The Diatom’s Shell

One of the most important physical features of a diatom is its unique cell wall, or frustule. Diatoms have a hard cell wall that is made almost completely from silica, a compound which also occurs in nature as quartz. The frustule is made up of two halves, or valves, that are arranged somewhat like a petri dish, with one half overlapping the other (Fig.1). The larger half that overlaps is older; the two halves are held together by a band known as the girdle.

Figure 1. Diatom showing the overlapping valves. (http://westerndiatoms.colorado.edu/images/page_images/15_Ellerbeckia.jpg)

Figure 1. Diatom showing the overlapping valves. (image source)

Diatoms are the producers in their food webs, and as such are eaten by many organisms. Any form of protection that improves survival rates for a prey species can be viewed as effective, and the mechanical protection that the frustule provides is a good example of this. Diatoms are more common than other algae with similar growth rates in algal blooms, because of the lower mortality rate associated with their frustule.

The strength of the frustule was tested in the lab using microscopic glass needles to press on the outer surface at known pressures until the frustule cracked. The frustules were shown to withstand a considerable amount of pressure, and even once the surface of the valve has cracked, more pressure is required before the frustule breaks. This process is similar to how bones fracture before they break. The girdle was shown to bend out of shape under pressure before cracking, if the pressure was removed whilst the girdle band was still intact then it regained its original shape. (Fig. 2)

(Hamm et al., 2003)

Figure 2. “Properties of an isolated girdle band. a, Sequence showing strong elastic deformation of a girdle band as a function of increasing force. b, girdle band deformed by a calibrated glass needle.” c, Field Emission Microscopy image of the girdle band comparing deformation with and without force (36 nN). Scale bars, 10 mm. (Hamm et al., 2003)

Two of the zooplankton species that feed on diatoms have been found to have very sharp, silica-edged ‘teeth’; this physical feature is likely to have co-evolved with the diatom’s frustule. If they are not crushed, then the diatoms can survive being eaten, and pass through their predator’s digestive system unharmed. Based on the amount of pressure needed to crack the diatom’s frustule, only a relatively large zooplankton would be able to produce the amount of force needed, and other, smaller, zooplankton have been found to choose other prey.

This suggests that the diatom’s frustule is a vital part of their success, as their mortality rates are improved by the mechanical protection provided. This mechanical protection limits the number of organisms capable of feeding on them.

Reference

Hamm, C., Merkel, R., Springer, O., Jurkojc, P., Maier, C., Prechtel, K. and Smetacek, V. (2003). Architecture and material properties of diatom shells provide effective mechanical protection. Nature, 421(6925), pp.841-843. (link – will need access to Nature to view)

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