Microscopic creatures could be helping reduce marine garbage on the ocean surface, not only by "eating" plastics but by causing tiny pieces to sink to the seafloor, Australian researchers said Thursday.

The plastic-dwellers appear to be biodegrading the millions of tonnes of debris floating on waters worldwide, according to oceanographers at the University of Western Australia.

They analysed more than 1,000 images of material drifting along Australia's coast in a study published in the scientific journal PLOS ONE.

The study is the first to document the biological communities living on the tiny particles of debris known as microplastics, and recorded many new types of microbe and invertebrate for the first time.

"Plastic biodegradation seems to happen at sea," oceanographer Julia Reisser said.

"I am excited about this because the 'plastic-eating' microbes could provide solutions for better waste disposal practices on land."

Scientists have warned that microplastics -- particles smaller than five millimetres (0.2 of an inch) -- are threatening to alter the open ocean's natural environment.

The United Nations Environment Programme estimated in 2012 that around 13,000 pieces of microplastic litter were found in every square kilometre of sea, with the North Pacific most badly affected.

While there has been previous research on microbes eating plastic at landfills, Reisser said her research found early indications that their marine counterparts could be just as effective on ocean garbage.

"If you use terrestrial microbes, you need fresh water to grow them and the process can be very expensive," she told AFP.

"But if you find marine microbes, they are growing in saltwater and that might be a cheaper way (to reduce landfills)."

Reisser said the research showed diatoms -- tiny algae that were the most commonly found microbe living on the microplastics -- were using the little pieces as a "boat" to move around on the surface of the ocean.

As more and more diatoms -- which are made of silica -- gathered on a plastic piece, they appeared to make it sink to the bottom of the ocean floor, she said.

The actions of the microbes could explain why the amount of plastic floating on the seas has not been expanding as fast as scientists expected, Reisser added.

But the researchers also found evidence of possible tiny bite marks on the microplastics, raising concerns that other small organisms could be consuming toxins found in the litter.

"It seems we have tiny animals grazing on these plastic inhabitants -- but we are not sure if this is good or bad," Reisser said.

"That's a hazard that we are very worried about, but we need far more research to see how big this problem is."