Chromium, a heavy metal that enters wastewater treatment plants (WWTPs) through industrial discharges, can be toxic to microorganisms carrying out important processes within biological wastewater treatment systems. The effect of Cr(III) and Cr(VI) on ammonia dependent specific oxygen uptake rate (SOUR), the transcriptional responses of functional genes involved in nitrification and changes in 16S rRNA level sequences were examined in nitrifying bacteria. The nitrifying bioreactor was operated as a continuous reactor with a 24 h hydraulic retention time. The samples were exposed in batch vessels to Cr(III) (10 – 300 mg/L) and Cr(VI) (1 – 30 mg/L) for a period of 12 h. There was considerable decrease in SOUR with increasing dosages for both Cr(III) and Cr(VI), however Cr(VI) was more inhibitory than Cr(III). Based on the RT-qPCR data, there was reduction in the transcript levels of amoA and hao for increasing Cr(III) dosage, which corresponded well with the ammonia oxidation activity measured via SOUR. For Cr(VI) exposure, there was comparatively little reduction in amoA expression while hao expression decreased for 1 to 3 mg/L Cr(VI) and increased at 30 mg/L Cr(VI). Nitrosomonas-like populations represented the dominant bacterial community in the bioreactor as determined by 16S rDNA and 16S rRNA levels. Based on 16S rRNA sequencing, there was a considerable reduction in Nitrosomonas activity upon exposure to 300 mg/L Cr(III) and a slight reduction in activity was observed at 30 mg/L Cr(VI) loading. Both Cr(III) and Cr(VI) were inhibitory to nitrification at concentrations near the high end of industrial effluent concentrations.

Chromium toxicity to nitrifying bacteria: Implications for wastewater treatment

251st National Meeting (2016)

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