Effects of Microcystin-LR on Metabolic Functions and Structure Succession of Sediment Bacterial Community under Anaerobic Conditions

Abstract

Microcystins (MCs), which are produced by harmful cyanobacteria blooms, pose a serious threat to environmental health. However, the effect of MCs on the bacterial community under anaerobic conditions is still unclear. This study examined the dynamic changes of MC-degrading capacity, metabolic activity, and structure of the bacterial community in lake sediment repeatedly treated with 1 mg/L microcystin-LR (MC-LR) under anaerobic conditions. The results showed that the MC-degrading capacity of the bacterial community was increased nearly three-fold with increased treatment frequency. However, the metabolic profile behaved in exactly opposite trend, in which the overall carbon metabolic activity was inhibited by repeated toxin addition. Microbial diversity was suppressed by the first addition of MC-LR and then gradually recovered. The 16S amplicon sequencing showed that the dominant genera were changed from Exiguobacterium and Acinetobacter to Prosthecobacter, Dechloromonas, and Agrobacterium. Furthermore, the increase in the relative abundance of Dechloromonas, Pseudomonas, Hydrogenophaga, and Agrobacterium was positively correlated with the MC-LR treatment times. This indicates that they might be responsible for MC degradation under anaerobic conditions. Our findings reveal the relationship between MC-LR and the sediment bacterial community under anaerobic conditions and indicate that anaerobic biodegradation is an effective and promising method to remediate MCs pollution.

Keywords: anaerobic condition; bacterial community; high throughput analysis; metabolic profile; microcystin.

Figure 1

Degradation curves of microcystin-LR (MC-LR) by different samples with different incubation time. Autoclaved sediment bacterial community was used as control to monitor abiotic degradation or adsorption of MC-LR. Data are shown as mean values ± standard deviation (SD) (n = 3).

Figure 2

Average well color development (AWCD) changes of sediment bacterial communities with different incubation time (a). Utilization of various carbon sources by the sediment bacterial communities in different samples after 168 h of incubation (b). Sample G0, which represented the original sediment bacterial community, was used as the control. Data are shown as mean values ± SD (n = 3), * p < 0.05 (sample G0 versus MC-LR treated samples).

Figure 3

The beta diversity of bacterial communities in different samples. Each point in this figure represents a treated sample and different colors indicate different treated groups (n = 3). Beta diversity was visualized using PCoA plot with Bray–Curtis dissimilarity distances.

Figure 4

Relative abundances of different groups at the phylum (a) and genus (b) levels. Phyla (a) and genera (b), whose relative abundance was below 1.00% in all groups or which cannot be assigned to a specific taxonomic category, were grouped into others. The data showed the integrated results of three replicated samples.

Figure 5

The changes in relative abundance in genera whose proportion continuously increased with increased treatment times: (a) Dechloromonas, (b) Pseudomonas, (c) Hydrogenophaga, and (d) Magnetospirillum. Data are shown as mean values ± SD (n = 3) and letters up the bars show significant statistical difference according to LSD test (p < 0.05).

Figure 6

Heatmap of the 30 most abundant operational taxonomic units (OTUs) at the genus level in all samples. The OTUs were annotated with taxonomic names based on the BLAST of the extracted genetic sequences.