Organic forms of mercury are recognized as persistent and highly toxic chemicals. Soil microbiota is an important player in assurance of soil functioning (nutrient cycling, organic matter decomposition, pollutants degradation, etc.). Hg methylation by soil microbiota plays a significant role in Hg global biogeochemical cycle and it should be considered. Generally, microorganisms are exposed mainly to bioavailable Hg in soil solution and fungi, but could be also through decomposing dead organic matter. At moment remained poorly understood the consequence on soil microbiota of exposure at different levels and forms of organic Hg. We presume that microbiota community structure and abundance could be changed as a consequence of exposure at organic forms of Hg, which in turn could alter soil functioning. In this study we used soil samples from a historically polluted site (Turda, Romania) with Hg, to assess Hg organic forms and their impact on soil microbiota community structure. To test our hypothesis, we performed also soil column experiments under controlled conditions (humidity, light exposure, etc.) and methylmercury exposure levels (5, 10, 25 and 50 µg⋅kg^-1, respectively). PLFA approach was used to assess microbiota community structure and abundance. Enzymatic activities related with C (CM-cellulase, invertase) and N (urease, protease) cycling were assessed also, as key functions of soil. We found that organic forms of Hg influence the microbiota community structure. Soil column experiments revealed that bacteria groups and abundance were negatively related to methylmercury amounts. Correlation between methyl mercury and monitored soil enzymatic activities were as follows: CM-cellulase 0.38, invertase 0.42, urease -0.79, protease -0.86, (p < 0.05).

These results provide a starting point for subsequent studies to investigate effects of organic forms of mercury on soil microbial community structure. Continued advancement in this area will allow for a more detailed understandings of methylmercury effects on microbiota and on soil functioning which will aid in understanding Hg organic forms effects on overall soil ecosystem health.