Microplastics bound up by bacteria

Bacteria release substances that bind microplastic particles into tasty-looking clumps, which fish may mistake for food, a new study has shown...

Microplastic pollution in our oceans has attracted significant attention in recent years with research suggesting that there may be harmful effects for marine life and even for human health as the material makes its way up the food chain.

The problem begins with filter feeders that draw in the invisible particles as they sieve seawater to extract morsels of food. The plastic burden can affect their feeding behaviour and may also introduce toxins and microbes that can cling to the plastic surfaces.

But this isn't the whole story, because now new experiments are showing that the action of aquatic microorganisms can turn particles of microplastic, and their even more diminutive "nanoplastic" relatives, back into larger plastic clumps that catch the eyes of fish and other bigger feeders.

“In the natural environment, microplastics readily form agglomerates, conferring the particles with properties different to their pristine counterparts,” explains Tony Gutierrez, from Heriot Watt University, Edinburgh.

The phenomenon had gone undiscovered previously because most experiments involving micro- and nanoplastics are set up in laboratories in a very controlled and "clean" way. But this doesn't mimick reality, because seawater is awash with microbes that interact with these plastic particles in a special way.

Writing in Marine Pollution Bulletin, the new study reveals that ocean microorganisms, like plankton, bacteria, and algae, secrete into their surroundings substances called biopolymers. “The biopolymers envelop the nanoplastic particles, which cause them to agglomerate into clumps,” says co-author Stephen Summers.

Using natural seawater was the key to the new discovery, enabling the researchers to more realistically mimick what is happening in marine settings. In a matter of just a few hours, the biopolymers secreted by the seawater microorganisms had coated the nanoplastics, causing them to aggregate into larger fragments. “The clumps of nano- or microplastics become visible to our naked eye, and fish could mistake them for food,” explains Tony Gutierrez.

The Heriot Watt team also saw that the clumps descend in the water column to the bottom of the ocean, depending on the density of the original plastics and the size of the clump. The authors call this effect "marine snow". This, they speculate, could affect a range of marine species, and even the food chain as a whole. They do not know yet though whether these clumps are, in fact, detrimental to animal health, and this has yet to be tested.

The group is planning to focus their next experiments on another aspect of the agglomeration of nanoplastics. They want to find out whether the biopolymers create a "hotzone" for plastic degradation. Simply put, does the bacterial biopolymer "goo" help to accelerate the breakdown of the nanoplastics? For the moment, scientists think that plastics break down into smaller and smaller pieces, but that these never go away.

According to Gutierrez, “you can’t study nano- and microplastics in isolation,” because it does not reflect what is happening naturally in the oceans. Gutierrez is part of the RealRiskNano Project, which focuses on the poorly-researched smaller plastic particles and, as the name suggests, the real risk posed by them...