During the last 60 years plastics have become one of the major sources of pollution on the planet and with annual production of plastic globally being approximately 245 metric tonnes per year this problem is likely to continue. It is hardly surprising then that some of this discarded plastic finds its way into our oceans, accumulating on our beaches and also in the centre of ocean gyres where it is held by currents. This paper looks at the effect that these accumulations of plastic have on microbial communities, in particular they are concerned about the longevity of plastic within the oceans when compared to more natural substances such as wood.
Zetter and his team were particularly interested in the plastic accumulation in the North Atlantic Subtropical Gyre as it does not appear to have increased in size since 1980 despite the rate at which plastic is being disposed of. Given that the hydrophobic surface of the plastic promotes microbial colonisation and biofilm formation this study wanted to show that the microbial communities found on the plastic would be distinct from those found in the surrounding waters.
Sampling and Visual Identification.
Three types of sample were taken: a sample of seawater surrounding the plastic debris, pieces of polyethylene (PE) and pieces of polypropylene (PP). The plastic chosen for the sample are two of the most common used in commercial packaging. After fixing the plastic samples were inspected using Scanning Electron Microscopy (SEM) which revealed 50 different morphotypes of eukaryotic and bacterial cells living on both types of plastic. The two most common organisms were diatoms and filaments however the third most common which is as yet unidentified was found embedded in pits on the surface of the plastic.
Through molecular analysis it was shown that there were distinct communities associated with each type of plastic and the seawater, (Fig 1). For the communities within the plastic the data showed the presence of phototrophes such as the cyanobacteria Phormidium and Rivularia, biofilm forming species including Navicula and Nitzschia. Vibrio species were also detected within the community but on the rRNA sequences used it was not possible to fully identify which species were present giving rise to concerns that the Vibrio strains could be harmful to humans or animals, this is of greater concern given the length of time that plastic can remain in the ocean and the distances it may cover.
Figure 1. Venn diagram showing bacterial OTU overlap for pooled PP, PE, and seawater samples; n = number of sequenced reads per group. Numbers inside the circles represent the number of shared of unique OTU’s for a gives substrate.
Of particular interest were the bacteria found only on the plastics that were known to be capable of degrading hydrocarbons. These were shown to be part of a widespread network within the microbial community linking to other microbes that have previously been found in areas of hydrocarbon contamination. Together with the visual examination of the plastic which showed individual cells embedded in pits within the plastic, this suggests that they may be working as a whole to possibly providing a sink for the recalcitrant carbon which is held on these plastic islands.
It will be interesting to follow the journey that this research team takes as it tries to establish if the microbial communities found within the plastic debris could be the solution to the problem.
Zettler, E. R., Mincer, T. J., & Amaral-Zettler, L. A. (2013). Life in the ‘Plastisphere’: Microbial communities on plastic marine debris. Environmental science & technology.