Did you know that the United Nations has declared 2014 the International Year of Crystallography? You might be thinking, “Wait a minute, I’m on an Invertebrate Zoology Blog. What do copepods, mollusks, and squids have to do with crystals?” To begin, each of those organisms actually possesses crystalline or semi-crystalline structures. For example, the armored exoskeleton of the copepod is composed of chitin and calcium; the shell of the mollusk is made of calcium carbonate crystals; and the beak of the squid is fabricated from a gradated, chitin-based composite. In contrast, the crystalline structures of vertebrates are, arguably, quite boring! That is, all vertebrate teeth and bones are composed of the same mineral, carbonated hydroxyapatite.
Unlike the vertebrates, diversity in composition and structure is a feature of the marine invertebrates. In this series, we’ll investigate some intriguing crystalline and semi-crystalline structures and tissues in different groups. We’ll also visit a bit of the modern crystallography research related to our organisms of interest, in addition to learning why other fields of science and engineering are interested in these marine organisms.
We kick off our series with diatoms, members of the phytoplankton (important photosynthesizers in the sea)! Diatoms come in a variety of geometrically stunning shapes, generally with bilateral symmetry, and they also serve as a major food source for many invertebrates, such as the copepods, mollusks, and squids referenced above. As delicious as diatoms are to filter feeders, they are also unlike other algae: diatoms grow into beautiful, complex shapes by constructing cell walls of hydrated silica (SiO2). The composition of the diatom skeleton was first identified in the mid-20th century with X-ray crystallographic investigations.
Although diatoms, as part of the phytoplankton, are a crucially important aspect of invertebrate zoology, they are also studied in many other fields of science and engineering. For example, diatoms represent an important component of Earth’s fossil history, with the earliest evidence of diatoms in the fossil record dating to the Jurassic period (201 – 45 Mya). Investigations of ocean cores and land rocks have led to the development of extensive marine records of diatoms, from which biostratigraphic records have been produced. Climate scientists also study these unicellular organisms: diatom communities are important indicators of water quality and other environmental conditions. Materials scientists have investigated diatoms as potential sources of high-purity silicon, for use in microchip fabrication technologies, and diatoms are regularly used in filter materials (“diatomaceous earth”). Finally, more recent analyses of diatoms using X-ray crystallography have focused on specific proteins, such as iron-storing ferritins and the anti-freeze proteins present in polar diatom species.
In short, diatoms are an excellent example of a semi-crystalline structure in a marine organism! And crystallography techniques have contributed significantly to our understanding of these visually striking, microscopic phytoplankton.
Next in this series of posts associated with the International Year of Crystallography, we’ll continue our exploration of the crystalline world of marine invertebrates by investigating the nautilus (Cephalopoda: Nautiloidea).
by Liz Boatman
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