All living organisms produce enzymes.  Enzymes speed up chemical reactions by converting another substance, called the substrate, into a molecule necessary for life. Microscopic organisms, or microbes, along with some plants and fungi, release enzymes into their environment to break down their food. As microbes eat, they exhale carbon dioxide gas (CO₂).  The faster the enzymes work, the more CO₂ gas is exhaled.  Therefore, we can evaluate the amount of CO₂ microbes are releasing by measuring how fast their enzymes are working.

In lakes and rivers, phosphorus and nitrogen are important nutrients that, just like fertilizer for your garden, help microbes grow. Enzyme activity is beginning to be thought to depend not just on the type and amount of nutrients, but also the chemical structure of their food. Food that has a complicated structure—and is therefore difficult to digest—may slow down enzyme activity.

We want to know exactly how both nutrients and food structure (“digestibility”) affect enzymes. To test this, we added nitrogen and phosphorus to water samples, from the smallest streams to the Arctic Ocean, and monitored the activity of four enzymes for eight hours. Polyvinylpyrrolidone (PVP), a chemical that makes organic material easier for the microbes to digest, was added to certain samples to see if this had an effect.

We discovered no clear patterns in response solely to nutrient addition. However, in the Kolyma River samples, enzyme activity was impacted by the combination of PVP and nutrient addition. This suggests that both the chemical structure of the organic material and the amount of nutrients are important in controlling enzyme activity.

The Kolyma River region is made up of permafrost, or permanently frozen soil. This soil, composed of plants and animals that died many thousands of years ago, has a different nutrient composition and a less complicated chemical structure than what the microbes are currently eating. If Arctic warming causes this permafrost to thaw, our research shows that this would be like opening an all-you-can-eat buffet for microbes. As a result, more heat-trapping CO₂ would be released into the atmosphere, creating a feedback that would lead to even greater warming.