Scientists engineered cyanobacteria to remove microplastics and extra nutrients from wastewater.
Introduction
Have you ever seen a plastic bottle floating outside in the water? While that is bad for the environment, there is more to the problem. When plastic sits in the sun and water, it breaks into tiny pieces called microplastics. These pieces are smaller than a grain of rice. Some are even too small to see!
These tiny plastics are everywhere. They are in the ocean, in the soil, and even in the water we drink. Fish and other animals (and humans) accidentally eat them, which can make them sick. Many wastewater treatment plants can remove big plastic pieces. But very small pieces can slip through. Filters can help but they can clog and cost a lot. They also make new waste that needs safe storage. Wastewater can also have extra nutrients, like nitrate, which is harmful to water systems.
We wondered: could we use Nature to help us? Some microorganisms make sticky layers that can trap particles. But this process can take a long time. We focused on cyanobacteria. They are algae-like microorganisms that use sunlight like plants do. We wanted to see if we could engineer them to help us. Could these engineered cyanobacteria remove microplastics fast? Could they also help with nitrate? And could they help us reuse the captured plastics?
Methods
First, we grew two kinds of cyanobacteria in the lab. The normal strain was our control. We used genetic engineering to produce a second strain. We changed the genes so the cells would produce limonene. This is an oily chemical that smells like oranges. Limonene acts like a tiny oily layer on the cell surface that repels water. Most plastics also repel water, so we hoped the cells would stick to the plastics.
For each kind of cyanobacteria, we mixed the cells with tiny plastic beads made of polystyrene, a common plastic. We mixed the samples and then waited about one hour. If the cells grabbed the plastics, we expected to see clumps sink. We also measured how “cloudy” the water looked (because cloudier water meant more particles still floating).
We then used different microscopes to look closely at the plastics and the cells. We checked whether plastic stuck to the cell surface. We also tested larger pieces of other common plastics.
Next we tested real-world water. We added microplastics to surface water and wastewater samples. We repeated the mixing and checking steps. In the wastewater tests, we also measured nutrients (like nitrate) before and after the cells grew.
Afterwards, we collected the material that sank (cyanobacteria cells + plastics). We used it to make thin plastic films. Finally, we tested how strong and stretchy those films were.
Results
Our engineered cyanobacteria removed microplastics quickly. In one hour, we removed about 91% of the polystyrene beads! The normal cyanobacteria removed much less (Figure 1). The water with engineered cells also looked clearer.
Under the microscope, we saw many plastic beads attached to the engineered cells. We also saw the engineered cells stick to larger pieces of other plastics. The normal cells did not stick as strongly to plastic.
In surface water and wastewater, our engineered cells still formed clumps, and many plastic pieces ended up in the clumps that sank. In wastewater, nitrate levels dropped after the cells grew.
What about the plastic films we made from the collected material? Some of these films stretched more and were harder to tear than our plastic-only control films.
Discussion
We think our engineered cyanobacteria worked well because they made microplastics clump and sink. The microplastics stuck to our engineered cells more than to the normal cells. As the clumps grew bigger, they became heavier. Then they sank to the bottom. This made the microplastics much easier to collect than trying to filter every tiny piece.
It also mattered that our method worked in real water, not just in the lab. Surface water and wastewater contain many other particles. We still saw sinking clumps even with that extra “mess”.
The wastewater results showed another benefit. Wastewater often contains extra nitrate, which can harm water systems. The cyanobacteria in our tests removed nitrate while they grew. So, they could work on two problems at once!
We also liked that we could reuse what we collected. Instead of storing the captured plastics, we turned the clumps into new films with useful properties.
Conclusion
Microplastics are tiny, so it is best to stop them before they reach water. There are many ways you can help!
Use fewer single-use plastics.
Pick up litter before it breaks into smaller pieces.
Skip glitter.
Avoid facial or body scrubs with plastic beads.
Wash clothes only when needed and clean the dryer lint trap. If you can, use a laundry filter or wash bag to catch any plastic microfibers.
Small changes can keep a lot of microplastics out of rivers and oceans.
