Bull kelp (Nereocystis luetkeana) is a marine macroalga that resembles a vine-like plant with a long, thin stipe (stem-like structure) up to 30 meters long, anchored to the sea floor by a holdfast (root-like structure). A gas-filled float at the stipe’s other end holds numerous photosynthetic blades close to the water’s surface. Although it helps expose the blades to sunlight, the buoyant float prevents the kelp from flattening against the seabed, thereby exposing the stipe to pulling forces from tidal currents, waves and surface chop. If the mechanical force exerted by fluid flow exceeds the breaking strength of a kelp stipe or holdfast, the kelp can break away and potentially die.
When pulled, the kelp stipe breaks at stresses (force per cross-sectional area) lower than other biomaterials such as wood and insect cuticle; however, it compensates for this low breaking strength by being stretchy. A bull kelp stipe can be stretched 40% percent of its length before breaking, absorbing energy as it stretches. Thus, kelp requires about the same amount of energy to break as wood or insect cuticle does, although it resists breaking through being extensible rather than being strong and stiff. What enables the bull kelp stipe to be so stretchy?
The stipe has an inner cortex made of cylindrical cells that bear most of the pulling (tensile) force. Similar to other plants, these cells are wound with a strong and inextensible cellulose fiber. The fibers wrap around in both left- and right-handed helices, creating an array that prevents cells from becoming too long and thin when pulled or too short and wide when compressed. This protects the cells from rupturing. The amount of shape change permitted depends on the angle between the fiber and the cell’s long axis: the smaller the fiber angle, the greater the cell width can change for any change in the cell’s length. Vascular plants typically have an average fiber angle of 20˚, whereas bull kelp has an average fiber angle of 60˚. This means that the cells can stretch considerably along their length with relatively small changes in width. This large fiber angle plays an important role in the high extensibility of bull kelp.
“The mean work per volume required to break Nereocystis stipes was 0.67MJ m^-3 (SD = 0.40, n = 6) which is similar to that of wood, bone, insect cuticle, and cast iron (Wainwright et al. 1976). Hence, Nereocystis stipes resist breakage by being stretchy rather than by having high breaking stress.” (Koehl and Wainwright. 1977:1068)
“…the stipe can absorb as much energy before breaking as can wood or bone because of its high extensibility. We suggest that the high extensibility is allowed by the crossed helical array at 60˚ to the stipe axis of cellulose fibrils embedded in a viscoelastic gel matrix in the cell walls of the cortical tissue.” (Koehl and Wainwright. 1977:1071)