Living Elements: Silicon

This post is the first in a three-part series on how living creatures use the elements of the periodic table. Learn more about the elements on NOVA's two-hour special, "Hunting The Elements."

When you think of silicon (Si), you may think about Silicon Valley and the fact that modern computing would not be possible without this element in computer circuits. You may also think about how silicon is the second most abundant element in the Earth's crust, after oxygen. You even see it (or don't see it) every time you look through a piece of glass.

What you probably won't think about are diatoms: tiny photosynthesizing algae that are the primary movers of silicon in the world's oceans. Diatoms depend on silicon. They flock to locations where silicon is available. In the process, they generate enormous blue-green blooms.

algae bloom
A bird's eye view of diatom blooms in the ocean. Credit: Norman Kuring/NASA Ocean Color Group, via NASA Earth Observatory.

Every year, diatoms use almost seven trillion kilograms of silicon. They collect silicon in the form of silicic acid--that's silicon plus four hydrogen atoms and four oxygen atoms--and convert it into silica, the primary constituent of glass. What could algae possibly be doing with all of this amorphous, natural glass?

Diatoms use silica to create protective cell walls called frustules that are strong and chemically inert. Essentially, this is glass armor--cheap glass armor. It costs a diatom only two percent of its total energy budget needed for growth to make its shell.

Their beautifully geometric and symmetric shells are tiny marvels of natural art. Furthermore, each of the estimated 100,000 of species of diatoms has its own unique glass shell design. Diatoms are extremely efficient micro- and nano-architects. They consistently create the same 3D structure over and over again.

diatom shells
A collection of various diatom species. Credit: Wipeter, via the Wikimedia Commons.

The intricate armor protects the diatoms and also acts as a chamber that helps the diatoms photosynthesize by boosting their surface area, facilitating the exchange of gasses between the air, seawater, and organism. The silica in the glass also speeds up the conversion of ocean bicarbonate to carbon dioxide by changing the acidity of the water, which facilitates the exchange of protons in vital chemical reactions. The diatom then uses this carbon dioxide for photosynthesis. Thanks to silicon, diatoms have been able to carve out a unique biological niche in Earth's oceans.

With such efficient photosynthesis thanks to their miniature greenhouses (silica shells), diatoms produce about one quarter of the world's oxygen--almost as much as all tropical forests combined. This process also removes great amounts of carbon dioxide from the atmosphere, potentially helping combat global warming. When diatoms die, their heavy shells cause them to sink down to the depths of the ocean, taking that carbon with them. So, not only do they play a vital role in the global silicon cycle, they are also responsible for removing carbon dioxide and producing the oxygen that we breath.

Since they have been using silicon for more than 110 million years, diatoms are a vital part of Earth's biogeochemical silicon cycle. Diatoms are the world's way of moving silicon between rocks, water, and life. They are a form of life that changes the chemistry of the world's oceans.

In a beautiful cycle, the tiny glass masterpieces created by living diatoms sink to watery depths when the diatom dies. Many of the silica shells become part of the rock record when they fall to a seabed. Over time, these rocks dissolve and become orthosilicic acid, which is reused by new diatoms. These rocks also erode and the old silica shells end up on beachy shores in the form of sand. For 5,500 years, humans have been making glass from the same sand that all came from diatoms.

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Deepa Rao

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