Systematic critical review of studies and methods assessing microbial degradation of plastics in the marine environment: evidence base and recommendations

When plastic enters the marine environment it is rapidly colonised by microorganisms, and with plastic pollution in our oceans increasing, interest in studying the potential of these microorganisms to degrade plastics has also increased. The validity and reproducibility of some of these studies has however been questioned, and with an ever-increasing number of studies in this area, a critical evaluation of past work and steer for future studies is urgently needed. Here we systematically and critically review studies investigating the microbial degradation of plastics in the marine environment, critically appraising the methods and strength of evidence used to link marine microorganisms to plastic degradation, and providing recommendations for future research. Bacteria from the genera Bacillus, Halomonas and Vibrio were the most commonly identified plastic degrading microorganisms across the studies included in this review. We find that whilst there is substantive evidence of microbially-mediated degradation of most plastic polymers in the marine environment, significant knowledge gaps and study design issues exist. Future studies should use environmentally relevant plastic polymers, concentrations and plastic type. A wide range of study designs have previously been used, each providing some insight, but a lack of method standardisation and reporting of controls and blanks is hindering the field. A lack of in-situ long-term studies means that an understanding of plastic degradation by marine microorganisms in the natural environment is lacking, as many studies only use isolates in short-term laboratory based experiments. Future studies should use multiple methods to assess plastic degradation, and consider the caveats of each method used to investigate plastic degradation and how findings actually link the degradation of plastic to microbial action. A move towards standardised methodology, controlled experimental conditions, and the utilization of modern analytical techniques will help unravel the role that marine microorganisms play in plastic degradation.