The resistance of biohumus microbiome to cobalt and nickel compounds

Heavy metals, particularly cobalt and nickel, are highly toxic and widely distributed in ecosystems. Sometimes, their concentration in natural ecosystems can increase sharply due to anthropogenic activities. Metal-resistant microorganisms are considered to be promising for their detoxification. The purpose of the work was to study the sustainability of microorganisms derived from the biohumus in the presence of toxic cobalt and nickel compounds as well as determine the maximum limit concentration of Co²⁺ and Ni²⁺ for them. The biohumus served as a model natural ecosystem free from heavy metals where microorganisms were not adapted to them. The resistance of microorganisms was determined by cultivation in the medium with a gradient of simultaneous Co²⁺ and Ni²⁺ from 0 to 1000 mg/L. The quantification of Co²⁺ and Ni²⁺-resistant microorganisms in the biohumus was determined by counting the number of colony forming units on nutrient agar. Using a Niton XL5 Plus manual XRF analyzer, it was determined that in metal missile fragments the concentration of cobalt ranged from 73 ± 22 to 589 ± 34 mg/kg, the concentration of nickel was 110 ± 15-577 ± 21 mg/kg. Cobalt was not detected in all soil samples. Nickel compounds were detected in two samples of the affected soil up to 408 ± 8 mg/kg and 36 ± 4 mg/kg in soil without shell explosions. On the example of the microorganisms of the biohumus, we confirmed that natural ecosystems contain microorganisms resistant to toxic Co²⁺ and Ni²⁺ compounds in high concentrations. The concentrations of simultaneous Co²⁺ and Ni²⁺ of 100 and 200 mg/L were established not to affect the growth of microorganisms, and the number of CFUs was (6.2 ± 0.2)×10⁵ and (6.1 ± 0.2)×10⁵ CFU/g. The maximum permissible concentration of simultaneous Co²⁺ and Ni²⁺ for the biohumus microbiome was 700 mg/L and the number of CFUs was (5.0 ± 0.1)×10² CFU/g after a month of cultivation. Moreover, microorganisms can adapt and maintain sustainable growth even after the increase in the concentration of metals from 500 to 2500 mg/L as well as to provide the detoxification of divalent metals by transforming into insoluble non-toxic sulfides.