DNA-studies of bacteria lead to significant insight into the nitrogen cycle

DNA-studies of bacteria lead to significant insight into the nitrogen cycle

The focus of the Center’s work is the study of bacteria that cannot be cultivated or engineered in labs but are rather found in the ‘real world’ – in the sea, in the human body, in wastewater treatment plants, biogas plants, aquacultures, etc.

Per Halkjær Nielsen explains:

- “In total, there are around 10 million bacterial species on the planet, of which only around 10,000 have been properly identified and described. So we have almost no idea what the vast majority is doing. Our group has established the first public database that collects all information about the most common microbes in the context of environmental biotechnology, such as wastewater treatment plants and biogas plants. The data is continuously being extended with the identity of novel bacteria – and what functions they perform in these contexts. This knowledge is based on many studies, including investigations of their genomes, which show us what each bacteria group is capable of.”

The database is open to the public, allowing for instance a wastewater treatment plant experiencing operational problems, such as increased foaming, to access a section of the database that deals specifically with bacteria that are present in wastewater treatment plants. Here, they will be able to see whether a specific bacterium has been identified as causing the foaming problem – and, if so, how other plants have solved the issue.

“The DNA-based classification of bacteria and the association with specific functions in the system enables us to solve problems, optimise processes and prevent infections, depending on the context in which the bacteria appear” Per Halkjær Nielsen says.


In 2014, one such context that was examined by the group in collaboration with their Austrian colleagues was the nitrogen cycle that appears in connection with fertilizing and in wastewater treatment plants; specifically the bacterial group of nitrifiers called Nitrospira. These have been thought to be highly specialised and dependent on the process of oxidizing nitrite into nitrate, a process which is extremely important for the breakdown of nitrite – and thus the amount of nitrate that is produced – before it enters our ground water.

- “Until now, the common conception was that if the Nitrospira were present, the nitrification process would occur, and nitrate, which is unwanted in our drinking water, would be produced. However, in our research we have examined the genome and gene expression of Nitrospira and proven that this is not the only function they have – in fact, they have markedly different abilities than anyone thought, including being able to live on hydrogen rather than nitrite” Per Halkjær Nielsen explains.

He has no doubt as to the impact the group’s results will have:

“This discovery has major significance for the understanding of the nitrogen cycle, since we can no longer take for granted that the nitrification process occurs when these bacteria are present. This concerns not only the nitrogen cycle in nature but also processes affecting our everyday lives where we want to limit the presence of nitrate, for instance with regards to agricultural fertilizing, the purity of our ground and drinking water and the purification of wastewater.”