Impact of trace elements (total and labile fraction) on the anaerobic digestion activity and microbial community structure

Trace elements (TEs) in anaerobic digestion (AD) are known to be essential for optimal biogas production, but inhibitive in excessive concentrations. However, the mechanisms of inhibition are not fully understood. The effects of the addition essential TEs (Co, Cu and Ni) and a non-essential TE (Cd) on the microbial community structure of AD were studied in lab-scale reactors, using total TE concentrations that ranged from 0 to 100 μM. Reactor performance was assessed by monitoring biogas production. The labile fraction of TEs (the most bioaccessible species) was determined by diffusive gradients in thin-films technique. Prokaryotic community composition was characterized through high throughput sequencing (HTS) targeting archaea and bacteria and qPCR to evaluate changes in methanogens and metal resistance genes. Only a minor fraction of added TEs was labile and it decreased over time, with Ni being the most labile. Although only a minor fraction of spiked concentration was labile, all TEs inhibited biogas production at the highest spiking concentration (100 μM), with higher inhibition observed for Cd and Ni. HTS and qPCR revealed changes, particularly in archaea, with reduced relative abundance at higher TE concentrations. Shifts in prokaryotic communities suggest alterations in AD metabolic pathways. High inhibition of biogas was linked to reduced diversity, dominance of the bacterial genus Klebsiella and changes in the ratio acetoclastic / hydrogenotrophic methanogens. This study addresses a research gap in understanding how TEs inhibit AD, and provides a strategy to improve TE dosing by monitoring the labile fraction of TEs to avoid overdosing.