Stable isotope probing of carbonyl sulfide and cyanate pathways in microbial thiocyanate biodegradation

Thiocyanate (SCN-) is toxic to many aquatic organisms at elevated concentrations. Historically, large amounts of SCN- were released to the environment by gold mining and coal coking. Microbial SCN- degradation provides a cost-effective approach to remediation; however, knowledge is still limited about the relative roles of the two known enzymatic SCN- biodegradation pathways. Here we applied stable carbon and nitrogen isotope probing of SCN--degrading microbial cultures to assess quantitatively the relative contributions of the cyanate (OCN-) and carbonyl sulfide (COS) pathways over a 24-hour experiment. In contrast to common assumptions, our results demonstrate that the OCN- pathway initiates before the COS pathway and can contribute up to 70% of early SCN- biodegradation. These results yield new insights into the dynamics of SCN- biodegradation, and also show that microorganisms can incorporate carbon and nitrogen from the SCN- into biomass, resolving the question of nitrogen mass balance. Our study holds implications for improving bioreactor design for SCN- bioremediation and for understanding the dynamics for sulfur, carbon and nitrogen flows from SCN- in natural environments into their respective biogeochemical cycles. ImportanceWhile the process of SCN- biodegradation has been well studied, relative contributions from the two known enzymatic pathways are still poorly resolved, hindering our ability to optimize bioremediation systems treating SCN--contaminated wastewater. Also, as SCN- biodegradation in nature impacts the biogeochemical cycling of sulfur (as well as carbon and nitrogen), a more quantitative understanding of enzymatic SCN- biodegradation pathways will yield insights into how the reactivity of this environmentally ubiquitous compound influences marine, terrestrial and atmospheric sulfur chemistry. Here we used carbon and nitrogen stable isotope probing to assess contributions to SCN- biodegradation from known enzymatic pathways in a microbial consortium grown from SCN--contaminated mine tailings. We found that the cyanate pathway dominates SCN- biodegradation initially, before activation of the carbonyl sulfide pathway. We attribute this ordering to the greater amount of bioenergy conservable via the cyanate pathway during the earliest stages of bioremediation.