When temperatures rise, algal blooms flourish across rivers and oceans worldwide. These blooms are well-documented ecological hazards, but a breakthrough study reveals they also rapidly accelerate the breakdown of freshwater plastic pollution into microplastics.

A research team led by Professor Jaewook Myung at the Korea Advanced Institute of Science and Technology (KAIST) has demonstrated for the first time that eutrophic water conditions – which trigger toxic algal blooms – significantly speed up the early-stage weathering of common plastics. 

This, scientists warn, is of increasing importance considering the wave of marine heatwaves we have faced this year, and the subsequent swathes of algal blooms facing our oceans: a prolonged marine heatwave off South Australia recently fueled a severe algal bloom that caused widespread marine mortalities, while record-breaking ocean temperatures in the English Channel and off the US West Coast are prompting active warnings for toxic bloom outbreaks.

According to the researchers, the findings, published in Water Research, suggest that water quality and plastic waste can no longer be managed as isolated issues.

When plastic debris enters an aquatic environment, it quickly becomes colonised by a complex matrix of microorganisms, creating a unique, localised ecosystem known as a plastisphere.

To better understand how algal blooms alter this ecosystem, the KAIST team built laboratory microcosms using water from the university’s campus pond. 

By manipulating light and nutrients, they artificially induced bloom conditions and monitored the material used to manufacture standard plastic shopping bags – low-density polyethylene (LDPE) – over a 42-day period.

In normal water, a thin layer of bacteria forms slowly on the plastic, but in nutrient-rich water experiencing an algal bloom, a massive amount of blue-green algae and specialised bacteria quickly takes over.

This algae produces large amounts of a sticky, mucus-like glue that bind the microbes together into an unusually thick, aggressive layer. Within this dense buildup, the types of bacteria shifted, and the population of microbes that possess specific enzymes capable of breaking down plastic grows rapidly.

As a result, the plastic’s surface rapidly oxidises and becomes covered in a network of tiny, hairline cracks. These microscopic fractures leave the plastic fragile and highly vulnerable to breaking apart into microplastics from everyday water movement.

Essentially, the heat creates conditions where, rather than a single rogue microbe doing the work, a highly coordinated ecosystem of bacteria were actively dismantling the plastic’s structural integrity.

As climate change drives warmer water temperatures and more frequent algal blooms, this toxic synergy is expected to intensify.

Myung said, “As algal blooms become more frequent because of climate change, we expect this work to provide an important scientific basis for integrated environmental management strategies that consider water quality management and plastic waste management together.” 

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