The use of zebrafish to assess water quality and remediation efforts.
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Date
2023
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Abstract
Although wastewater effluents continue to be significant polluters of aquatic ecosystems in
developing countries with limited water resources, little is known about the ecotoxicity induced by
these effluents on fish throughout their early life stages. Several wastewater treatment plants
(WWTPs) in South Africa (SA) do not adequately meet the minimal wastewater treatment
requirements established by the country's Department of Water and Sanitation (DWS). Moreover,
contaminants of emerging concern (CECs) originating from synthetic or natural sources, are
widely distributed in aquatic environments of SA. This includes a broad range of natural and
chemical compounds, such as aspirin (44243 ng/L), Fluoroquinolones (27100 ng/L), Atenolol
(25900 ng/L), Nalidixic acid (25234 ng/L) and Ciprofloxacin (20514 ng/L). In addition to chemical
compounds, endocrine disrupting chemicals, pharmaceuticals and personal care products are
also distributed in the water systems. In the process of wastewater treatment, agents such as
flocculants, coagulants, chemical precipitants (e.g., calcium hydroxide or sodium hydroxide) and
chlorine disinfectants are utilized in wastewater treatment settings. However, research to
understand the adverse effects that can be caused by these agents on aquatic organisms is still
ongoing in SA. In order to bridge this knowledge gap, advanced techniques could be employed
to help reveal adverse effects of wastewater as well as any shortcomings of current water
remediation techniques. Using an appropriate aquatic model organism with highly conserved
physiological pathways present in higher vertebrates (including humans), a rich behavioural
repertoire, and occurrence in a variety of habitats would be a novel approach. To this effect, this
study employed zebrafish with the aim to monitor six distinct wastewater samples from various
regions of SA and to assess the effectiveness of currently used water remediation techniques
such as chlorination. Two wastewater effluents, namely, Southern Works Final Effluents (SWFE)
and Jacob’s Incoming (JB) alerted potential toxicity during chemical characterization with suboptimal
pH (SWFE = 9.02 ± 0.16 and JB = 5.65 ± 0.02) and total alkalinity of zero (0 mg/L)
detected for both effluents. The lethal toxicity of these effluents was seen by the elevation of
mortality rate up to 77 ± 2.89 % and 100 ± 0.00 %, respectively for SWFE and JB at 40 %, with
corresponding LC50 values of 17.77 % and 16.46 %. The zebrafish jaw and face, heart, brain, fins,
notochord, somite and tail were significantly deformed (p < 0.05) post-exposure to these effluents,
as revealed by morphological scores upon the analysis of the zebrafish’s body structure.
Moreover, there was a delay in development due to the aforementioned effluents, unsuccessful
hatching, craniofacial abnormalities, pericardial and yolk sac oedema, notochord abnormality
somite defects and spinal cord curvature. In addition, locomotor activity of zebrafish was inhibited
following observation of distance travelled, frozen moments, acceleration rates, swimming
trajectories and exploration rate. Surprisingly, safety of these wastewaters was restored by
chemical precipitation revealing non-lethal pH ranges of 6.02 - 8.02 and 6.65 - 7.65 for SWFE
and JB, reducing the mortality rate to non-significant levels (p > 0.05) compared to the control.
Also, sodium bicarbonate (NaHCO3) at 120 mg/L was found effective at supplementing the
wastewater total alkalinity. In contrast, Amanzimtoti water before and after chlorination (TB and
TA), Incoming Badulla (IB) and Chatsworth Incoming (CI) exhibited no consistent lethality effects
on zebrafish and induced no apparent stress as demonstrated by insignificant expression (p >
0.05) of the stress protein: heat shock protein 70 (HSP70). However, the insignificant mortality rate (p > 0.05) in the water tested before (TB) and after (TA) chlorination appeared to be the same
(~25 %) indicating that chlorination is not enough at completely remediating wastewater. Our
study is a pioneer in evaluating the ecotoxicological impact of wastewater effluents from localized
regions of a developing country like South Africa in relation to the adjustment of water quality
parameters for the neutralization of contaminants. To better understand emerging contaminants
released as effluents in SA's water bodies and their interactions with aquatic organisms at the
adult stage, more studies needs to be developed.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.