Toxicity of cyanobacteria and cyanobacterial compounds from Tri An Reservoir, Vietnam, to daphnids

Cyanobacteria are of human and environmental concerns as many of them are known to produce toxic metabolites. Tri An Reservoir provides drinking water for millions of people in Southern Vietnam. Therefore, cyanobacterial composition and toxicity assessment and an early warning system for the toxins from the reservoir are in need to protect the local ecological and human health. The occurrence of potentially toxic cyanobacteria was common in Tri An Reservoir. The seven isolated strains of cyanobacteria from the reservoir did not produce microcystins (MC) under culture conditions. However, in the cyanobacterial scum samples, high concentration of MC were detected and two MC variants (–LA and –LY) were recorded for the first time for Vietnam. The genus #Microcystis# in Tri An Reservoir was assumed to be a MC producer. Occurrence of MC in the reservoir poses a risk to local people, who use the water for domestic purposes. Water samples from Tri An Reservoir and cultures of #Microcystis# and #Aphanizomenon# did not induce behavioral changes of daphnids. However, cultures of #Cylindrospermopsis# and Anabaena increased daphnids’ swimming speed and decreased the daphnids’ distribution and distance between the animals. These adverse effects were supported by the significant elevation or inhibition of antioxidant enzyme (CAT) and biotransformation enzyme (GST) activities in daphnids exposed to non-MC cyanobacterial crude extracts. Survivorship decreased during chronic exposure with increasing MC concentration. Low concentration of MC slightly affected the growth and reproduction of parent daphnids. In the offspring, effects were more pronounced. Age to maturity of the offspring increased whereas their survival decreased after parent generation was exposed to the toxin, even if the offspring were raised in non-toxin medium. Besides, cessation of the eggs/embryos was observed and malformation of neonates caused by cyanobacterial toxins was firstly recorded. Thus, even if the first generation and by acute toxicity testing revealed only small effect, it became clear that environmentally realistic exposure scenarios would adversely impact #D. magna population#.