Microalgal Bioremediation of Microplastics: Current Advances, Challenges and Environmental Applications

The world is facing a triple global crisis: climate change, loss of biodiversity, and pollution. Plastics, man-made polymers from primary fossil fuel sources, have pervasively entered almost every sector and thereby caused huge environmental pollution. Microplastic (MPs) sizes range between 1 μm and 5 μm, and nanoplastics are less than 1 μm in size, respectively. Behaviour, accumulation and movement of plastics in soil, water and air depend on their size. Smaller sizes of microplastics and nannoplastics can easily enter cells, tissues, and finally, the food chain, and pose growing ecological and health concerns due to their persistence. It also acts as an adsorbent and has the ability to penetrate food webs. Their diverse shapes, especially fibres, increase their dispersal and bioavailability in aquatic ecosystems. Recent findings indicate that microalgae contribute to the mitigation of microplastic pollution. Microalgae, diverse photosynthetic microorganisms ranging from 0.5 to 200 μm, interact with microplastics through processes such as adsorption, aggregation, and potential biodegradation mechanisms, including toxin systems or enzymes produced by the microalgae, utilising plastic polymers as carbon sources. The filamentous morphology of some species, such as Spirulina sp., increases the entrapment and potential degradation of microplastic fibres. It is essential to understand these interactions to construct sustainable means of mitigating microplastic pollution and recovering the aquatic ecosystem health. Microalgae produce enzymes such as lipases, esterases, and cellulases, which facilitate the biodegradation of plastics. Microalgae form biofilm on the surface of microplastics. They also secrete sticky extracellular polymeric substances (EPS) that cause the microplastic to sink to the bottom of the water body, which helps to remove microplastics from surfaces. This review summarises knowledge on microalgae–microplastic interactions, with a focus on their potential for bioremediation of microplastics and also sustainable conservation of the environment.