Biochemical and biotechnological insights in the adaptation of soil microrganisms to high Zinc concentrations

Zinc is one of the essential trace elements for life but also toxic for microbial communities of agricultural soils when occurring at high concentrations. Microbial nitrification demonstrated in past studies to be a good indicator of heavy metal toxicity in soil due to its sensitivity. Potential nitrification assays (PNR) and amoA gene (encoding the A subunit of the ammonia monooxygenase enzyme that catalyzes the first step of nitrification, the oxidation of ammonia to hydroxylamine) quantification are considered reliable indicators of the microbial nitrification activity. In a past study, PNR measurements of soil indicated an adaptation of previously exposed to toxic zinc concentrations soil microbial communities compared to unexposed ones. In an attempt to identify possible adaptation mechanisms, microcosms generated from a corn field soil were exposed to zinc EC50 concentration (350 ppm for the particular soil) until almost absolute nitrification restoration in comparison to non exposed microcosms (about four months). Concomitantly, sub-samples of adapted soils were used to inoculate sterile soil treated with zinc at various concentrations ranging from 0 to 5000 ppm and microbial growth rates were further monitored. PNR activity measurements were compared all through the experiment with total bacterial and crenarchaeal amoA content and bacterial amoA expression in soil. Moreover, microbial diversity and soil microbial community functional organization were assessed with performance of denaturant gradient gel electrophoresis (DGGE) on both isolated DNA and RNA. Finally, gene copy numbers and transcripts of the merA (mercuric reductase encoding) gene, previously suggested to be an “early warning” indicator of zinc contamination of nitrifiers in pure cultures, were assessed through out the experiment. Results of this study and possible mechanisms involved are discussed.