Metagenomic study to evaluate functional capacity of a cyanobacterial bloom during oxidation

Highlights

• Evaluation the functional response of cyanobacterial bloom during oxidation.

• Progressive shift for chlorination functional response from metabolic activity at low exposure level toward dormancy and sporulation genes at high exposure.

• Relative abundance of cyanobacterial biomarkers decreases after high H2O2 exposure.

• Digital droplet digital PCR could not evaluate the toxin removal efficiency after oxidation.

Abstract

Pre-oxidation can be used against cyanobacteria at the water treatment plant intake to improve cell removal efficiency in down flow processes and reduce cyanotoxins concentrations. In this study, shotgun metagenomic sequencing was used to describe the functional capacity of a cyanobacterial bloom (at Lake Champlain, southern Quebec, Canada) before and after pre-oxidation using Cl₂, KMnO₄ and H₂O₂. The bloom samples were associated with two functional profile assemblages: that of August 1st (onset of the bloom) characterized by enrichment of genes related to nutrient uptake and that of August 13th-29th (towards the end of the sampling) associated with competition for resources and repair such as Photosynthesis, Protein metabolism and DNA metabolism. Different functional profile responses to oxidation with Cl₂, KMnO₄ and H₂O₂ was also identified as two-time points during the bloom (at the August 1st, and August 29th). On August 1st, chlorinated samples showed a progressive shift in functional profile: starting by acquiring and sequestering nutrient sources (e.g. Iron acquisition, carbohydrates) at low chlorine exposure (CT, concentration X contact time) level, followed by showing a stronger tendency toward dormancy and sporulation genes at high CT. Our results showed that following high CT of H₂O₂, the relative abundance of the cyanobacterial biomarkers decreased, regardless of the dominant cyanobacterial genus. The toxicity of the bloom before and after oxidation samples was assessed by droplet digital PCR (ddPCR) to measure the mcyD gene. Our results showed significant positive correlation between the mcyD gene copies number and microcystin concentrations in the bloom samples (before the oxidation). However, such correlation was not observed after oxidation. These results suggest that ddPCR can only be used to evaluate bloom toxicity before oxidation.