Diatoms (pronounced di-a-TOMs) may be humble single-celled algae, but they have been around for at least 75 million years, can be found in every environment where there's light and moisture, come in up to 200,000 species and form roughly 25% of the world's biomass. What's more, they could form the basis for a new branch of nanotechnology. That's because the creatures grow themselves amorphous silica shells that end up outside their cells and often have complex and beautiful shapes.
"The fundamental problem diatoms might solve is that nowadays nanotechnology is practised 99.9% by lithography techniques, where you make layer after layer," explained Gordon. "If you want to try to do something three-dimensionally you have to dissolve layers and so on. Diatoms make 3D structures by themselves."
Diatom shells consist of a nanoporous assembly of silica particles about 10 nm in diameter, and sport mesoscale (around 100 nm to 1 µm) features such as ridges, protuberances and pores. The diatoms themselves range in size from less than one micron to more than 100 microns, and each species has a unique shell design.
If technologists could breed diatoms that grow shells in the shapes they require, they could use them as the basis for a new manufacturing technique. "The most exciting thing to me was produced by Ken Sandhage of Ohio State University," added Gordon. "He's done something almost like the replicator on Star Trek - the device that makes anything you want by assembling atoms."
Sandhage and his team put diatom shells in an atmosphere of magnesium at 900°C for four hours. The result was tiny particles that looked just like diatoms except they weren't made of silica, they were magnesium oxide. "He did an atom for atom substitution of magnesium for the silicon," explained Gordon. And the same technique could work for about a dozen reactions, producing diatom-shaped structures made of ceramics, ceramic/metal complexes or various metal oxides.
"If you can get the organism to build a structure and then change it to the material you need, it might be a better, more modular way of approaching the Star Trek replicator 'placing atoms where you want' idea," added Gordon.
Since diatoms have a cell cycle of about one day, you can double their numbers overnight. So once researchers can get diatoms to make a component they want, they will be able to get any number of copies just by growing them up in a batch culture.
Shaping up
But there's still some work to be done on getting the diatoms to grow into the desired shapes. "Nobody really knows what you can get at this point," explained Gordon. "There has been no attempt that I'm aware of to try to mutate them like you would with a domestic animal - that is, do selective breeding - let alone any genetic engineering."
A few years ago, Gordon himself proposed a technique for directing diatoms to build designated structures. He dubbed the method the compustat. "It's basically a selective breeding and culling of micro-organisms based on shape," he explained. "The idea was to do it visually by growing diatoms in a chamber with an image coming through a microscope to a computer. The computer would find each cell in turn and compare it with an ideal that you'd like it to achieve. If the cell was anywhere near that ideal you'd leave it alone and if it wasn't, you'd kill it by zapping it with a laser."
So what could these diatom shapes be used for? "The field's so new it's really just at the ideas stage," said Gordon. "Whether anyone will succeed in building anything that's of real engineering importance is not yet obvious, to me at least. But scientists wouldn't be doing the work if they didn't think they could get there."
Specially tailored diatoms could have applications in making devices such as optical gratings, sensors, filters and membranes, high-surface-area catalysts, masks for lithographic patterning, structural composites and possibly even to encapsulate drugs for delivery to the body, although this would need careful investigation as to whether it would be safe - inhaling silica has been found to produce a disease known as silicosis. Some researchers, meanwhile, are taking a slightly different tack and using diatom enzymes to encourage silica deposition on surfaces for holographic applications.
But what if these specially tailored diatoms should run amok and self-replicate wildly, in the same way that the swarms of nanoparticles did in Michael Crichton's recent book Prey? "They're plants, all you'd have to do is turn out the lights," explained Gordon, half reassuringly. "If you're only modifying them in terms of their shapes, which is what the major interest in nanotechnology would be, I find it a little difficult to figure out how they could harm anything if they got released, including other diatoms."
That said, "if you introduce other characteristics at a molecular level then you're in the same business as genetic engineering any micro-organisms or crop plant," Gordon continued. "If the genes can propagate better than the natural ones then you might indeed change them and what the effect would be is hard to tell. Diatoms are at the bottom of the food chain - they're everywhere and they're eaten by everyone."
Meeting the future
There are around 13 groups worldwide working on aspects of diatom nanotechnology, including researchers in the US, Canada, UK, Italy, the Netherlands and Germany. Along with co-organizers Ken Sandhage and diatomist Frithjof Sterrenburg, Gordon hopes to attract at least one researcher from each group to the diatom nanotechnology meeting.
The meeting will take place at the North American Diatom Society conference, where there will be diatomists, cell biologists, taxonomists and researchers specializing in areas such as water quality analysis, stratigraphy - diatom fossil records are used in predicting the presence of oil - or forensics, as diatoms in the lungs can indicate whether a person found dead in water actually drowned.
"It's a unique opportunity to get industrial people together with those kinds of people and see what happens," said Gordon. "We're trying to make sure that everybody explains their jargon so that some communication does occur!"
So let's hope to see some exciting diatom nanotechnology developments soon.