UNC-TV Science: Good From Bad

UNC-TV Science: February 18, 2014
Good From Bad: A New Technique Determines which Organic Wastes May be Harmful to the Environment

This is a story about a clever use of assays, bacteria, gene expression and the intricacies of the nitrogen cycle... and poop.

Ok, stay with me here. Poop is a challenge. We make a lot of it: hundreds of pounds per person per year. And all of that waste has to go somewhere. The first stop is the waste treatment plant, but there’s no way they can store and completely process all of that waste. So commonly, waste treatment plants will sell a recycled version of sewage called biosolids to farms for use as fertilizer.

It’s a win-win, right? Sewage stays out of the water supplies where it can harm humans and animals, and the plants can make use of the all the organic matter and nitrogen in the biosolids.

As it turns out, however, the story is a little more complex, because biosolids contain hundreds of different types of organic molecules, the stuff made primarily of carbon and hydrogen. Some of those organic molecules don’t help plants grow, in fact they may actually prevent the plants from getting the nutrients they need to survive.

Enter Claudia Gunsch, a civil and environmental engineer at Duke University. She and her colleagues recently invented a method to test which of those organic molecules may be harmful to plants and to the environment in general.

To understand how it works, you need to know a little about the nitrogen cycle. Plants need nitrogen to make proteins. The good news is, there’s nitrogen everywhere. It’s in soil, it’s in biosolids and it even makes up 78% of our atmosphere. The bad news is that plants need it to be packaged up in a very specific way to make use out of it. Think about it like being trapped on a desert island, surrounded by coconuts but you don’t have anything to open then with. Technically, you have all the food you need; it’s just not in a form that you can eat.

For plants, the coconut form of nitrogen they need is the ion, nitrate. The air is full of nitrogen gas, while soil and biosolids are full of ammonia and nitrite, which are as inedible to plants as hard coconut shells are to us.

And just as we would use a tool like a rock or a seashell to open a coconut, plants have a tool to access nitrate. Two families of bacteria that live in the soil, called nitrifiers and nitrogen fixers, use ammonia and nitrogen gas for fuel, and when they’re done, they discard nitrate as a waste product. Then, plants absorb that waste nitrate to build their proteins.

Somewhat counterintutively, there are bacteria called denitrifiers that make their living doing the exact opposite. They take up nitrate and make it into nitrogen gas and nitrous oxide and these denitrifiers are just as important to the environment. If a field has too much nitrate, that excess will flow into streams and rivers and help algae grow. But that algae is harmful, both to fish and to humans who drink the water.

But getting back to poop, some of the organic molecules in the biosolids are actually antimicrobial agents, that is, rat poison for bacteria. So as biosolids get spread out over a field, they can kill off the bacteria that make nitrate for plants and keep it from getting into the rivers.

Here’s where Gunsch’s test fits in. She makes vials of a specific dentirifier called Paracoccus denitrificans in a medium of their nitrate food. Then, she adds a single component of the biosolid waste, waits a day, and measures how much of the food got eaten. This way the test shows how much of an effect the organic molecule had on the bacteria’s ability to thrive and make nitrate.

The new method is fast, effective and most importantly, cheap. As new organic molecules are found in biosolids, water quality testers can use Guncsch’s method to quickly determine whether it might be a problem for bacteria in the soil. The work was published in the journal Environmental Science and Technology.

- Daniel Lane

Daniel Lane covers science, medicine and the environment as a reporter/writer. He is currently pursuing a master's degree in medical and science journalism at UNC - Chapel Hill.