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By Ian Halim

Yeast gives us a way to start understanding nitrogen’s role in living things. But I’ll need to start by explaining why yeast is so important. Not only does it make bread rise and ferment sugar into alcohol, but it also offers a powerful way to begin investigating many key questions in biology.

Before the systematic observation of modern science, a lot of evidence seemed to support the idea that living things could arise from non-living things. Maggots, for instance, seemed to burst forth from rotting meat. People didn’t notice the tiny fly eggs. And since yeast floats in the air and dust – settling by chance on grape skin or bread dough –it wasn’t always clear that it was really something separate from the grape juice that it ferments into wine, or the barley mash that it turns into beer. The Dutch scientist and microscope-maker, Antonie van Leeuwenhoek (1632-1723), was the first to see yeast globules under a microscope, but mistakenly thought they must be a component of crushed cereal grains. In the fourth century BCE, Aristotle expressed the theory of spontaneous generation, that living things could arise from inanimate matter. And for more than two thousand years, many believed it. 

The French scientist Louis Pasteur wasn’t the first one to disprove spontaneous generation, but he was the most persuasive – undertaking multiple experiments, with different variations, showing that dust introduced tiny living things into previously sterile material. He found that if he extracted grape juice from a grape using a sterile syringe, that juice would not ferment. In doing so, he showed that yeast is separate from the grape. Without yeast, there is no wine. And yeast doesn’t just wink into being. Much as humans only arise from other humans, yeast only arises from yeast.
Pasteur also found that yeast would only grow in the presence of certain nutrients – sugar, minerals, and a source of nitrogen. The sugars provide an energy source for the yeast – getting split into carbon dioxide gas and alcohol. But why should this little organism need nitrogen to grow and reproduce?

Yeasts, actually, are not special in this respect. No living thing can survive without nitrogen. All protein and all DNA contains it.

For plants, nitrogen tends to be scarce. Without enough, a plant may stop growing, or wither and die. Nitrogen scarcity even helps drive fall color change. In the autumn, most deciduous trees break down the biological machinery that has been harnessing sunlight all spring and summer. Part of the reason why they bother doing this is in order to recapture nitrogen. Since the light-capturing chlorophyll pigments are green, this nutrient recycling drains the summer color out of the leaves, unmasking the bright pigments that give us the familiar orange and yellow brushstrokes of autumn. Understanding nitrogen scarcity helps us see fall color change in a new way – from the tree’s point of view.

Almost 80% of the air is nitrogen, so you might wonder why it’s so scarce for plants. The nitrogen gas in the air, it turns out, is in a form that’s very stable, called dinitrogen, with nitrogen atoms paired up and bonded together in twos (hence di-). Like pairs of happy dancers reluctant to change partners, it’s very hard – and takes a great deal of energy – to yank a nitrogen atom free from its doubled state. But it’s only once dinitrogen has been cleaved that nitrogen may be used by living things, as a building block for making the new biomolecules that make up our flesh, blood, bone – or twigs, fruits, and stalks.

How, then, do living things harness atmospheric dinitrogen? While humans breathe dinitrogen gas in and out unchanged, special bacteria in the root nodules of some plants can cleave it – a process called nitrogen fixation. This is the basis for millenia-old crop rotation systems. One season, a farmer would plant a crop that hosts nitrogen-fixing bacteria its roots – often a legume, such as peanuts, chickpeas, or soybeans. The next season, the farmer would plant a different crop that would feast upon the nitrogen-enriched soil left behind.

This is the main way that nature introduces nitrogen into living systems, through specialized bacteria that cleave dinitrogen gas. Herbivores eat plants and access their nitrogen. And humans, as omnivores, get nitrogen from both plants and animals. In fact, our diet is so rich in nitrogen that we end up with excess – a fact about us that reflects our position in the food chain, and which turns out to explain why blood nitrogen levels can be used to measure kidney function.

One of the jobs of the kidney is to eliminate excess nitrogen as waste, packaged in a molecule called urea. When our kidneys fail, they stop moving urea into the urine, and the level of urea in our blood rises. Blood urea nitrogen (BUN) thereby serves as an easy way to see if the kidneys are working – a little like a “check engine” light.

Very high levels of blood urea are known as uremia – named for the molecule urea and the Greek word haima for blood. Uremia makes us sick, but the urea itself doesn’t seem to be the culprit. When the kidneys fail and urea accumulates in our blood, so do other waste products – making us confused, nauseous, increasing blood pressure, and sometimes irritating the membrane around the heart. Urea serves as a proxy for these toxins, allowing us to estimate how much other stuff the kidneys are failing to remove from our blood – and giving us an invaluable way to monitor kidney disease.

To many doctors and nurses, this is the sole meaning of blood urea nitrogen (BUN) – a standard laboratory test that acts as a marker of kidney function. But our overabundance of nitrogen is a direct result of our position in the system of life. All living things need nitrogen, and we only have it in such abundance because we get it from other living things that have painstakingly acquired it – a biochemical stamp of our position in the system of life, dissolved in our very blood.

Somerville Bagel Bards member and physician-humanist, Ian Halim, writes about how medicine relates to everything from ethics to botany – aiming to make science accessible to the widest possible audience. Ian earned his PhD in Greek & Latin literature and his MD at Columbia University in New York City and is now training at a hospital in Boston.