Microbes to Clean the Oceans

Bacterial opportunities for plastics biodegradation

Plastic pollution in a few figures

About 400 million tons of plastic are produced annually, with 9 to 14 million tons ending up in the oceans on top of the already 150 million tons already there, causing adverse effects in all marine ecosystems.

“In a business-as-usual scenario, the ocean is expected to contain one tonne of plastic for every three tonnes of fish by 2025, and by 2050, more plastics than fish (by weight),” — Ellen MacArthur.

The web is full of articles about plastic pollution, and it is impossible to address this issue without addressing our model of consumption at the root, and proposing solutions like new business models, new packagings, the elimination of single-use plastic packaging, reuse, recycle, and modify the plastic qualities to make them compostable. The objectives of the Global Commitment for plastics reduction aims for a yearly reduction of 17 to 19% by 2025.

We need a bigger effort than that though, and we need to target not only the open tap that keeps feeding the problem, we also need to address the quantity that’s already there.

A problem for the whole biosphere

If you have seen the insides of dead whales and dolphins filled up by plastic (Animal, Seaspiracy…) you know how heartbreaking the plastic situation is. Marvellous species are on the verge of extinction, and what’s more, we really need them for the functioning of ecosystems, at global scale. Whales’ poop is a key nutrient to surface critters whose metabolism participate in a huge part to the oxygen in the atmosphere.

Worse than plastics are microplastics. Microplastics are of particular concern to health — we could be the next whales ending belly up filled with microplastics — and they prove impossible to remove from the marine environment by the floaters, boats and filters that have been proposed to clean up the sea surface. They are too small to be caught, and big proportions are denser than saline water and fall onto the seafloor.

Microbes to the rescue: can they help?

Scientists have been searching for solutions in the enzymatic toolbox of bacteria for roughly a decade. A 2019 review lists identified bacteria that can degrade different types of plastic. For example, Ideonella sakaiensis produces PETase, an enzyme degrading Polyethylene terephthalate (PET): the bacterium uses this form of plastic as the main energy and carbon source. Other bacteria use other sources of plastics as shown by the figure here-under. However, such biodegradations occurred in optimal laboratory conditions that do not represent what happens in the natural environment.

At sea, highly dense and hydrophobic polymers (common plastics) have low biological degradability. Famous soil microbiologist M. Alexander explained it well already back in 1975: he says a compound fails to be degraded microbiologically if:

• the molecule is too large to penetrate the cell
• the compound concentration in aqueous solution is extremely low
• or the cleavage sites of the compound are hard to access

All of which is the case for plastic pollution.

What do we know about bacterial communities growing on plastic?

Bacterial communities form a slimy layer at the surface, which composition has been studied. Some authors warned about the abundance of Vibrio subspecies in such biofilms (a group including the agent of cholera), questioning the disease-causing risks, as well as about the increase in gene exchange and, consequently, the prevalence of antibiotic resistance genes in such communities.

If there are microorganism communities there already, could they adapt evolutionarily to degrade plastic in the future?

The PET degrader I. sakaiensis isolated from outside a recycling facility (Yoshida et al. 2016) indicates that higher concentrations of plastics can lead to the evolution of plastic degradation in bacteria. However, the authors of a major review, Sonja Oberbeckmann and Matthias Labrenz concluded just last year that:

“It is unlikely that bioavailable synthetic polymers will reach sufficiently high levels in the oceans to promote the evolution of bacteria that can effectively degrade microplastics.” — Sonja Oberbeckmann and Matthias Labrenz

We’re already in the future: widespread evolution of plastic-degrading microbes: a good thing or a bad thing?

New studies are coming out, based on advances in environmental DNA sequencing and computational algorithms, which enable large-scale metagenomic analyses of microbial genes in their natural habitats.

A recent, global study analyzing samples from 169 sites across 39 countries revealed a massive adaptation to plastics in oceans and soils: it found 12 000 enzymatic genes relative to plastic degradation in oceans and 18 000 in soils. An average of 1 in 4 organisms in the analyzed global microbiome was found to carry a potential plastic-degrading enzyme!

In the oceans, plastic degrading enzymes abundance increases with depth, confirming plastic falls down and is not concentrated only on the surface.

“The earth’s microbiome might already be adapting to current global plastic pollution trends” say the authors.

This could be helpful, of course — although these genes don’t cover the degradation of all types of plastics. But this is also frightening: it means plastic is more ubiquitous, more abundant and more impactful than we had previously estimated. It means plastic already has had a huge impact on shaping ecosystems.

How would you feel if bacterial genes for degradation of plastic were found in your gut microbiome? Wouldn’t you wonder what are the hidden implications? You would have an indicator that plastic is present in your gut, but wouldn’t you ask if it could also have transferred to your bloodstream, to your brain? With which consequences?

I believe we should feel the same about oceans. There’s potential help from microbes to clean plastic pollution into microbial biomass, which is great, but it’s also a worrisome indicator that we failed to prevent infiltration of plastic in all the biosphere, with consequences that we’re about to witness.

What can you do at your level?

Reduce the use of plastic, reuse, and recycle of course. If you work in the industry, see how you can improve your products’ or tools’ packaging types or life cycle. Buy bulk products or vegetables packaged in compostable plastic. Drink tap (filtered) water, and coffee in your own cup. Avoid synthetic clothing: microplastics derive from each laundry to the sewers and environment — privilege cotton, linen and natural fibers.

Launch your own startup to transform plastic waste into energy with microbes. If you do that, call me :-)