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Diatoms, an Evolutionary Mystery, Come into Nano-Focus

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Photo credit: CSIRO, CC BY 3.0 , via Wikimedia Commons.

The other day a friend introduced us to a novelty among household conveniences — kitchen and bath mats made from diatomaceous earth, rich in fossil diatoms. Get water on the mat while washing the dishes or stepping out of the shower, and it dries almost immediately. One less reason to worry about dampness and mold, in case that’s a concern for you.

The world, indeed, should know more about the microbes called diatoms. When you hear the word “microbe,” what comes to mind? For most, it’s probably a misshapen amoeba or a disease germ. In fact “microbe” is a generic term that incorporates representatives from life’s three domains, Archaea, Bacteria, and Eukaryota — any single-celled microorganism. Lumping diatoms in with disease bacteria should be considered an affront. Sure, the bath mats are clever. If diatoms could be magnified to fit in the palm of your hand, though, they would be treasured gems, sparkling in a rainbow of colors and sold in jewelry stores. Or, at least at Target. If they could be magnified further, so as to tower over people, crowds of tourists would gather (masked and observing social distancing, of course) in their shadows, admiring the magnificent architecture. And to be told that our lives depend on these little modernist cathedrals — well, that would be almost too much for visitors to contemplate.

Take a Deep Breath

Scientists estimate that a fourth of the oxygen we breathe comes from diatoms. There are some 100,000 species of them, inhabiting the ocean, freshwater, and soil. They live almost everywhere. Diatoms are eukaryotes, a form of photosynthetic algae. Unlike other microbes, they build hard-shell houses to live in, composed of silica glass which they gather from the environment and cement with proteins. Because of their ubiquity, they play a major role in the earth’s silicon cycle. The shells, called frustules, fit onto each other like two parts of a pill box held together by a girdle. The variety of diatom shapes is staggering. Some frustules are round, some are rod-like, some are triangular, and some are even shaped like a five-pointed star. Their surfaces are decorated with intricate patterns. Humans did not know of their existence before the microscope was invented. 

Light microscopes since the 19th century disclosed something of their variety and beauty. Observers made drawings the best they could. As microscopes improved, and cameras joined them, the wonder of diatom variety and composition became clearer. One could imagine admiring a magnified one in the hand. In the late 1930s, electron microscopes ushered in a new era of detailed imaging. Now in the 21st century, the era of super-resolution microscopy and other techniques allow the full glory of diatoms to come into focus (see, “‘Resolution Revolution’: Intelligent Design, Now at the Atomic Level”). It’s as if we can walk inside them, looking up at the pillars and struts and artistic patterns high overhead.

The joy of discovery for nine microscopists from Poland and Germany, who wrote about this in Nature Scientific Reports, is palpable, not lessened at all by their charming broken English. 

The diatom shell is an example of complex siliceous structure which is a suitable model to demonstrate the process of digging into the third dimension using modern visualization techniques….

Nature is a source of complex materials which possess a wide range of complementary or synergistic properties. Multi-scale structures in biological organisms determine their behaviour, simultaneously this elegant perfection makes scientists awestruck. Beyond delightengineers make attempts to emulate solutions as well as designs invented by Nature in man-made innovations. A range of technological advances inspired by living organisms also known as biomimicry is broad and become widespread. There is no doubt that solutions presented by Nature are well ahead of every engineering material. [Emphasis added.]

Izabela Zgłobicka et al. remind readers that diatom shells are extremely hard for their size. They are also remarkably resistant to fractures, able to withstand hundreds of billions of pascals of pressure (one pascal is one kilogram per square meter squared). This fact would no doubt astonish the polymath Blaise Pascal (1623-1662), who investigated pressure, mathematics, theology, probability, and numerous other subjects.

The morphology of their siliceous shells (= frustules) is highly elaborated. Substantially, many details responsible for the properties of these structures, e.g. mechanical ones, may be observed by high-resolution imaging. According to the literature, the mechanical properties of the diatom frustule depends on the location. Experiments conducted on Navicula pelliculosa showed that the highest values of the elastic and hardness modulus, respectively up to hundreds of GPa and up to 12 GPa, have been obtained at the central part of the frustule. These results have been confirmed by Subhash et al. conducting investigations of hardness and fracture modes of the frustule of Coscinodiscus concinnus, higher values have been obtained in the central nodule which is solid. In addition to this, the fracture resistance also depends on the size of the diatoms. This values are regarded as remarkably high, for shells made of relatively soft bio-silica and discussed in terms of guarding cells inside frustules against predators.

A Question Arises

So, does protection from predators require this level of engineering and beauty? 

Let the reader visit this open-access paper and look at the figures. As the photographs proceed from light microscope images to the latest ones using EM, focused ion beam, and X-ray nano tomography, the comparison to cathedrals becomes apparent. There’s artistic design all the way in! 

“The natural architected materials show great variety in shape, morphology and structure,” they conclude. Building upscaled diatoms might be a worthwhile art project. Someone could make jewelry in the shape of diatoms. An architect could build a crystal palace based on both the support structures and the artistic patterns shown in Figures 3 to 5. The paper even hints at that suggestion in the last sentence: “The usage of these sophisticated visualization techniques, like XCT and TXM, have a special meaning of possible upscaling such biogenic based solution by 3D printing.” Great idea! And if builders could imitate the fracture-resistance and hardness of diatom material, there would be little risk of breakage. 

An Evolutionary Mystery

Back in 2012, Michael Gross wrote about “The Mysteries of the Diatoms” in Current Biology. Darwinians think that diatoms emerged suddenly in the Jurassic and then quickly diversified into the 100,000 forms we have today in a “short fraction of the geological timescale.” They believe this not because a progressive series has been found, but because diatoms are so successful, they must be somehow “fittest” for survival. Maybe endosymbiosis was involved, one theory goes. None of these speculations explain the high-quality craftsmanship in the frustules. Gross writes,

The silica frustules with their intricate nanoscale patterns can make any nanotechnologist jealous. Nature can produce such structures at ambient temperature and under benign conditions, an achievement that our technology cannot match yet.

Engineering Expertise

Other researchers have noted functional designs in diatom shells that suggest high-quality engineering. One theory is that the shells protect the cells inside from ultraviolet light by means of “photonic bandgap structures” due to the “periodic 10–100 nm patterns of holes, slits and ribs.” Another suggestion is that the pores in the shells direct the right wavelength of light to the photocenters. Before suggesting their theory of UV light protection, Luis Ever Aguirre et al., writing in Nature Scientific Reports, see analogies to human inventions in diatoms:

The frustules from the genus Coscinodiscus act as lenses focusing light in a small volume at a distance from the frustule, and this volume is constant with changing direction of light illumination. It has also been noted that the frustules may act as small spectrographs, focusing photons into specific volumes inside the diatom, possibly for absorption in chlorophyll and other chromophores.

Those impressive engineering features, additionally, are independent of the amazing strength of the diatom frustule. Caltech scientists in 2016 found that tiny diatoms “boast enormous strength.” Their silica shells have the highest specific strength (resistance to breakage per unit density) “of any known biological material, including bone, antlers, and teeth.” This is a remarkable property for a glass object!

“Silica is a strong but brittle material. For example, when you drop a piece of glass, it shatters,” says Greer. “But architecting this material into the complex design of these diatom shells actually creates a structure that is resilient against damage. The presence of the holes delocalizes the concentrations of stress on the structure.”

The work by Zachary Aitken et al. was published in PNAS.

Unifying Design Answers Go Beyond Local Function

One has to wonder, gazing at these images, how and why the microscopic “brainless” diatoms do it. The patterns, unique for each species, seem to go far beyond any need for survival, protection, or species recognition. The photos show that the intricate patterns are found not only on the outside, but on the inside as well. Is the organism gazing up at its own cathedral? Or is this beauty meant for us human beings, as exceptional minds and souls, to ponder the level of engineering in biological systems all around us? If so, is it possible the evidence was purposely planned to unfold over time as our technology improved, so that we would never be without evidence the closer we looked?

Some of the diatom patterns appear to reveal similar Fibonacci spirals as found in sunflower heads and nautilus shells. Watch Cristobal Vila’s Nature by Numbers again. Discovering mathematically precise, beautiful patterns at multiple scales in disparate phenomena will suggest to many a unifying, undeniable signature of a designing intelligence. A design perspective requires no storytelling about a blind tinkerer finding silicon useful for survival. Rather, it supports the God Hypothesis: the best explanation for ubiquitous engineering, beauty, and purpose surrounding us.