Not many organisms would choose to live on a wave-swept shore, yet many species of seaweeds cling to the rocks, battered by thousands of waves a day. A particular section of the shore, the marine intertidal zone, is especially hostile since as the tide goes in and out, it is exposed to the air and submerged underwater every day. Here the waves are strongest, and water velocities up to 35 m s-1 subject seaweeds to `truly stupendous forces,' says Katharine Mach of the Hopkins Marine Station at Stanford University. Wondering how organisms survive in this environment, Mach and her colleagues wanted to find out if repeated poundings by the waves would eventually lead to seaweeds breaking due to material fatigue.
To find out what forces a seaweed can take, scientists previously used standard `pull to break tests'; which involve fastening the seaweed in a clamp and measuring how hard they had to pull to break it. Comparing these forces with the forces exerted by waves in the field led them to predict that seaweeds shouldn't be breaking. But seaweeds encounter many waves in one day, not just one big one, and it's also clear from the piles of seaweed washed up on the shore after a big storm that they break frequently. To try and close the gap between predictions and what is actually happening, Mach took a new approach and turned to engineering theory, specifically fracture mechanics (p. 2213). `It describes how materials cope in the presence of cracks and under fatigue, also how cracks form and then propagate before the material finally breaks,' she explains, `[we're] using engineering theory and applying it to understanding how all the forces that a seaweed experiences in one day will affect its breakage'. First the team tackled the engineering literature, bringing together theories developed for a variety of materials, especially rubbers, and putting them into a biological context. `We wanted to standardise terms and make it something that [biologists] can use,' says Mach.
Having worked out which fracture mechanics theories were best suited to squashy biological tissues, the team chose four seaweeds to study: two species of Mazzaella, and one each of Porphyra and Ulva (p. 2231). `They were a good place to start because they are relatively flat and simple, and can be considered as 2D structures, which makes things simpler from the modelling point of view', Mach explains.
The team cut sections of seaweed and placed them in grips each connected to an arm in a metal frame. Moving the top arm pulled the seaweed: the sample's change in length relative to its starting length told them how much strain it was under. The fixed bottom arm measured the forces acting on the seaweed.
Using the standard `pull to break tests', the team tugged on the seaweeds with large continuous forces until they broke. They did this with complete samples and samples with small cracks, to measure the maximum stresses and strains the seaweeds were under, and then used their model to calculate the rate at which released strain energy contributed to making the cracks bigger. Mach explains that how cracks move through a material depends on the properties of the material; for example if it is elastic, or if it is stiff. As the crack grows, a material also releases energy as the area around the crack relaxes slightly, and this released energy goes into making the crack bigger.
Having measured the maximum stresses and strains the seaweeds could take, they next looked at how the seaweeds responded to smaller forces applied thousands of times over. Using a telemicroscope, they periodically scrutinised the progress of cracks moving through the seaweed pieces.
The team found that the seaweeds behaved differently. Mazzaella was the most robust of the three, tolerating cracks that tended to grow slowly in the material. It lives in the most exposed environment, so not only is it more likely to get damaged but also has to be better able to withstand damage once it has occurred. `Ulva was the hardest to study,' says Mach. Either it didn't crack at all or the cracks shot through the material, suggesting that once damaged, Ulva doesn't last long if struck by a large wave.
By knowing how strain energy is released under the different conditions, the team then predicted how long seaweeds would last in the field when subjected to wave forces. Using wave speeds and water forces measured from Mazzaella's environment they calculated that wave speeds over 8 m s-1 would lead to accumulated damage. Depending on how many waves a day reach that speed, many seaweeds would only last hours or days if they were damaged and subjected to a relentless pummelling. Next, it is back to the shoreline, taking measurements of wave forces to better predict how long a seaweed would last in the field.
- © The Company of Biologists Limited 2007