###### Abstract  
A microorganism isolated and identified as _Acinetobacter couralini_ was found to be able to perform sequential free cyanide (CN\({}^{-}\)) degradation, simultaneous nitrification and aerobic denitrification (SNaD); this ability was associated with the multiphase growth profile of the microorganism when provided with multiple nitrogenous sources. The effect of CN\({}^{-}\) on SNaD including enzyme expression, activity and protein functionality of _Acinetobacter couralini_ was investigated. It was found that CN\({}^{-}\) concentration of 1.9 to 5.8 mg CN\({}^{-}\)/L did not affect the growth of _Acinetobacter couralini_. Furthermore, the degradation rates of CN\({}^{-}\) and ammonium-nitrogen (NH\({}_{4}\)-N) were found to be 2.2 mg CN\({}^{-}\)/L/h and 0.40 mg NH\({}_{4}\)-N/L/h, respectively. Moreover, five models' ([PERSON], [PERSON], Generic Rate law, [PERSON], and [PERSON]) ability to predict SNaD under CN\({}^{-}\) conditions, indicated that, only the Rate law, [PERSON] and [PERSON]'s models, were suited to predict both SNaD and CN\({}^{-}\) degradation. The effect of CN\({}^{-}\) on NH\({}_{4}\)-N, nitrate-nitrogen (NO\({}_{3}\)\({}^{-}\)) and nitrite-nitrogen (NO\({}_{2}\)\({}^{-}\)) oxidizing enzymes indicated that the CN\({}^{-}\) did not affect the expression and activity of ammonia monooxygenase (AMO); albeit, reduced the expression and activity of nitrate reductase (NaR) and nitrite reductase (NiR). Nevertheless, a slow decrease in NO\({}_{2}\)\({}^{-}\) was observed after the supplementation of CN\({}^{-}\) to the cultures, thus confirming the activity of NaR and the activation of the denitrification pathway by the CN\({}^{-}\). These special characteristics of the _Acinetobacter couralini_ isolate, suggests its suitability for the treatment of wastewater containing multiple nitrogenous compounds in which CN\({}^{-}\) is present. aerobic denitrification; bio-kinetics; free cyanide; nitrate-nitrogen; nitrification; nitrite-nitrogen 2020 10 4823 10.3390/app10144823 10.3390/app10144823 10.390/app10144823 10.  
plants (WWTPs) [1]. This type of wastewater may be treated by either physicochemical or biological methods. Most municipal and industrial wastewater treatment plants use biological methods such as simultaneous nitrification and aerobic denitrification (SNaD) for the treatment of contaminated water due to the cost-effectiveness of such processes [2; 3]. However, SNaD is sensitive to toxicant loading [4; 5; 6], particularly free cyanide (CN\({}^{-}\)) [5]. Physical and chemical methods have been used to reduce CN\({}^{-}\) effects on SNaD; however, these methods are relatively expensive, complex and produce undesirable by-products such as hypochlorite, and toxicant loaded sludge [7]. Hence, biological removal of CN\({}^{-}\) prior to SNaD has recently received the most attention. Biological CN\({}^{-}\) removal has been proven to be an ideal option because of its cost effectiveness and environmental benignity [8]. Although, biological CN\({}^{-}\) removal reduces the production of undesirable by-products, conducting CN\({}^{-}\) degradation in a separate reactor with SNaD in sequential two stage reactor systems, may result in the escalation of costs associated with the operation of the multiple independent reactors dedicated to the biodegradation of CN\({}^{-}\) and TN. [PERSON] et al. [9] proposed that CN\({}^{-}\) degrading and resistant microorganisms can be used for SNaD in order to lower the inhibition effect of CN\({}^{-}\) towards SNaD in a single reactor system. [PERSON] et al. [10] reported that _Bacillus_ species were capable of degrading up to 300 mg CN\({}^{-}\)/L and subsequently utilize the by-products which were NH\({}_{4}\)-N, NO\({}_{3}\)-N and NO\({}_{2}\)-N. Additionally, [PERSON] et al. [11] isolated a CN\({}^{-}\) tolerant strain capable of simultaneous CN\({}^{-}\) degradation and SNaD.