Guest Post by Pippa Strickland from iDiveblue.com

It has been estimated that over five trillion pieces of plastic currently litter our ocean; the result of our addiction to the convenience and durability of this long-lasting product. Plastic production flourished during the second half of the 20th century and now we have more than 6.9 million tons of plastic waste to deal with (much of which eventually makes its way to the open ocean). The World Economic Forum has even gone so far as to say that, if current trends continue, there will be more plastic than fish in the sea by 2050.

Images of turtles trapped in six-pack rings and whale stomachs filled with trash have gone viral as have beaches so littered with plastic waste that there’s not a grain of sand in sight. While these scenes are visibly disturbing, even more so is what is going on beneath the water’s surface – and often on a scale that’s difficult to observe with the human eye.

The science of what plastics are doing to our coral reefs is of grave concern (on both large and small scales) and could have far-reaching consequences for not only our ocean-dwelling species but also for human life on Earth.

Plastic waste makes corals more vulnerable to potentially fatal diseases

A recent study at Cornell University in Ithaca, New York, found that the likelihood of disease on coral reefs impacted by plastic pollution was significantly higher than those that were plastic-free. More than 150 reefs across the Asia-Pacific region were surveyed for signs of disease by examining the health of more than 124,000 corals and recording the presence of plastic larger than 0.5 centimeters. Diseases such as white syndrome and black band disease were recorded, both of which have the potential to rapidly kill corals.

Findings from the study revealed that reefs free from plastic waste had only a 4% chance of being diseased while those impacted by pollution were at an 89% risk. While the study didn’t confirm that plastic was the cause of any of the diseases found, it’s thought that the plastic can carry pathogens and act as a vector for diseases to flourish.

For example, the bacteria that trigger outbreaks of white syndrome (which is characterized by a rapid loss of tissue) likes to colonise plastic. At the same time, the low-light, low-oxygen conditions created when plastic settles on coral is ideal for microbes that cause black band disease to flourish, resulting in complete tissue degradation. In addition, hard debris can damage corals, which may trigger infections, or it may block sunlight from reaching the corals, leading to low-oxygen conditions that allow disease-causing bacteria to thrive.

The study also found that some types of coral are more likely to be impacted than others, largely due to their physical structure. Staghorn and branching corals were found to be eight times more likely to trap plastics, which is bad news for the juvenile marine life that uses their structures as refuges and nurseries.

Plastic breaks down into microplastics that end up in our food chain

While large pieces of plastic waste and abandoned fishing nets are among the visible evidence of the devastating impact humans are having on marine life, even more troubling is what is occurring once this breaks down. As plastic waste is transported from populated areas via rivers, tides or storm drains, it’s gradually broken down into smaller pieces by sunlight and heat. When exposed to ultraviolet light from the sun, a chemical reaction takes place in the plastic that leads to the severing of big polymer molecules and the gradual degradation of the product.

Once they become smaller than 5mm, they are referred to as microplastics and, rather than floating at the surface of the ocean, they permeate through the water column. Aside from the hazardous ingredients that are used in the manufacturing process of these plastics, they also absorb oily pollutants during their journey. These concentrated toxins are then ingested by fish and other marine life before being absorbed up the food chain and biomagnified. In some cases, these food chains include humans, with tiny pieces of plastic found in the stomach of Southern Bluefin tuna caught off the coast of Tasmania that was otherwise intended for human consumption.

Chemicals leaching from plastic products are harming oxygen-producing bacteria

On an even tinier scale is the impact that plastic waste is having on some of the smallest marine organisms: oxygen-producing bacteria. As plastic is broken down, it begins to release some of the chemical compounds that are added during manufacture to improve the performance of the product. This leachate is also known as plastic “tea”.

A research study at Macquarie University in Sydney looked at the effects of plastic toxicity on Prochlorococcus, a genus of marine cyanobacteria that produce around 10% of our oxygen needs. They number around an octillion (equal to 1 followed by 27 zeros!), making them the most abundant photosynthetic organisms on Earth.

Of concern to the researchers was how plastic leachate or “tea” impacts the ability of Prochlorococcus to grow, reproduce, and produce oxygen, with negative impacts already recorded in zooplankton. The researchers looked specifically at how these chemical additives impacted oxygen-producing cyanobacteria by preparing two different types of plastic leachate. One was contaminated with high-density polyethylene (HDPE), which is commonly found in plastic shopping bags, while the other was laced with polyvinyl chloride (PVC), which is found in a variety of plastic materials.

After measuring the impact on the cyanobacteria over several days, they found significant population losses compared to the control Prochlorococcus in plastic-free water. In addition, their photosynthesis rates were reduced with the PVC preparation having the most toxic effect.

It’s a stark warning of the devastating impact our plastic addiction could have on human life and the need to take drastic measures soon to reduce the amount of plastic waste ending up in our oceans.

A wake-up call for our coral reefs

While the results from these studies are confronting, they’re a first step in driving change on a consumer and governmental level. Not only do individuals need to reduce their plastic usage and make a greater effort to recycle, but legislation needs to be enacted to limit the industrial use of harmful chemical additives in plastic products. Aside from minimising our own plastic use and supporting large-scale projects like the Ocean Clean Up and 4Ocean's plastic removal initiative, we need to invest in chemical removal technologies to eliminate harmful leachate from our oceans and make them a habitable place for marine life to thrive.