Adhesion of freshwater microorganisms to microplastics
Millions of metric tonnes of plastic debris have entered oceanic and freshwater environments. Among these, microplastics (plastic debris which are less than 5mm in diameter) are of particular concern as they can be ingested by birds, fish, etc. Previous research has shown that microplastics provide a substratum for bacterial adhesion in the oceans. However, there is a paucity of information on the adhesion of freshwater bacteria to plastics, especially microplastics. There are several forms of recyclable microplastics which we are examining: polypropylene (PP), polystyrene (PS), high-density polyethylene (HDPE), and low-density polyethylene (LDPE). Each type of plastic possesses specific surface morphology and characteristics which we predict will influence adhesion of bacteria. For example, polyethylene has a net negative charge while polypropylene has a net neutral charge. The purpose of this study was to examine colonization of different plastics by freshwater bacteria. Bacteria found in freshwater were allowed to colonize different forms of microplastics and were visualized using scanning electron microscopy (SEM) and enumerated using confocal microscopy with DAPI staining. Specifically, spherical microplastics (2-2.5mm in diameter) were incubated with water from Lake Erie for 8 weeks and colonization was examined.
Colonization of the spherical microplastics composed of PP, PS and PE (representing both HDPE and LDPE) was visualized with the help of SEM at weeks 1, 4, and 8. Imaging showed a gradual increase in attachment of bacteria, diatoms, and adherent biofilm matrices on all three forms of microplastics. PS exhibited high microbial attachment from week 1, which gradually increased at weeks 4 and 8. PE and PP had minimal microbial attachment at week 1; however by week 8, biofilm matrices were apparent. Subsequent testing will involve growing known, individual species of freshwater bacteria on flat (rather than spherical) microplastics generated from recyclable plastics which will then be imaged and enumerated. In conclusion, microplastics are chemically persistent in the environment, providing a stable substratum for bacteria and potentially transporting them across water bodies, which might impact the microbial ecology of freshwater ecosystems. Information about bacterial adhesion generated from this study will help us understand how different forms of microplastics act as substratum within freshwater environments.