Low-level thiocyanate concentrations impact on iron oxidation activity and growth of Leptospirillum ferriphilum through inhibition and adaptation

Abstract

Leptospirillum ferriphilum is the dominant iron-oxidising bacterium in traditional microbial communities utilised in bioprocesses for gold recovery from sulfidic minerals. Ferrous iron oxidation activity and growth of unadapted and thiocyanate-adapted L. ferriphilum HT was studied in batch culture across increasing thiocyanate (SCN−) concentrations in the range 0–2 mg/L to assess the feasibility of recycling remediated cyanidation wastewaters. Thiocyanate concentrations of 1 mg/L and 1.4 mg/L induced an inhibitory effect in the unadapted culture wherein ferrous iron oxidation rate and cell growth were compromised. A substantial lag in the onset of ferrous iron oxidation occurred at concentrations above 0.5 mg/L SCN−, with no oxidation activity above 1.75 mg/L SCN−. The adapted culture, however, was uninhibited across the SCN− concentration range investigated and demonstrated a higher specific ferrous iron oxidation rate owing to reduced growth. It is postulated that SCN− exposure in the absence of adaptation induces osmotic stress. Moreover, upregulation of genes associated with the synthesis of osmo-protectants may be responsible for the preservation of activity observed in the adapted culture. As L. ferriphilum is dominant within the biooxidation tank community, evidence of sustained iron oxidation activity at low-level SCN− concentrations affirms the potential of recycling bioremediated cyanidation wastewater.