# Ecotoxicological Assessment of Microplastics in Freshwater Sources--A Review
## 5 Ecotoxicological Effects on Microplastics
### Cumulative Effects of MPs and Anthropogenic Molecules  
MPs released in the environment are exposed to other hazardous substances (e.g., heavy metals, CECs) and can act as vectors for their further transfer. MPs have strong adsorption capacity due to small particle size, large specific surface area, and high hydrophobicity [147; 148]. PE, PP, PS, and PVC particles showed high sorption potential for hexachlorocyclohexanes, polycyclic aromatic hydrocarbons, chlorinated benzenes, and other CECs [149]. MPs sorption capacity depends on physical-chemical characteristics, composition, color, shape, and size [54]; e.g., colorless MPs showed greater adsorption potential for polychlorinated biphenyl (PCBs) comparing to colored ones [150].  
The ingestion of MPs with hazardous substances adsorbed can have various adverse effects on organisms since multiple modes of action can occur. The combined toxicity effect (synergism) of MPs and adsorbed substances [131; 151; 152] must not be excluded  
Figure 5: Effects of microplastics on zebrafish.  
from consideration as well. The most studied anthropogenic molecules adsorbed on MPs include heavy metals and CECs. It was found that PS with cadmium adsorbed exhibited increased toxicity to _D. rerio_. Thus, MPs contributed to the accumulation of cadmium in fish livers, guts, and gills, and caused oxidative stress and inflammation [153]. In the case of 17\(\alpha\)-ethinylestradiol (EE2) adsorbed, genotoxicity, reproductive and behavioral effects were observed [124]. [PERSON] et al. [154] studied the synergistic action in adverse effects of simultaneous exposure of early juveniles of goby fish (_Pomatoschistus microps_) to PE microspheres and chromium(VI). A significant decrease of predatory performance (\(\leq\)67%) and the inhibition of AChE (\(\leq\)31%) were noticed, which was not observed when species were exposed to each pollutant separately. Endocrine disruptors are often used in studying potentially combined effects with MPs. [PERSON] et al. [155] studied adverse effects of PS with PCBs adsorbed on lugworm _Arenicola marina_; however, they concluded that PS does not act as a vector for PCBs, as demonstrated for cadmium [153] or 17\(\alpha\)-ethinylestradiol [124]. A similar conclusion was given by [PERSON] et al. [156] who tested the influence of PS and PE microspheres with adsorbed PCBs on Norway lobsters (_Nephrops norvegicus_) and found no significant increase in PCBs bioaccumulation. [PERSON] et al. [118] examined the behavior of _D. magna_ exposed to 1 and 10 \(\upmu\)m PS, pharmaceutical roxithromycin, and their mixture, reporting the strongest biological response during simultaneous exposure.  
It is interesting that reports related to MPs particles with pesticides adsorbed [157; 151] are rather scarce despite the facts that: (i) many pesticides are considered as CECs and are included in the list of priority substances [158] and their watch list [159]; and (ii) the agriculture, which is among the major sources of MPs and pesticides in the environment [160; 161], makes that these two pollutant classes are commonly present in the environment.  
MPs can also adsorb various microorganisms, including those able to degrade MPs, or, more concerning, pathogens. Once formed, surface biofilm can cause changes of surface properties (e.g., decreasing hydrophobicity), resulting in the increased MPs density, and consequently yielding MPs sinking to deeper layers of aquatic ecosystems, exposing a filtering organism to MPs intake as well [15]. This is particularly concerning for low-density MPs (e.g., PE and PP) that mostly float.