# Microbial Life on the Surface of Microlgistics in Natural Waters
## 3 Microplastics as Substrate for the Microbial Colonization in Marine and Freshwater Environments  
The main kinds of plastic, approximately 75% of total plastic production, are polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalalate (PETE), polyvinyl chloride (PVC), and polyurethane (PU) [1]. Polyethylene, polystyrene and polypropyleneparticles were identified in the majority of plastic debris found on the surface of North Sea (UK) [59]. Different kinds of microplastic have their own density, g/cm3: polypropylene and polyethylene (0.91); polystyrene (0.96-1.05); polyurethane (1.13); polyamide (1.13-1.35); polyethylene terephthalate (1.38), and polyvinyl chloride (1.47). The difference in plastic density specified its distribution in the water column: plastic with density lower than density of water, particularly, polypropylene and polyethylene floated on the water surface, while polystyrene, polyethylene terephthalate, and polyvinyl chloride with a higher density than water were deposited through the water column [60]. So, presence of microplastic can be expected in every layer of marine systems. However, the character of plastic and microplastic distribution in the layers of water can be changed in time due to its colonization by microorganisms such as bacteria, fungi and algae forming biofilm on plastic surfaces that cause an increase of microplastic density. For example, it was shown that the befouling of plastic particles can result in their settling down and, according to the model based on settling, biofilm growth, ocean depth profiles for light, water density, temperature, salinity, and viscosity, the authors predict that the larger particles start settling last, so these particles are present longer at the ocean surface [61]. At present time a number of studies of microbial biofilms covering the microplastics particles in the marine environment have been conducted. However, almost every study has its own research design and detected parameters, and it is difficult to compare obtained data. Microorganisms in seawater are able to rapidly in a few hours attach to a microplastic particle and colonize its surface [62, 63]. It was suggested that the development of a microbial community on microplastics progresses in a certain succession which can be divided into early, mid and late colonization periods [64]. It was observed that representatives of _Gammaproteobacteria_ (genera _Pseudomonas_, _Acinetobacter_ and _Alteromonas_) dominated in the microbial community during 0-9 h on acryl samples placed in seawater; meanwhile the members of _Alphaproteobacteria_ (genera _Loktanella_, _Methylobacterium_, and _Pelagibacter_), became predominated during 24-36 h of biofilm formation. However, the same succession was observed for biofilms developed on glass and steel, notwithstanding that bacterial communities were different in biofilms developed on different substances [65]. The study of biofilm development on different substances such as polycarbonate and stainless steel in estuarine water showed that at the beginning the representative form _Alpharotobacteria_, mainly the order _Rhodobacterales_, were abundant members in both stainless steel and polycarbonate biofilms, but after one week of growth the differences were observed [66]. Members of the _Gammaproteobacteria_, namely from the genus _Olebiucter_, dominated up to 59.1% among microbial communities in biofilm during the first day of their formation in coastal marine on the samples of polyvinyl chloride, but were quickly replaced by the representatives of _Alphaproteobacteria_, mainly represented by _Rhodobacteraceae_, and _Flavobacterium_[67]. The composition of microbial communities of biofilms attached to polypropylene and polyvinyl chloride microplastics in seawater varied depending on location and time of exposure, but bacteria from the _Alpharotobacteria_ class, mainly _Rhodobacteraceae_, dominated, followed by the _Gammaproteobacteria_ class [68].