Cyanobacterial Blooms in City Parks: A Case Study Using Zebrafish Embryos for Toxicity Characterization

Abstract

Cyanobacteria are photosynthetic prokaryotes that play an important role in the ecology of aquatic ecosystems. However, they can also produce toxins with negative effects on aquatic organisms, wildlife, livestock, domestic animals, and humans. With the increasing global temperatures, urban parks, renowned for their multifaceted contributions to society, have been largely affected by blooms of toxic cyanobacteria. In this work, the toxicity of two different stages of development of a cyanobacterial bloom from a city park was assessed, evaluating mortality, hatching, development, locomotion (total distance, slow and rapid movements, and path angles) and biochemical parameters (oxidative stress, neurological damage, and tissue damage indicators) in zebrafish embryos/larvae (Danio rerio). Results showed significant effects for the samples with more time of evolution at the developmental level (early hatching for low concentrations (144.90 mg/L), delayed hatching for high concentrations (significant values above 325.90 mg/L), and delayed development at all concentrations), behavioral level (hypoactivity), and biochemical level (cholinesterase (ChE)) activity reduction and interference with the oxidative stress system for both stages of evolution). This work highlights the toxic potential of cyanobacterial blooms in urban environments. In a climate change context where a higher frequency of cyanobacterial proliferation is expected, this topic should be properly addressed by competent entities to avoid deleterious effects on the biodiversity of urban parks and poisoning events of wildlife, pets and people.

Keywords: bloom evolution; cyanotoxins; developmental delay; mortality; oxidative stress.

Figure 1

Schematic representation of the steps involved in the preparation of cyanobacterial extracts and filtered samples. Methodology based on [30].

Figure 2

Mortality and hatching effects in zebrafish larvae exposed to cyanobacteria extracts. Mortality data was fitted to a logistic curve and symbols represent mean values. Hatching data are represented as mean values ± standard error. Asterisks denote significant differences toward the control indicated by the Dunn’s method.

Figure 3

Developmental effects in zebrafish embryos/larvae exposed to cyanobacteria extracts from site 1. The legend indicates the stage(s) of development in hours (hours post-fertilization (hpf)) according to Kimmel et al. (1995) [31]. The images in the legend depict the respective stage or a representative stage within the indicated interval and were adapted from [40].

Figure 4

Behavioral effects in zebrafish embryos/larvae exposed to cyanobacteria extracts. Symbols represent mean values ± standard error. Bars represent mean values. Asterisks denote significant differences toward the control indicated by the Dunn’s method.

Figure 5

Biochemical effects in zebrafish embryos/larvae exposed to cyanobacteria extracts. Symbols represent mean values ± standard error. Asterisks denote significant differences toward the control indicated by the Dunnett’s method (site 1—ChE and LDH) or Dunn’s method (site 2—ChE).

Figure 6

Mortality and hatching effects in zebrafish larvae exposed to filtrate. Mortality data was fitted to a logistic curve and symbols represent mean values. Hatching data are represented as mean values ± standard error. Different letters above the symbols denote significant differences between treatments indicated by the Dunn’s method.

Figure 7

Behavioral effects in zebrafish embryos/larvae exposed to cyanobacteria filtrates. Symbols represent mean values ± standard error. Bars represent mean values. Asterisks denote significant differences toward the control indicated by the Dunn’s method.

Figure 8

Biochemical effects in zebrafish embryos/larvae exposed to cyanobacteria extracts. Symbols represent mean values ± standard error. Asterisks denote significant differences toward the control indicated by the Dunn’s method.