The secret of survival lies in genes and the restructuring of the microbial community: some microorganisms are able to remove toxic metals from the cell, while others die and are replaced by other more stable ones. The object of study was chemozems in the area of the former Lake Sornoye in the floodplain of the Seversky Donets River in the Kamensk-Shakhtinsky district of the Rostov region. From the 1950s to the mid-1990s, this place was a sludge storage facility for wastewater from the Kamenskvolokno chemical plant, which is why the zinc content in the soil in this area turned out to be two thousand times higher than normal.

According to Elizaveta Pulikova, a junior researcher at the Rhizosphere Bioengineering frontier Laboratory:

"Extremely polluted soils represent a unique model for studying the balance of the nitrogen cycle, where nitrification, as an indicator process, should be completely stopped due to the toxicity of heavy metals, but activity persists. The mechanisms of adaptation of nitrifiers to such extreme conditions remain unknown."

Nitrification is the process of converting nitrogen in the soil, which is considered an indicator of ecosystem health. Extreme long-term heavy metal pollution should have stopped these processes completely, but life in the soil continued.

To understand this, the scientists applied a set of methods: analysis of the chemical and physical properties of the soil, study of the enzymatic activity of microorganisms, isolation and sequencing of soil DNA, as well as bioinformatic analysis of the data obtained. This made it possible to study in detail the structure of the microbial community and the mechanisms of its adaptation.

 It turned out that prolonged pollution triggered natural selection.

 "Heavy metal pollution reduces the activity of nitrification, reduces the number of microorganisms sensitive to contamination, thereby changing their structure: it replaces weak ones with strong ones," explains the researcher.

 For example, according to her, archaea usually dominate in clean soils, but they turned out to be too sensitive to pollution. The study showed that comammox bacteria are beginning to dominate in polluted soils. They are unique in that they are able to perform a complete nitrification cycle within a single cell, which makes them more efficient and competitive, as this process is usually divided between several cells. In addition, their genomes contain genes responsible for removing heavy metals from the cell, which ensures resistance to toxic environments.

"Comammox bacteria, which were able to survive in the soil, have 18-27 genes responsible for resistance to metals, while archaea, whose abundance has decreased, have only two genes," notes Elizaveta Pulikova.

 In parallel with comammox bacteria, the abundance and activity of heterotrophic nitrifiers in the soil increases. Unlike autotrophic nitrifiers (these are organisms capable of creating organic substances from atmospheric carbon dioxide and receiving energy during the nitrification process), heterotrophic nitrifiers receive organic carbon from the external environment and do not extract energy from the nitrification process. Their dominance is due to both their resistance to metals and their ability to detoxify toxic products, the consequences of a disrupted nitrogen cycle.

 The work of SFedU scientists is of great importance for solving the global pollution of fertile lands with heavy metals. Such pollution disrupts natural processes in the soil, including nitrogen cycle processes. An imbalance of nitrogen leads to the leaching of nitrate from the soil into groundwater and the emission of nitrous oxide, a greenhouse gas.

 "Studying the mechanism of adaptation of microorganisms to high concentrations of heavy metals will make it possible to create a consortium of microorganisms resistant to pollution while preserving ecosystem functions," Elizaveta Pulikova adds.

 In the future, such microbial communities can be used to restore the disrupted nitrogen cycle in polluted soils. In addition, scientists suggest the existence of an additional mechanism for the detoxification of toxic nitrogen compounds in polluted soil — the conversion of nitric oxide into nitrate with the participation of the enzymes nitrite reductase and nitrite oxidoreductase.