Group Post; A plastic house for microbes?

It is well documented that plastic is the most common form of marine debris, and its presence in our waters continues to increase, thus the term ‘plastisphere’ has been introduced. The effects of plastics on animals are relatively well documented; the same however cannot be said for microbes. Plastics provide a substrate to which microbial communities can attach, that said plastics may elicit a negative impact on these communities, this has not been investigated. The aim of this study was to characterize the microbial communities (with particular focus on the bacteria) of two different plastics, and compare these to the open ocean.

Samples were collected from the surface waters of the open ocean. Plastic fragments were sorted in to two groups; those destined for SEM and those DNA analysis. In order to confirm the identity of the plastics, ramen spectroscopy was undertaken, these were identified as polypropylene (PP) and polyethylene (PE). Using amplicon pyrotag sequencing, the microbial communities were categorised through formation of operational taxonomic units (OTUs) and cluster analysis was employed.

Zettler et al. (2013), analysed 2 types of plastic; polypropylene and polyethylene. Both plastics showed signs of degradation in terms of cracks and pitting. These pits contained unidentified round cells of 2µm diameter with signs of division, suggesting active growth. They named the communities associated with the plastic the ‘plastisphere’. In this plastisphere they found rich eukaryotic and microbial community with evidence of phototrophy, symbiosis, heterotrophy and predation. Diatoms and filaments were the most common morphotype observed on the plastic. As expected there was a distinction between the communities associated with the plastic compared to that of the seawater. A high concentration of Vibrio species were found in the plastishpere communities, which whilst not identified to species level, could show possibility of animal or human pathogens. This could show potential for the transmission of disease via these plastishpere communities and the importance of finding out more about plastic associated communities.

From the analysis, the authors identified a huge range of diversity or OTUs from a single fragment of PE and PP. They observed two distinct differences between diversity patterns between the plastics fragments and surrounding seawater. Firstly, the average observed richness was much higher in surrounding water compared to plastics. Secondly, seawater have higher average richness and polyethylene with the lowest.  However, plastic substrates showed greater evenness than seawater.Nevertheless, there is much ambiguity in quantifying the relative richness between seawater and plastic substrates. First off, sample size difference might be one of the big considerations which would result in the skew of the data. Even with normalisation of the results with respect to sampling effort, it still raises the question if the way of quantification is appropriate. Richness correlates with substrate area. When comparing plastic substrate of different sizes, they observed bigger size plastic to have higher diversity. Evenness on the other hand, was consistently higher on plastics compared to seawater and the brown alga Sargassum.    

Some of the bacteria identified in the ‘plastosphere’ were thought to be capable of degrading hydrocarbons. The identified taxa were recognised as being associated with hydrocarbon degradation such in the environment, such as in the horizon oil spill. The author suggested that the bacteria had formed a network that together was degrading the plastic. They indicated that through either physical or metabolic processes, bacteria degradation could be a possible sink for plastic in the ocean.

Jack, Freya, Li and Kat

Zettler, E.R., Mincer, T.J. and Amaral-Zettler, L.A. (2013) Life in the ‘Plastisphere’: Microbial Communities on Plastic Marine Debris. Environmental Science and Technology. 47, pp7137-7146.