Plastic degradation and biofilm formation

Plastic became an omnipresent material in the everyday life these days and one has to deal with the waste. A lot of plastic is not recycled, therefore, a lot of plastic occurs in the environment, also in the ocean waters (like in the Great Pacific Garbage Patch). Plastic and its particles can be ingested by animals or can be incorporated in the sediments. Consequently, plastic debris is a significant problem in environmental pollution and can also be ingested by humans who consume e.g. fish that on its part has ingested plastic particles.

Normally, plastic is positively buoyant and floats at the seawater-air interface. As a result of fouling processes and degradation it can lose its buoyancy and start sinking and therefore e.g. can be incorporated in the sediments. Attaching algae to the plastic and other organisms can increase the degradation process and make the plastic neutrally buoyant. The attaching organisms can form biofilms, whereby firstly bacteria will attach to the plastic, followed by unicellular eukaryotes, larvae and spores.

The study by Lobelle & Cunliffe (2011) aimed to examine the early biofilm formation on plastic debris to get a better understanding of the mechanisms of the behaviour of plastic in the oceans.

For that reason, plastic food bags (polyethylene) were secured to boards and brought into the coastal waters in Plymouth, UK at 2 m depth for a duration of 3 weeks in summer. The plastic bags were sampled weekly and brought to the laboratory. Lobelle & Cunliffe used crystal violet to dye the samples and to make the attached biofilm visible. The stained plastic samples were incubated in ethanol, which was then transferred into a cuvette and its optical density at 595 nm was measured. Buoyancy was measured by placing the washed plastic samples in sterile seawater and noting its position after 10 min. Furthermore, the hydrophobicity was measured and the attached biofilm was diluted and plated out on Marine Agar 2216 (Difco) and polyethylene marine agar plates and incubated for 3 weeks to determine the number of culturable bacteria on the plastic samples.

After the first week of the experiment a visible biofilm has started to form and significantly increased during the 3 weeks. Coinciding with this, a decrease in hydrophobicity as well as an increase in hydrophilicity after 2 weeks were detected. Additionally, the plastic lost its positive buoyancy after about 2 weeks and floated below the seawater-air interface when tested in the laboratory and showed neutral buoyancy in week 3.

Apparently, the plastic samples were positively buoyant again and floated at the seawater-air interface as the control plastic in week 3, which is contradictory to the finding presented above. The authors mention that physicochemical effects of biofilm can be reversible and plastic can be positively buoyant when rapidly defouled, but do not really explain the finding and it remains unclear.

Over the duration of the experiment an increasing number of culturable bacteria has been found but no polyethylene-degrading bacteria were detected. Previous studies have shown that plastic-degrading bacteria like Pseudomonas or Arthrobacter have only been isolated from plastic after 12 weeks. Consequently, it takes longer to “infect” plastic with plastic-degrading bacteria.

The paper by Lobelle & Cunliffe is a bit hard to follow because it has no subheadings to structure it more. Moreover, it lacks some information to clarify the results. It is not clear, why the plastic was placed at 2 m depth. I think that, firstly, the plastic would float in the SML and would start to sink after a while. I think that the composition of the bacterial communities differs between the SML and at 2 m depth, so it would appear more logic to me to place the plastic bags directly at the surface, because plastic-degrading bacteria may be more abundant in the SML. A possible reason for the placement could be that the plastic cannot be ingested by e.g. birds during the duration of the experiment. Additionally, I have not found the results of the optical density measurements. In some points the study seems to be incomplete or some parts were not chosen to be published in this 4 page long paper.

All in all, this study provides a good base for understanding the influencing factors on plastic debris behaviour. It has been cited nearly 100 times since its publication and shows that plastic-degradation may start after more than 3 weeks and is a long process that includes different factors. It would be interesting to see how the results change when the study is conducted with the plastic floating directly at the seawater-air interface and with a different kind of plastic. 

Reviewed paper:

Lobelle, D., & Cunliffe, M. (2011). Early microbial biofilm formation on marine plastic debris. Marine Pollution Bulletin, 62(1), 197-200.

http://ac.els-cdn.com/S0025326X1000473X/1-s2.0-S0025326X1000473X-main.pdf?_tid=e830865e-b55f-11e6-b984-00000aab0f27&acdnat=1480333600_8074b98c59785186b7da541593992c3d