Effects Of Microplastics As Carriers On Metal Bioaccumulation In Marine Medaka Oryzias Melastigma

Microplastics are plastics smaller than 5 mm in size and have been found in many ecosystems.Microplastics are difficult to degrade;the amount of microplastics in the environment is expected to increase continuously.The size of microplastics is similar to many natural feeds;microplastics thus are easy to be ingested by aquatic organisms and can be transferred in the food web,posing great potential risks to the ecosystem health.Microplastics coexist with a variety of pollutants in the environment,easily adsorb other pollutants in aquatic environments,and then become important carriers of pollutants.These characterists and environmental behaviors should be considered when evaluating microplastic ecological risks.At present,there are few studies on the mechanism,capacity and biological effects of the adsorption of heavy metals by microplastics,which impedes the accurate ecological risk assessment of microplastics.In this study,we simulated the processes of metal adsorption onto microplastics in the real aquatic environment and explored the associated adsorption mechanism.We selected the marine medaka Oryzias melastigma as the model organism to study the bioaccumulation and toxicity of copper(Cu)under single Cu exposure and combined exposure of Cu and microplastics.Firstly,six different laboratory-ground microplastics(LDPE,PET,PVC,PS,PP,Nylon)and a tire wear particle were placed in Jiulong River Estuary,Tongan Bay,and Shenhu Bay for 50 days for aging.The surface properties of the aged microplastics were analyzed by using stereomicroscope and scanning electron microscope.Heavy metal concentrations of aged microplastics were measured.Substantial differences exist in heavy metal concentrations measured on different microplastics.Concentrations of microplastic-adsorbed heavy metal also varies greatly among sites.Microplastics aged in Jiulong River Estuary show the highest metal concentrations,reflecting its status of metal pollution.The microbial community on the surface of microplastics was analyzed.Microplastics aged in Jiulong River Estuary was found to have the highest abundance of microorganisms.There was a strong positive correlation between microbial abundance and heavy metal pollution.The adsorption of 65Cu by microplastics showed that aging significantly increased Cu adsorption capacity of microplastics,The above results show that microplastics can be important carriers of heavy metals in aquatic envrironments.Secondly,the effects of salinity on Cu bioaccumulation and toxicity were studied.At low salinity(5 to 10),the uptake rate constant of Cu is higher,and the Cu bioaccumulation potency is higher,indicating the higher bioavailability of Cu,which is due to lower water chemistry effects(e.g.,complexation and competition effects)from salinity constituents.At salinity 5,15,and 30,the 96-h median lethal concentration of Cu to the marine medaka O.melastigma was 191 μg L-1,4780 μg L-1,and 587 μg L-1.It shows that with the increase of salinity,Cu toxicity decreased first and then increased.This trend was further confirmed in 96-h acute toxicity tests at a fixed Cu concentration of 500 μg L-1,which also showed that the marine medaka have a higher survival rate at intermediate salinities.The higher tolerance of marine medaka at the intermediate salinities could be attributed to its lower intrinsic sensitivity to Cu measured by the two toxicodynamic paratemers(i.e.,internal threshold concentration and killing rate).Finally,we studied the effect of microplastics on Cu bioaccumulation.When O.melastigma was exposed to a mixture of microplastics and Cu,it was found that the presence of microplastics increase the bioaccumulation of Cu,but the effect of microplastics decreased with the increase of microplastics concentration.There were no significant differences in the influence of microplastics with different particle size ranges on Cu bioaccumulation.The comparison between O.melastigma and the filterfeeding mussel Perna viridis showed that the influences of microplastics on organisms with different feeding strategies were different:the presence of microplastics reduced the accumulation of Cu in P.viridis.The comparison between mieroplastics and natural suspended particulate matters shows that the effect of microplastics on Cu bioaccumulation is much lower than that of suspended particulate matter.In conclusion,microplastics can adsorb heavy metals in aquatic environment and become the carrier of heavy metals.Microplastics can be ingested by aquatic organisms and increase the bioaccumulation of heavy metals.The extent of microplastic carrier effects is expected to vary among different biological species.Although the carrier effects of natural suspended particulate matter in increasing metal bioaccumulation is much higher than that of microplastics,the additional risks generated by microplastics cannot be ignored.In this study,the ecotoxicoligical effects of microplastics,Cu,and their combination have been studied from sevral aspects;the findings will provide a scientific basis for the assessment of ecological risks of microplastics.