in real life. ‘We were quite surprised that such a simple model could do such a good job of predicting the material's behaviour’, says Denny, adding, ‘If a simple model works well, it's usually a good sign that you've captured the important aspects of a system’.
Interpreting their measurements in terms of the molecular structure of the joint cells, the duo suspects that the cellulose fibres in the genicular cells perform like stretchy springs, while the two shock absorber components of the model simulate the intrinsic viscosity of the long flexible cells and the viscosity as the cellulose fibres slide past each other in the cell wall, shearing the gel-like material in which they are embedded. And Denny is optimistic that his model will help us to better understand how shoreline ecosystems might change in the face of increasing wave activity. ‘Our ability to predict when and where joints will break will allow us to predict how the “forest” in the low intertidal zone will change in response to any changes in the wave environment’, he says.