Short-term responses of ammonia oxidizers to increasing Zn concentrations: a soil microcosm approach

Nitrification activity belongs to the soil properties quite often used for soil quality assessment. Ammonia oxidation (the turnover of ammonia to nitrates), being the rate-limiting step of nitrification, is considered to be performed to its greatest extend in soil by two taxonomically distinct microbial groups. These are the β-proteobacterial ammonia oxidizers and the more recently accredited for that activity crenarchaeal ammonia oxidizers both of which produce the enzyme ammonia monooxigenase (AMO, encoded by a gene group encompassing the amoA gene). Due to the central functional role of these microbial groups in nitrification they were consolidated in the past years as model groups for eco-toxicological studies and particularly for trace element eco- toxicity. Zn is popular for such studies due to it being an essential element for life when found in low concentrations (related to enzymatic functions, transcription factors etc) while it becomes toxic in high concentrations and it also has a relatively high solubility (easily accessible by soil living organisms). In the present study the system referred above was utilized for the thorough study of short-term responses of soil ammonia oxidizers to Zn toxicity. Eight soil microcosms were generated from loamy soil with ~85 mg/kg background Zn (and low concentrations of other trace elements with toxicity potential) and were spiked with Zn in final nominal concentrations ranging from 0 to 5000 mg/kg. Potential nitrification measurements demonstrated a value decrease following the sigmoidial model and being almost depleted at Zn nominal concentration of 500 mg/kg. amoA gene counts did not demonstrate significant shifts for both β-proteobacterial and crenarchaeal ammonia oxidizers for the various applied Zn concentrations apart from the case of 5000 mg/kg nominal Zn concentration of the β-proteobacterial amoA. On the other hand amoA transcript counts and also the 16S rRNA copies of β-proteobacterial ammonia oxidizers were correlated to the potential nitrification results with the bacterial ammonia oxidizers being more sensitive to Zn than the crenarchaeal ones. DGGE transcriptional profiles were obtained and genotypes responding significantly differently among various Zn concentrations were sorted.