bacteria and marine fungi, plastic degraders?

Polyethylene (PE) Is the most commonly produced plastic that is discarded after use. A large amount of plastic eventually ends up scattered throughout the oceans. Polymer type, environmental conditions and season can affect the composition of a microbial biofilm. Plastic debris has a hard, hydrophobic surface which is an ideal environment for settlers. It can take as little as one week for a biofilm to form on plastic debris with the biofilm community significantly differing from that of the surrounding the environment. The degradation of PD is extremely slow and it is thought that microorganisms may degrade PD in the environment. A few bacterial species have been identified as PE degraders and recently marine fungus have also been shown to have the potential to degrade PE.

Current studies on biofilm formation on plastics are short term studies that do not assess the bacterial communities present on the plastics. These studies are also carried out on random pieces of collected plastic of unknow origin.

Long term exposure experiments (44 weeks) were carried out in the Belgian part of the North Sea at two locations in the harbour of Ostend and offshore, at the Thornton windmill park. The Harbour is affected by land run-off, ship discharges and pollution through waste pipes while the off-shore site is affected by activities from a wind farm and Fisheries.

Two types of PE with different colours and shapes were chosen. Transparent plastic sheets and orange dolly ropes were used. Three pieces of each were attached to an anchor and sent to the seafloor in both locations and Samples were collected at different time points. Half of the plastic was used for DNA extraction and half used in biofilm assays. Samples of sea water and sediments were also collected at the same time.

From the first week, a coating containing a biofilm, sediment particles, algae and macro-fouling species could be observed on plastic sheets at the harbour. Bacterial and fungal communities were analysed through metabarcoding and sample richness was assessed using OTU’s. Both showed that the plastic sheets and dollies had similar assemblages of bacteria and fungi in the harbour. At each timepoint the bacterial richness of the plastics was higher than fungal richness.

The bacterial community in the harbour showed a gradual change in abundance of bacterial classes over time. Alpha- and Gammaproteobacterial are primary biofilm colonizers while Bacteroidetes are secondary biofilm colonizers. There was a gradual shift between the two types suggesting that there are time points in the development of biofilms. This was not however seen on the dolly ropes.

25 bacterial core OTUs were identified on both plastic types. These were then placed into 4 groups based on their core members. 1 – neutral, without a clear period of high abundance, 2- higher abundance in the beginning, 3- higher abundance in the middle and 4- OTUs with highest abundance at the end of the exposure period.

Most of the fungal communities on the plastics could not be assigned using the UNITE database. Some of the reads could be identified as fungi using BLAST. This showed that Ascomycota was highly abundant but there was no core group of fungal organisms showing that fungi had high variability over time.

Offshore plastics had biofilm formation but it was much less pronounced compared to the harbour. The amount of biofilm seen at 22 weeks on the offshore site was similar in amount to the harbour site after 1 week. Bacterial OTUs remained low in offshore compare to harbour up to week 18. Fungal OTUs were similar between the two sites. Offshore sites did not have a gradient of primary and secondary colonizers with levels remaining steady throughout.

10 OTUs were identified in the offshore sites which were dominated by Flavobacteria and Gammaproteobacteria. When assessing the fungal communities more of the offshore sequences were unassigned than in the harbour. Fungal communities were like harbour communities in that there were no core members and that Ascomycota and Basidiomycota dominated.

Overall three fungi identified to species level were found on the plastic samples. These were more abundant on the harbour site and have been previously identified as PE biodegraders. No previously identified bacterial PE biodegraders were found in any of the plastic samples. This study found that substrate degradation efficiency increased when a biofilm is formed on the substrate and microbial populations had a higher metabolic activity when in a biofilm compared to when in the planktonic form. Overall there was no significant increase in biodegradation of plastics by microorganisms.

This study was limited by the lack of knowledge of marine Fungi. Many marine fungi are not functionally characterized and little is known about their biological functions and so their role in biodegradation of plastics is little understood.

Temporal Dynamics of Bacterial and Fungal Colonization on Plastic Debris in the North Sea

Tender et al., (2017) ‘Temporal Dynamics of Bacterial and Fungal Colonization on Plastic Debris in the North Sea’, Environ. Sci. Technol 51, 7350−7360. DOI: 10.1021/acs.est.7b00697