Millions of metric tons of plastic wash into the sea each year, and studies have shown that we simply don't know where the vast majority of it is. The corrosive forces of the ocean break these bottles, cigarette lighters and pieces of packaging down into tiny fragments that are difficult to track, but scientists have come up with a technique that could reveal their whereabouts by way of a glowing fluorescent dye.

Microplastics can measure as little as the width of a human hair, and one recent study estimated there to be somewhere between 93,000 and 236,000 metric tons of the stuff floating around on the surface of the ocean. Sure, that's a lot, but the scientists behind that study estimate that it accounts for just one percent of the total plastic waste in the ocean, so where is the rest of it? Erik van Sebille, the lead author on that study, told us earlier in the year that it's a bit like astronomers dealing with the unknowns of dark matter.

"We don't know how much of it is on the seafloor, we don't know how much of it is on the coastlines or beaches, trapped in mangrove forests, those kinds of things, and we don't know how much is in the guts of marine animals and organisms," he said. "As long as we don't know that, we don't know where marine life interacts with those plastics and we also don't know where is best to take out the plastic or to clean it up."

Some scientists are concerned about the chemicals within these plastics being released into the water as the materials are broken down. These could be ingested by sea creatures, which is a problem because we know little about their safety when it comes to marine organisms, and in turn the humans that eat them.

Before we can even think about how to clean up this monumental mess, we first need to get a much better understanding of it. Sebille has secured funding from the European Union for the next phase of his research, which involves building 3D distribution maps of plastic circulating through the ocean. And a new technique developed at the University of Warwick could form an equally important piece of the puzzle.

Researchers at Warwick's School of Life Sciences have found that Nile red, a type of fluorescent dye, will bind to plastic particles floating in water. These plastic particles are then illuminated when viewed through a fluorescence microscope, enabling scientists to tell them apart from natural materials and count them with much more ease.

The team tested out this approach by collectinng samples of surface water and beach sand from a coastal area of England, around the port city of Plymouth. Using the dye approach, it was possible to detect microplastics within the samples at sizes as small as 20 micrometers (around the width of a human hair).

Although just one relatively small sample, this exercise did return a couple of alarming findings. The team found a much larger amount of microplastics than would've been detected by traditional methods, where pieces are manually picked out one by one. It also found that the most common type of plastic making up the debris of this size was polypropylene, a material commonly used for packaging and food containers.

There are still a number of questions to be answered about this approach, namely how it could be applied on a larger, more meaningful scale, but the team says the early results are promising. Together with image analysis software, the work does raise the prospect of an automated process for detecting microplastics in the water, which would not only be faster, but would also avoid the problem of subjectivity inherent in manual sorting.

"Using this method, a huge series of samples can be viewed and analyzed very quickly, to obtain large amounts of data on the quantities of small microplastics in seawater or, effectively, in any environmental sample," says study author Gabriel Erni-Cassola. "Current methods used to assess the amount of microplastics mostly consist in manually picking the microplastics out of samples one by one – demonstrating the great improvement of our method."

The study was published in the journal Environmental Science & Technology.

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