Early microbial biofilm formation on marine plastic debris

Plastic debris in marine environments can cause multiple problems for the marine ecosystems. The problems include; ingestion by animals, transporting bound organic pollutants and animal entanglement. Pollution risks from plastics are determined by behaviours of plastic debris, such as buoyancy. Most plastics are positively buoyant and are dispersed by wind and ocean currents, however some become neutrally buoyant and sink below the surface. It has been found that biofueling can play a significant role in controlling plastic debris buoyancy.

Marine biofilms, which can lead to biofueling, can occur on any surface in the marine environment. The degree of biofueling can be increased via the attachment of specific invertebrates and algae. The objective of this study was to characterise early biofilm formation on plastic debris with the aim of contributing to a systemic understanding that maybe useful for biofueling.

Polyethylene plastic food bags were secured to weighted Perspex boards and suspended 2 m below the water’s surface. Samples were taken weekly for 3 weeks. The plastic samples were stained, washed and dried and then the optical density was measured. The buoyancy of the plastic pieces was determined. Attached biofilms were dislodged, diluted and plate on Marine Agar 2216 and polyethylene marine agar plates. The hydrophobicity of the plastic’s surface was measured.

Biofilm formation was visibly apparent on submerged plastic after 1 week and continued to increase throughout the experiment. Along with biofilm formation, the surface of the plastic became less hydrophobic during the experiment. Surface hydrophilicity significantly increased after 2 weeks. The buoyancy assessment showed that after the plastic was initially very hydrophobic and remained at the seawater-air interface. By the end of the experiment, the plastic started to sink below the surface and showed signs of neutral buoyancy.

The number of culturable heterotrophic bacteria on the submerged plastic increased from 1.4 x 10⁴ cells cm^-2 after the first week to 1.2 x 10⁵ cells cm^-2 by the end of the experiment. No polyethylene-degrading organisms had grown under the incubation conditions provided. Previously it has been shown, two strains of the common soil bacteria genera, Pseudomonas and Arthrobacter, have been able to degrade plastic. These were obtained from enrichments that had been maintained for 12 weeks, which was longer than the time that plastic was submerged for in this study (3 weeks).

Plastic debris in marine environments is now an international problem, the factors that influence the behaviour of the plastic debris in the environment need to be identified. It has been shown that microbial biofilms develop rapidly on submerged, concordant with significant changes in the physicochemical properties of plastic. There is still no evidence of potential plastic-degraders during early attachment. I think more studies should concentrate on this subject as it is such a huge problem. Maybe one such study could be one similar to this but over a longer time frame to see if the previously mentioned strains of plastic degrading bacteria become present.

Lobelle D. and Cunliffe M. 2011. ‘Early microbial biofilm formation on marine plastic debris’. Marine Pollution Bulletin. 61 (1). 197-200.

http://www.sciencedirect.com/science/article/pii/S0025326X1000473X