The Detrimental Mechanism Of Ichthytoxic Microalgae Species On Marine Medaka (Oryzias Melastigma)

Harmful algae bloom of ichthyotoxic species have been implicated in massivemortality of fish and invertebrates, which has caused serious damages to mariculutre.However, the causative agents responsible for the event remain in discussed. Thetoxicity unit of judging ichthyotoxic algae toxicity level also has not been established.Hemolytic toxins and reactive oxygen species are considered to be the essentialcauses of fish mortality. Chattonella marina, Heterosigma akashiwo and Prymnesiumparvum are typical and common ichthytoxic algae species. To investigate thedetrimental mechanism of ichthytoxic algae on fish, effect of C. marina, H. akashiwoand P. parvum, digitonin substituted for hemolytic toxins, as well as hydrogenperoxide substituted for reactive oxygen species on Oryzias melastigma were studiedin this paper.The results showed that no fish died in the group of digitonin concentration of0.2μg ml-1within48h. LC0of digitonin on Oryzias melastigma was0.2μg ml-1. LC50ofdigitonin on Oryzias melastigma was3.44μg ml-1within30min, and the LC50was1.25μg ml-1within60min. LC50of digitonin decreased with the prolonged survivaltime of fish, which showing a good exponential relationship. LC50of hydrogenperoxide on fish was144.5mmol·L-1within60min. By using methods of Toxic Unit,Additional Index, Mixture Toxic Index and Similarity Parameter in combination, theresult indicates that there was a synergy between digitonin and hydrogen peroxide tothe death of Oryzias melastigma.Fish unit (FU) was established according to the toxic experiment in this paper,which provided a standard for judging toxicity of ichthyotoxic algae. One FUrepresented the digitonin concentration needed to kill a fish weighed20mg at60daysof age, which was3.44μg ml-1digitonin. Hemolytic units (HU) was1.02μg ml-1, and one FU was equivalent to3.4HU. Morphological fish gill damage, activity of SODand CAT and contents of MDA in Oryzias melastigma incubated with or without threeichthytoxic algae, digitonin and hydrogen peroxide were also observed or detected inthis paper. It was suggested that hemolytic toxins play a predominant role in ichthyotoxity, and reactive oxygen species further exacerbated the death of fish.