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Ocean Plastics can now kill oxygen-producing bacteria

This planet has a problem with plastic. Not only the large masses of them gather in the Pacific Ocean, but with the small pieces that blow into pristine mountainous habitats. The spots appear in a series of marine animals. The dots even materialize in human excrement.

Now scientists have uncovered a potential new consequence of the plastic threat: the toxins leaching the material into seawater inhibit the growth and photosynthetic efficiency of the bacteria Prochlorococcus, which is responsible for the production of an estimated 20 percent of the oxygen we breathe. That means Prochlorococcus is also responsible for 20 percent of the carbon capture on this planet (one molecule of carbon goes in, one molecule of oxygen goes out), theoretical spelling problems for humanity's quest to keep CO2 out of the atmosphere. However, this is an early investigation and comes with a number of important comments. Indeed, it exposes the challenges of studying a threat as new and pervasive as plastic pollution.

Prochlorococcus is a type of cyanobacteria (derived from their blue color) that floats in oceans around the world. We are talking about many single-cell organisms, with an estimated population of 1027. Like a plant, Prochlorococcus uses photosynthesis to produce its own food, absorb carbon and spit out oxygen, making it a major part of the carbon cycle that people have been turning around in such an uncontrolled manner.

Unfortunately, the researchers discovered that in addition to carbon, the bacteria also absorb plastic toxins that are leached into the water, known as leachate. They did this in the lab by mixing different amounts of plastic in an artificial seawater base in which they grew Prochlorococcus. They compared the results with a check of Prochlorococcus grown in unaffected artificial seawater.

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The researchers found clear responses that varied based on the concentration of leachate, suggesting a toxicological reaction. At low concentrations there was no difference with the control. But when they increased the percolate concentration, they saw that the physiological response of the bacterium gradually became confused. "As the leaching of plastic leachate increases, you see the cells grow less well, and in fact they die at the highest concentrations," says microbial oceanographer Lisa Moore of Macquarie University in Australia, a co-author of the paper.

Moore and her colleagues were also able to measure photosynthetic activity at these different concentrations with an instrument that looks at the fluorescence intensity of the cells. "We saw parallels with what we saw with growth: a decrease in photosynthetic efficiency, and in fact a fairly dramatic decrease with higher concentrations," says Moore.

Further, the researchers looked at the genes of these bacterial populations, whether they were more or less expressed in the presence of leachate. A large proportion of those that are less expressed are associated with photosynthesis, "which was absolutely consistent with what we saw in terms of reduced photosynthetic efficiency, and then growth declined," Moore says.

Could be a number of things in plastics. Flame retardants, for one, and other additives that give plastic its flexibility. In particular, zinc may have an excessive effect on the bacteria – it is used in plastic parts ranging from dyes to heat stabilizers.

Now we have some reservations about this study. The biggest thing is that the work was done in the lab, not in the ocean, and it is. To do such an experiment, the researchers had to carefully check the samples of artificial seawater so that they would not be contaminated with other pollutants that could shed the results.

"That's a limitation, trying to equate what we used in the laboratory compared to what's in the oceans," says Moore. "So what we were trying to estimate at least in the article was the number of particles found in the ocean, relative to the number of cells found in the ocean."

Have plastic percolates had a surprising effect? Prochlorococcus in the lab? Yes definitely. But that does not mean that the effect necessarily takes place in nature. "We know that plastic is bad," said Luiz Rocha, the curator of fish at the California Academy of Sciences, who studies ocean plastic. "This article shows that it can adversely affect one of the most common photosynthetic organisms on Earth, but we don't know if the concentrations of these chemicals will ever reach this high in the ocean."

"I have exactly the same problem with sunscreens," Rocha adds. "All studies analyzing the effects on corals were conducted in very high aquariums that may only occur on beaches that visit thousands of people every day. So actually, sunscreen banning does little to nothing to save coral reefs."

None of this means that plastic is not terrible for the planet. Large pieces, known as macroplastics, end up in the stomachs of all kinds of sea creatures. And although microplastics (pieces smaller than 5 millimeters long) are everywhere, science does not yet know what their effects can be. "I am the first to say that our plastics problem is macroplasty," says University of Michigan eco-toxicologist, Allen Burton, who studies plastics. The bits that strangle sea birds and hide beaches and even sink to the bottom of the ocean. "We all know how terrible the effects of so many species have been through macroplastics. Microplastics, not so many. Percolates, probably even less."

The core of the problem is that plastic pollution is a whole new science. Researchers still feel their way through an environmental threat, the epic nature of which is probably only the second after climate change. About 8 million tonnes of plastic come into the sea every year. And between 2015 and 2025 the amount of plastic that flows into the ocean can increase tenfold. That is objective, monumental, terrible, but exactly what ways it will be, science is not yet certain.

"Everyone acknowledges that we need more research, but the sky is not falling due to percolation water or microplastics," says Burton.

At least not yet. This is humanity we are talking about – just give us some time to make it worse.

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