Microbes munching on microplastics in mangrove mud

Microplastic pollution in marine ecosystems is a contemporary issue that has been at the forefront of environmental research and the media in recent years. Roughly 4.8 million tonnes of plastic enter the marine environment every year with microplastics constituting 92.4% of this waste. Microplastics are often made deliberately for use in cosmetics that then pass through wastewater treatment or by the weathering and breakdown of large plastic debris.

Microplastics have a global ocean distribution and we are still only just beginning to discover the ill effects that they cause to marine animals and environments. Microplastics are often found within filter feeding invertebrates, marine mammals and seabirds causing pathological stress, stunted growth, and false satiation and even facilitate the accumulation of heavy metals in marine sediments. Even with ever-increasing evidence of the damage to marine life microplastics cause, less attention is given to strategies for environmental remediation. Mangrove forests support a high diversity of microbes and are often subjected to high plastic pollution, could members of these microbial communities be a solution to our growing plastic problem?

The aims of the reviewed study were to provide a remediation solution to microplastic polluted environments, using bacterial isolates, and to evaluate the potential of marine bacteria to degrade microplastics.

Microplastic polyethylene, Polypropylene (PP), Polystyrene and Polyethylene terephthalate were collected and treated with UV-radiation. Sediment samples were taken from the top 4 cm from six mangrove forests around the Peninsular Malaysia from which bacteria were isolated, plated onto nutrient agar (NA) and incubated. Pure cultures were obtained after incubation for species identification and plating onto a mineral salt medium (MSM) with the UV-treated microplastics as the only carbon source. Species that could grow on the MSM were inoculated into an MSM broth containing one of the four microplastics and incubated for 40 days. Cell growth was monitored every 10 days and after 40 days microplastic weight loss, the rate of degradation and half-life was calculated. Changes in the plastic structure were assessed by Fourier transform infrared and scanning electron microscopy (SEM).

Only two isolates obtained from incubation could grow in the presence of plastics, these were Bacillus cereus and Bacillus gottheilii. Both species showed similar growth patterns over the 40-day incubations with exponential growth between day 0 and 20 followed by a die off. However, B. gottheilii could grow in the presence of PP where B. cereus could not. B. gottheilii shows a wider capacity to degrade microplastics than B. cereus while also degrading plastics at a higher rate. Microplastics were also shown to be oxidised when inoculated, these bacterial species are able to alter the chemical structure of plastic polymers to make them easier to adhere to and degrade. SEM observations also revealed that plastics inoculated with B. cereus and B. gottheilii had much rougher surfaces and bore crevices and holes.

This study has revealed two bacterial species with a capacity to degrade plastic and could serve as a solution to boost remediation of polluted mangrove sediments. Adding bacteria to sediments may be an “environmentally safe” method of plastic clean-up, but future work should focus on what effect if any, adding these microbes has on the community dynamics of the sediment. Also, subsequent work should try to establish the molecular and genetic pathways these microbes possess that are involved in microplastic degradation.

Paper reviewed:

Auta, H.S., Emenike, C.U., & Fauziah, S.H. (2017). Screening of Bacillus strains isolated from mangrove ecosystems in Peninsular Malaysia for microplastic degradation. Environmental pollution, 231, 1552-1559.