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3, Nov 2022

Chemicals from plastics can kill oxygen-producing ocean bacteria

A new study has found that the chemicals leaching from plastics can change the mix of microbial life in seawater and can harm microbes critical to oxygen production in our oceans

Courtesy of Macquarie University’s Lighthouse. Story by Fran Molloy 

Each year, around 9.5 million tonnes of plastic waste enters the oceans. Larger plastic items cause immense harm to marine wildlife that get tangled in it or mistakenly eat it, and the degraded microplastics end up in the marine food web.

‘This study shows that the chemicals that leach out of plastic can also disrupt the health of microbial ocean communities, harming the tiny life-forms that we can’t see but which play a vital role in the health of our oceans, says the CoESB’s Macquarie University microbial ecologist, Dr Sasha Tetu.

The latest study builds on earlier lab-based research that showed that chemicals leaching from plastic were harmful to a single-organism community of Prochlorococcus, the most abundant plankton in the ocean.

Now, researchers have tested the effects of chemicals that leach from common plastics on a live ocean microbial community – and the news is concerning.

 

Leachate changes the microbe mix

‘We found that our plastic leachate mix harmed a number of different micro-organisms, including both bacteria and eukaryotes, says Dr Tetu.

She said the research aimed to show which microbial groups might be ‘winners’, and which likely to be ‘losers’, when exposed to plastic leach.  While certain microbes were unaffected and even thrived in the leachate mix, this came at the expense of other groups which declined significantly, she says.

‘Photosynthetic microorganisms were strongly affected and they showed significant decline in numbers, and also in their photosynthetic efficiency and their diversity.’

 These photosynthetic marine microbes play a crucial ecosystem role, contributing to carbon cycling and oxygen production and supporting the marine food web, says  Dr Tetu.

‘Research into the impacts of human pollutants on microorganisms doesn’t always distinguish  between which flourish and which do not, and microbes like heterotrophic bacteria, may tolerate stressors that photosynthetic microbes cannot.’

That’s a problem, she says, because risks to marine microbes and the marine ecosystem as a whole, can be downplayed if we don’t understand the impacts of pollution on all community members, especially photosynthetic microbes.

 Testing ocean water microbe communities 

The researchers used samples of surface seawater from a standard monthly collection taken at a marine reference station off the Sydney coast in 2019. 

‘It’s a good reference point because microbial data has been regularly collected from this site over a long period,’ Dr Tetu explains.

There are more than 10,000 chemical compounds which are known to leach from plastic, with at least 2,400 of these chemicals recognised by the EU as substances of concern.

‘We created a leachate fluid that was a kind of plastic tea, containing the mix of chemicals that leached from a common type of plastic, polyvinyl chloride (PVC), as well as looking at the effects of a specific plastic additive,  says Dr Tetu.

The microbial communities were exposed to the leachate and monitored over six days. Negative effects on the photosynthetic community were seen within 48 hours.

Dr Tetu points out that the ocean is a big place, and both the make-up of microbial communities, and the concentration and composition of plastic leachate, varies hugely between locations.

‘Instead of trying to mimic particular ocean plastic concentrations, we looked at past studies performing similar tests and similar concentrations. We would like to extend this work, looking at more concentrations, and more types of plastic, in the future.’

Future impacts

Dr Tetu says that another interesting aspect of this study is that the substances causing harm are often the additives put into plastic polymers to colour them or to make them flexible, for example.

‘That’s important to know because there’s a lot of effort going into creating plant-based plastics, and it will be critical to consider the additives that are used in these,’ she says.

‘They might be biodegradable but still leach out chemicals that can harm microbial ocean life.’

Despite ongoing work in many countries to reduce production of single-use plastics and to stop plastics entering the ocean, the OECD predicts that global plastic waste will almost triple by 2060.

“We need to understand the impact of plastic waste on these organisms, which are at the very base of our food web and which play a major role in carbon cycling, and produce oxygen for our planet, says Dr Tetu.

‘If plastic waste is causing these critical organisms to decline, we can expect that to have flow-on effects and it’s crucial that we get a handle on what these are.’

‘The other important finding in this work is that some marine microorganisms thrive under plastic leachate exposure conditions. We were able to identify genes in these bacteria which likely give these organisms the ability to use substances leached from plastic as a source of energy. These are exciting findings, and we can now apply this knowledge to synthetic biology applications, investigating ways we might use select groups of microorganisms to clean up environmental pollutants. ‘

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