Biofilm development as a factor driving the degradation of plasticised marine microplastics

Biodegradation of microplastics facilitated by natural marine biofouling is a promising approach for ocean bioremediation. However, implementation requires a comprehensive understanding of how interactions between the marine microbiome and dominant microplastic debris types (e.g., polymer and additive combinations) can influence biofilm development and drive biodegradation. To investigate this, polystyrene (PS) and polyvinyl chloride (PVC) microplastics (< 200 µm in diameter) were prepared either without any additives (i.e., virgin) or containing 15 wt% of the plasticisers diethylhexyl phthalate (DEHP) or bisphenol A (BPA). Each polymer-plasticiser microplastic combination was exposed to environmentally relevant conditions in a simulated seawater mesocosm representative of tropical reef waters over a 21-day period to allow for natural biofilm development. Following this, microplastic degradation and the colonising bacterial biofilm was assessed as a function of time, polymer and plasticiser type using infrared, thermal, gel permeation and surface characterisation techniques, as well as 16S ribosomal RNA bacterial gene sequencing, respectively. Together, these analyses revealed time-, polymer- and plasticiser-dependent degradation, particularly of the PS-BPA microplastics. Degradation of the PS-BPA microplastics also coincided with changes in bacterial community composition and an increased total relative abundance of putative biodegradative bacteria. These findings indicate that the metabolic potential and biodegradative capability of the colonising marine biofilm can be significantly impacted by the chemical properties of the microplastic substrate, even within short timeframes.