Triple-threat algae engineered to turbocharge microplastic bioremediation

Engineered algae are a bioremediation triple threat, synergizing microplastic removal, upcycling and wastewater nutrient utilization to combat environmental contamination.

By genetically engineering cyanobacteria to produce limonene, researchers from the University of Missouri (MO, USA) and Texas A&M University (TX, USA) have created a system that could be used to remove microplastics from wastewater. With refinement, this approach could one day be used for bioremediation and upcycling efforts to help tackle our environmental pollution problem.

Microplastics pose a major threat to ecosystems, with negative impacts on a broad spectrum of living organisms, including microbes, plants, animals and humans. Their removal from the environment requires efficient, cost-effective and sustainable remediation strategies – however these are hard to come by.

Conventional methods are expensive and often subject to clogging and water chemistry variations that impact their effectiveness. Meanwhile, current state-of-the-art remediation primarily focuses on designing systems to remove microplastics from the aqueous phase, which leaves a waste stream that requires storage or further processing. Moreover, microplastic removal capacity generally decreases when particle sizes get smaller – all of which means we are in need of new and improved microplastic remediation technologies if we are to curb its environmental contamination.

It is against this backdrop that scientists have developed RUMBA – a technology for the remediation and upcycling of microplastics by algae.

The team engineered Synechococcus elongatus UTEX 2973 to produce limonene – the chemical that gives oranges their zesty scent – which is enriched on the cell surface, increasing hydrophobicity. This, in turn, enhances interactions between the cell and microplastics, which are also hydrophobic, enabling rapid aggregation and removal.

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Cell aggregation and self-sedimentation were observed in the hydrophobic cyanobacteria cell samples, while a BATH assay confirmed the cells’ elevated hydrophobicity by demonstrating that a larger portion of engineered cells attached to a hydrophobic hexadecane layer compared to wild type.

To investigate whether the hydrophobic cell surface could aid microplastic removal, the researchers studied its affinity for polystyrene. Dry weight measurements suggested that sediments accounted for 82.5% of the total mass for hydrophobic cyanobacteria cell samples in the absence of polystyrene, which increased to 90.3% when the polymer was added. Then, thermogravimetric analysis demonstrated that microplastic dry weight constituted 37.8% of the total suspension mass of engineered algae suspensions. Finally, the removal rate was calculated based on these two statistics, revealing that 91.4% of polystyrene microplastics were removed by engineered S. elongatus within 1 hour. This efficiency was superior to previously developed microplastic bioremediation methods that rely on extracellular polymeric substances.

The team also established an upcycling strategy that converts microplastic-enriched cyanobacteria into plastic composites and can integrate microplastic removal with cyanobacterial bioproduction and wastewater treatment.

Ultimately, they conclude, RUMBA provides a viable and sustainable pathway to address microplastic pollution.

“By removing the microplastics, cleaning the wastewater and eventually using the removed microplastics to create bioplastic products for good, we can tackle three issues with one approach,” study author Susie Dai noted. “While our research is still in the early stages, our eventual goal is to integrate this new process into existing wastewater treatment plants so cities can clean their water more effectively and reduce pollution while creating useful products at the same time.”