Nitrous oxide-emitting bacteria can help us predict climate impacts

When we talk about greenhouse gasaes, CO₂ usually steals the limelight. Most people have also heard of the problem of methane-burping cows – but have you ever heard of the nitrous oxide-emitting bacteria?

Maybe not. And in fact, we don’t know much about the bacteria or how much nitrous oxide they emit, says Bo Thamdrup, professor of Geomicrobiology at the Department of Biology.

– Nitrous oxide is a very potent greenhouse gas. In fact, it is about 300 times as potent as CO₂, and emissions are presently increasing, he says and continues:

How much in Danish waters?

– So even though nitrous oxide concentrations are relatively low compared to CO₂, they still play a role as a climate gas. Having said that, we really don’t know how much nitrous oxide is emitted by these bacterial processes.

In a new research project supported by the Independent Research Fund Denmark, Bo Thamdrup and his colleagues will, therefore, investigate nitrous oxide emissions and, not least, the bacteria that create nitrous oxide.

– If we are to reduce nitrous oxide emissions from the marine environment, it’s essential that we know more about the processes behind it and, not least, the actual scale of the problem, says Bo Thamdrup about the project, which, among other things, will result in the first estimate of nitrous oxide emissions from Danish waters.

Nitrous oxide can be a by-product

There are two different groups of bacteria that produce nitrous oxide. One of the groups is called nitrifying bacteria, and in a nutshell, they eat nitrogen.

The second group of bacteria is called denitrifying bacteria. They use nitrogen in their breathing process. Just as we use oxygen, they use nitrate and turn it into free nitrogen – that is, the form of nitrogen found in our atmosphere.

Emissions depend on, for example, oxygen depletion

– And although this gas is harmless, the way to becoming so happens through nitrous oxide – thus making nitrous oxide an intermediate step. And during that process, some of the nitrous oxide escapes, says Bo Thamdrup.

These processes are the very reason why researchers are having a hard time calculating how much nitrous oxide escapes.

– If it were the end product, we could relatively easily calculate the conversion and thus the emissions. But because it is a by-product, emissions depend on many different factors and environmental conditions – including oxygen depletion, Bo Thamdrup points out.

Boosts the nitrogen system

When, for example, a farmer fertilises his fields, part of that nitrogen ends up in the ocean, causing algae blooms, and when the algae die, they consume oxygen. Therefore, Bo Thamdrup and his colleagues are measuring nitrous oxide levels in a handful of Danish coastal areas poor in oxygen.

– At the same time, it also results in increased nitrous oxide emissions, because more nitrogen speeds up the natural nitrogen cycle, says Bo Thamdrup and continues:

– It’s an aspect of nitrogen emissions that research hasn’t looked into before. Today, we calculate emissions per hectare of farm land, but nitrous oxide keeps coming from the fertiliser – we just don’t know how much. We would like to put figures on this.

A new nitrous oxide meter

Only a few measurements of nitrous oxide have been done in Danish waters, Bo Thamdrup says, and none of them have measured the level over a whole season, as the researchers do in his new project. In general, according to him – also internationally – very little research has been carried out in this particular field.

However, their pilot measurements show that in connection with oxygen depletion, very high concentrations of nitrous oxide may be found in the water.

The researchers measure the amount of nitrous oxide by taking water samples in small bottles for lab testing back home. In addition, they will also test a new nitrous oxide sensor, developed by colleagues from Aarhus University, which can measure the concentration of nitrous oxide directly in the water.

What do the bacteria eat?

Bo Thamdrup and his colleagues will be busy with more than just taking measurements. They will also investigate and analyse the processes that form nitrous oxide – i.e. the eating and breathing habits of the tiny bacteria.

– If we are to predict nitrous oxide emissions, it is important to know which processes actually produce nitrous oxide – and how quickly they do so. We can determine this by taking water samples home to the laboratory and analysing the nitrogen conversion, Bo Thamdrup says.

In this regard, they will conduct various experiments to see what happens if, for example, they add ammonia or nitrite, increase the temperature or lower the oxygen concentration.

Pushing the green transition

– This will help us understand the mechanisms behind it, and based on this, we can calculate how different environmental changes affect nitrous oxide production and thus emissions, says Bo Thamdrup.

Researchers will be able to use these calculations in different environmental and climate models – and not just for Danish waters.

– Hopefully, we will be able to use the results from the project to understand not only what is happening in the Danish marine environment, but also in the marine environment in general at an international level. Perhaps it can help nudge the green transition in the right direction.