Denitrification performance and microbial community under salinity and MIT stresses for reverse osmosis concentrate treatment

[Display omitted] •Nar was more sensitive to salinity than Nir, Nor and Nos.•Suppressed reductase activity stimulated the synthesis of nar and nor.•40 mg/L MIT was close to the threshold of denitrifiers’ tolerance.•Thiols reaction and organic supply for heterotrophs contributed to MIT degradation.•10 mg/L MIT inhibited Halomonas and continuous addition inactivated Micavibrio. Salinity and biocide in reverse osmosis (RO) concentrate can affect the subsequent biological treatment. Effects of salinity and methylisothiazolinone (MIT) on denitrification performance, denitrification genes and microbial community were investigated. The salinity from 10 to 40 g/L NaCl and 10 mg/L MIT did not have impact on denitrification. Nitrite reductase, nitric oxide reductase and nitrous oxide reductase could tolerate higher salinities than nitrate reductase. The salinity improved the gene abundances of narH, narI, norB and norC as a negative feedback towards unfavourable conditions. Both Thauera and Marinobacter possessed all denitrification genes with the highest abundance at 0–20 g/L and 20–40 g/L NaCl, respectively. Continuous or intermittent MIT addition at 10 mg/L did not affect the denitrification rate, whereas 40 g/L MIT inhibited denitrification. MIT degradation was attributed to both the reaction with intracellular thiols and the organics uptake by denitrifiers, thus low dosage could not exhibit the inhibitory effect. After a long acclimation with continuous MIT addition, microorganisms tended to have a stronger tolerance to MIT shocks. 10 mg/L MIT generally had no influence on the denitrification genes and relevant microbial community, but it still suppressed the denitrifier Halomonas and continuously adding MIT could inactivate the predator Micavibrio.