Drinking water treatment residuals from cyanobacteria bloom-affected areas: Investigation of potential impact on agricultural land application

Abstract

In cyanobacteria bloom-affected areas, drinking water treatment processes are optimized to ensure the absence of cyanotoxins in their finished water. A concern about the sludge generated from water treatment has emerged because the removed cyanotoxins and cyanobacteria can get concentrated in the sludge, called water treatment residuals (WTR), and these WTR are often applied on land for beneficial purposes. However, the impact of WTR from bloom-affected areas on the agricultural application and public health is hardly reported. The objective of this study was to characterize bloom-affected WTR by focusing on cyanotoxins, toxin-producing cyanobacteria, microbiomes, and resistome profiles. In addition, the fate of WTR-originated microcystin in crops and soil was examined. WTR samples were obtained from a bloom-affected area in Ohio, USA in November 2017. Cyanotoxins and toxin-producing cyanobacteria were quantified with the enzyme-linked immunosorbent assay and droplet digital PCR, respectively. Microbiome and resistome were determined with Nanopore sequencing. Cyanotoxin concentrations were measured: microcystin (259 μg/kg), saxitoxin (0.16 μg/kg), anatoxin-a (not detected), and β-Methylamino-L-alanine (BMAA) (575 μg/kg). MC-producing cyanobacteria concentrations were determined: Planktothrix (5.3 × 10⁷ gene copies/g) and Microcystis (3.3 × 10³ gene copies/g). Proteobacteria was the most predominant and Planktothrix phage was a remarkably dominant virus in the WTR microbiome. Aminoglycoside resistance was the most abundant class, and antibiotic resistance was found in multiple pathogens (e.g. Mycobacterium). WTR land application was simulated by growing carrots with a mixture of WTR and soil in a greenhouse. At harvest, ~80% of WTR-originated microcystin was found in the soil (83-96 μg/kg) and 5% accumulated in carrots (19-28 μg/kg). This study provides the first insight into the cyanotoxin, microbiome, and resistome profile of bloom-affected WTR. Our finding suggests that careful WTR management is needed for the beneficial use of WTR for protecting agricultural environments, especially soil and groundwater, and food safety.

Keywords: Antibiotic resistance; BMAA; Drinking water sludge; Microbiome; Microcystin; Nanopore sequencing; Resistome.