Apart from birds, most onshore creatures need to find rafts to cross the open sea.
Plastics are light and tough. They can float longer than natural rafts like driftwood because they have a lighter density than seawater.
Moreover, plastics are basically not broken up by microorganisms, so they do not disappear. Therefore, plastics create a habitat that sustains various creatures, much like small islands (Harrison et al. 2011, Zettler et al. 2013)．
We call this unique ecosystem related to plastics the “plastisphere” (Zettler et al. 2013)．
Plastics floating on the sea accelerate to spread of unbidden alien and invasive species (Jokiel 1990, Minchinton 2006, Bryan et al. 2012). This can include harmful microorganisms and pathogenic organisms (Keswani et al. 2016)．
Hence, plastic waste is believed to be the factor expanding the habitat of certain organisms (Keswani et al. 2016)．
Microorganisms stick to artificial and natural objects and form “biofilms”. For example, if we put plastics such as polyethylene (PE) and polyethylene terephthalate (PET) into seawater, the microorganisms multiply and a biofilm is immediately formed on the surface of the plastic (Webb et al. 2009, Lobelle & Cunliffe 2011)．
What Is Biofilm?
Biofilm is organic matter such as glycoprotein secreted by microorganisms (Flemming et al. 2007). Thanks to biofilm, microorganisms can stick to the surface of various objects (O’Toole et al. 2000)．Research has revealed that microorganisms on biofilm can live longer than microorganisms in seawater.
On the coast of Europe, researchers have found 150 different bacterial strains, including Escherichia coli and disease-causing bacteria such as Pseudomonas anguilliseptica (Van Der Meulen et al. 2014).
Microplastic in the river has a bacterial community that is different from the bacteria multiplying on the surface of natural objects. When microplastics end up in the sea and downstream, they become a carrier of these different bacterial communities (McCormick et al. 2014).
As microplastics have become a carrier it has been pointed out that the opportunity for people who enjoy swimming in the sea to touch pathogenic bacteria is increasing (Keswani et al. 2016)．
However, we still cannot clarify whether microplastics actually make pathogenic bacteria propagate as well as carry them. Therefore, further research is needed (Caruso 2015).