The Hitchhiker’s Guide to the oceans

The growing abundance of microplastics is a major cause of concern due to the most likely harmful effects on ecosystems and human health. Plastic debris can disperse over long distances as wind, waves and currents all contribute to the transport the particles. Moreover, synthetic polymers are more durable than biopolymers, which leads to their accumulation in the environment. Furthermore, microplastics can also serve as surfaces for microbial biofilms that form so called ‘plastispheres’ which are distinct from the surrounding water. These plastispheres could in turn lead to the transport of microbes over longer distances.

Previous studies (Zettler et al. 2013) (De Tender et al. 2015) have identified potential pathogenic Vibrio spec. sequences in plastispheres. However, the scientists were not able to identify the exact Vibrio species present. In their study Kirstein et al. (2016) set out to detect potentially pathogenic Vibrio species on microplastic biofilms in the North and Baltic Seas. 

Samples of both microplastics and seawater were collected from various sites in the North Sea (39 sites) and the Baltic Sea (5 sites). Subsequently, the samples were analyzed with selective enrichment for pathogenic Vibrio species and Mass Spectrometry for species identification. The presence of regulatory and virulence-associated genes was examined using PCR-amplification. The collected synthetic polymers were identified using Optical Spectrometry. 

Out of the sampled microplastics, most were withering and at least partially covered with biofilms. Polyethylene was the most abundant synthetic polymer in all sample sites (> 40%, however, it is also one of the most widely produced synthetic polymers. The analysis of the associated biofilms showed 13 % of the microplastic samples to house cultivable Vibrios, among them the potential pathogen V. parahaemolyticus. Although the microbial communities in plastispheres weredistinct from their surroundings, in most cases the Vibrio species occurred both in the surrounding water and the plastispheres. Analysis of the water isolates showed V. parahaemolyticus to be the most abundant species. However, the scientists were only able to detect a virulence associated gene in one V. parahaemolyticus strain out of the 104 strains cultivated in total.

The findings on the distribution of the potentially pathogenic Vibrios reflected the results of previous studies. V. alginolyticus and V. parahaemolyticus were dominant in the North Sea sites, with the latter also reported once in microplastics in the Baltic Sea. In contrast, V. vulnificus and V. cholerae were more abundant in the Baltic Sea. In this study, only V. parahaemolyticus and V. fluvialis were detected on both microplastic and seawater samples. Moreover, most Vibrios occurred in coastal and estuarine environments. 

Polymer type, along with season and geographical location, seems to influence the plastisphere communities. However, free-living and attached Vibrios appear to be equally influenced by environmental conditions. 

In conclusion, this study confirms the presence of potentially pathogenic Vibrios on microplastic particles. Surrounding seawater is suggested to be the likely source for the colonization of microplastics by microbes. However, conclusive evidence for hitchhiking, pathogenic Vibrios is beyond the scope of this paper. The authors acknowledge that the distribution of microplastics would have to be researched in a more systematic way in order to provide such evidence.
Nevertheless, with Vibrio abundance on the rise due to climate change (Vezulli et al. 2016) and the accumulation of microplastics on shores, the potential danger of the long-distance transport of pathogens is very worth examining. 

Reference:

Kirstein, I. V., Kirmizi, S., Wichels, A., Garin-Fernandez, A., Erler, R., Löder, M., & Gerdts, G. (2016). Dangerous hitchhikers? Evidence for potentially pathogenic Vibrio spp. on microplastic particles. Marine Environmental Research, 120, 1-8. Link: http://www.sciencedirect.com/science/article/pii/S014111361630112X

Further Reading:

De Tender, C. A., Devriese, L. I., Haegeman, A., Maes, S., Ruttink, T., & Dawyndt, P. (2015). Bacterial community profiling of plastic litter in the Belgian part of the North Sea. Environmental science & technology, 49(16), 9629-9638. Link: http://pubs.acs.org/doi/abs/10.1021/acs.est.5b01093

Vezzulli, L., Grande, C., Reid, P., Helauet, P., Ewards, M., holfe, M., … Pruzzo, C. (2016). Climate influence on Vibrio and associated human diseases during the past half-century in the coastal North Atlantic. Proceeding of the National Academy of Sciences of the United States of America, 113(34), E5062–E5071. doi:10.1073/pnas.1609157113 Link: http://www.pnas.org/content/113/34/E5062.abstract

Zettler, E. R., Mincer, T. J., & Amaral-Zettler, L. A. (2013). Life in the “plastisphere”: microbial communities on plastic marine debris. Environmental science & technology, 47(13), 7137-7146. Link: http://pubs.acs.org/doi/abs/10.1021/es401288x