Plastic pollution in the ocean is a hugely serious issue that affects not just sea creatures but the global ecosystem as a whole, us included. You might think that this is only in terms of the fish we eat, but it appears ocean plastics have another major impact: they might reduce the number of organisms on Earth that make the oxygen we breathe.

An international team of researchers recently conducted a study, published in Communications Biology, on the effects of plastic pollution on a particular group of bacteria known as Prochlorococcus. These organisms are very important because they are capable of photosynthesis. They are the most abundant photosynthetic organisms on Earth. There are so many in the ocean, about 3 octillion (~10²⁷), that they are responsible for the production of 10 percent of all the oxygen on Earth.

The researchers took two strains of Prochlorococcus from different depths and tested how they dealt with chemicals leached by two common plastic products, high-density polyethylene (which makes plastic grocery bags) and PVC matting. The lab experiments showed that the bacteria were badly affected by both of them.  

"We found that exposure to chemicals leaching from plastic pollution interfered with the growth, photosynthesis and oxygen production of Prochlorococcus, the ocean's most abundant photosynthetic bacteria," Dr Sasha Tetu, lead author and Macquarie University researcher, said in a statement. "Now we'd like to explore if plastic pollution is having the same impact on these microbes in the ocean."

This is the first study of its kind to focus on the effects of plastic on photosynthetic microorganisms in the ocean. Exposure to sunlight, ultraviolet light in particular, and just being in salty water causes plastic to break down and degrade. Items are turned into smaller and smaller fragments and the materials release harmful chemicals over time.  

"This pollution can leach a variety of chemical additives into marine environments, but unlike the threats posed by animals ingesting or getting entangled in plastic debris the threat these leachates pose to marine life has received relatively little attention," said co-author Dr Lisa Moore.

As if contaminating fish and potentially affecting our oxygen supply wasn’t enough, plastic pollution is also a costly affair. Every year marine ecosystems suffer more than $13 billion' worth of damage, and that's before the effects on microorganisms are even considered.

"Our data shows that plastic pollution may have widespread ecosystem impacts beyond the known effects on macro-organisms, such as seabirds and turtles," added Dr Tetu. "If we truly want to understand the full impact of plastic pollution in the marine environment and find ways to mitigate it, we need to consider its impact on key microbial groups, including photosynthetic microbes."