## 1 Introduction
### Distribution, Abundance, and Importance of Microl plastics in the Aquatic Environment  
There is increasing evidence of ocean-based microplastics present in all marine environments, including the deep seas/trenches [10; 11; 12; 13; 14; 15]. One of the most commonly reported polymer types in marine surface water and sediment is polyethylene (PE) [13; 16; 17; 18]. Moreover, fibers and fragments were noted as prevailing microplastic shapes in the ocean and sediment [13; 19; 20; 21]. However, the abundance, size, and concentration of observed microplastics can vary significantly across different sampling times and regions of the sampled marine environment [22; 23; 24; 25].  
Although there has been a lot of research on marine microplastic pollution, several studies have revealed the presence and distribution of microplastics in fresh open waters comparable to marine ecosystems [26; 27; 28; 29]. The contamination with microplastics has been detected in natural freshwater systems and wastewater treatment plants at various locations around the European, Asian, North American, and South American continents, albeit at very diverse concentrations [30; 31; 32; 33; 34; 35]. Such variability suggests that a variety of locations, anthropogenic activities and environments, and sampling strategies were employed [36]. Owing to their long-term persistence and long-range transportation, approximately 1.15 to 2.41 million tons of microplastics are estimated to be annually transported downstream from their initial sources (mainly wastewater treatment plants) by rivers to seas, with consequences for the aquatic organisms and environments along their transport routes [28; 34; 37; 38].  
Microplastics are thought to be ubiquitous by now, and besides their physical presence in the environment, other characteristics of these materials such as their toxicity, durability, and persistence could pose a potential threat to the environment and ecosystems [20; 39]. Ingestion of or exposure to microplastics could cause negative consequences to organisms. The similarity of microplastics' size, shape, and color to natural sediments and feedstuff can mislead organisms to ingest microplastics instead of their natural diet. This, in turn, can cause malnutrition and, in extreme cases, even starvation effects, observed as the altered growth rate, reduced fitness, and changed behavior of the affected organisms [40; 41; 42; 43; 44; 45]. Irregularly shaped and sharp-edged microplastics could cause abrasion and disrupt the integrity of gastrointestinal mucosa in living organisms [46]. Further, ingested microplastics may release different additives used in plastic production due to changes in the digestive tract (pH, enzymatic actions, etc.), and components such as plasticizers, halogen stabilizers, lubricants, and flame-retardants can be introduced to microplastic-eating organisms, causing additional harm [47; 48; 49]. More importantly, there is increasing evidence that microplastics can serve as a vehicle or concentrator for certain chemical and biological agents (micropol-lutants), and their ingestion could facilitate their transfer to organisms and cause adverse health effects or even death [50; 51; 52].