A Chinese-German team of scientists has developed a new approach to tackling nitrogen - one of the major pollutants in freshwater and wastewater.

Nitrogen is an essential nutrient for plant growth, but too much of it, in the wrong place, can cause overstimulation in the growth of aquatic plants and algae which causes multiple problems - from stealing light and oxygen from other species, to clogging water intakes and disrupting treatment processes. The researchers from the Chinese Ministry of Natural Resources in Xiamen and the Leibniz Institute of Freshwater Ecology & Inland Fisheries (IGB) in Germany have identified a combination of natural fungi and bacteria that can purify nitrogen highly effectively.

This natural fungus-bacteria consortium, as it is known, could be crucial for the development of new biotechnological water treatment approaches and provides crucial evidence of the important role of fungi in aquatic ecosystems.

Biological nitrogen removal is known as denitrification. It is an important biochemical process where microorganisms convert two of the most important nitrogen compounds, nitrate and nitrite, into gaseous nitrogen. This occurs naturally in water bodies through metabolic processes of the organisms living there and is seen as self-purification.

This principle is already widely used in water treatment. A variety of bacteria and fungi that can break down nitrogen, with and without oxygen, have long been deployed. For water treatment, nitrogen degradation in the presence of oxygen is particularly relevant as it is more cost-effective than other techniques and can be implemented at a large scale.

However, isolating individual strains of bacteria or fungi is time-consuming and expensive. Combinations of the two microbial consortia, are considered a promising alternative to pure strains, but are still little researched in the field of denitrification where oxygen is present.

The researchers took this as an opportunity to investigate this potential, as microbial consortia have been used for a long time in the fermentation of food and beverages. Fungi in particular have the advantage of being very robust to environmental stressors such as acidic pH and high temperatures.

The research team identified a natural bacteria-fungi consortium from aquaculture that removes nitrate from water very efficiently and consistently. In the presence of oxygen, nitrate removal is up to 100%, and denitrification efficiency - how well microorganisms are able to convert the nitrogen - is 44 per cent.

High-throughput sequencing (HTS) was used to identify the bacterial and fungal taxa involved in this process. This involves using technologies that tell scientists the kind of genetic information that is carried in a particular DNA segment. HTS involves sequencing multiple DNA molecules in parallel so hundreds of millions of DNA molecules can be sequenced at a time.

Further analysis showed which of the bacteria and fungi species interact positively with each other and are suited for use in combination.

"We succeeded in identifying denitrifying bacterial-fungal groups that have the potential to remove nitrate more effectively," explained Professor Hans-Peter Grossart from IGB, co-author of the study. This is an important step in putting together microbial consortia for optimal water treatment."

The search for suitable microbial communities of bacteria and fungi is still a very young field of research and so far there are no practical applications. However, the authors are certain that microbial consortia will significantly shape biotechnology in wastewater treatment in the future.