Identifying Algicides of Enterobacter hormaechei F2 for Control of the Harmful Alga Microcystis aeruginosa

Abstract

Eutrophication has become an increasingly serious environmental issue and has contributed towards an explosion in harmful algal blooms (HABs) affecting local development. HABs can cause serious threats to ecosystems and human health. A newly isolated algicidal strain, Enterobacter hormaechei F2, showed high algicidal activity against the typical HAB species Microcystis aeruginosa. Potential algicides were detected through liquid chromatograph-mass spectrometer analysis, revealing that prodigiosin is an algicide and PQS is a quorum sensing molecule. RNA-seq was used to understand the algicidal mechanisms and the related pathways. We concluded that the metabolism of prodigiosin and PQS are active at the transcriptional level. The findings indicate that E. hormaechei F2 can be used as a potential biological agent to control harmful algal blooms to prevent the deterioration of the ecological and economic value of water bodies.

Keywords: algicidal bacteria; prodigiosin; quorum sensing molecular; transcriptome.

Figure 1

Electron microscopic images of Microcystis aeruginosa co-incubated with Enterobacter hormaechei F2. (A) Phenotype-based images of algicidal effects. (B) Scanning electron microscopy images, bar = 1 μm; to enable easier distinguishing of bacteria from algae, the photo was processed in color, with cyanobacterial cells in green and bacteria in yellow. (C) Transmission electron microscopy images, bar = 0.5 μm.

Figure 2

Algicidal effect of extracellular substances and potential algicides. (A) Algicidal efficiency of Enterobacter hormaechei F2 extracellular substances from different extracts. (B) LC–MS analysis to detect the presence of 2-heptyl-3-hydroxy-quinolone (PQS). (C) LC–MS analysis to detect the presence of prodigiosin; “*” indicates characteristic peaks of similar retention times.

Figure 3

Algicidal activity of prodigiosin and 2-heptyl-3-hydroxy-quinolone (PQS). (A) Phenotype images that were taken from the bottom of the flask; (B) scanning electron microscopy images, bar = 1 μm; (C) transmission electron microscopy images, bar = 0.5 μm.

Figure 4

Algicidal activity of PQS from Enterobacter hormaechei F2. (A) Cyanobacterial cell concentration; (B) chlorophyll a content; (C) phenotype images that were taken from the bottom of the flask. “*” indicates a significant difference (0.01 ≤ p value ≤ 0.05), “**” indicates a very significant difference (p value ≤ 0.01).

Figure 5

Effects of PQS from Enterobacter hormaechei F2. (A) Growth curve of E. hormaechei F2; the curve was prepared by plotting the logarithmic values of OD₆₀₀ (optical density at 600 nm) vs. incubation time. Mean ± S.E. (B) Laccase activity in E. hormaechei F2; “*” indicates a significant difference (0.01 ≤ p value ≤ 0.05), “**” indicates a very significant difference (p value ≤ 0.01). (C) Relative gene expression fold-change, as calculated based on 16S rRNA as a housekeeping control.

Figure 6

Gene Ontology (GO) functional analysis of differentially expressed genes (DEGs) for (A) 0 vs. 4 d (B) 0 vs. 7 d, and (C) 4 vs. 7 d. “*” indicates a significant enrichment of the GO term or KEGG pathway (p-value ≤ 0.05).

Figure 6

Gene Ontology (GO) functional analysis of differentially expressed genes (DEGs) for (A) 0 vs. 4 d (B) 0 vs. 7 d, and (C) 4 vs. 7 d. “*” indicates a significant enrichment of the GO term or KEGG pathway (p-value ≤ 0.05).

Figure 7

Transcript profiling of differentially expressed genes (DEGs). The color of the grids within a row indicates expression value of a DEG at 0, 4, and 7 d.