Marine Biology 162:725-732
THE MANY FACES OF MARINE DEBRIS
Marine debris can be highly variable both in size and composition. Debris may represent large products, such as cargo containers lost from ships, as well as small microdebris no larger than phytoplankton (10 micrometers)! Since plastic debris are not broken down biologically they will fracture into smaller and smaller pieces and may take centuries to fully remineralize. Marine debris are bad for marine life, and may end up in the stomachs of birds, seals, and fish causing health problems and death (Read more about plastic ingestion in Laysan Albatross).
Well, we now know vertebrates and megafauna are not the only animals ingesting marine debris. A new study published in the journal of Marine Biology (Hall et al. 2015) found that corals may ingest microplastics debris.
CORAL PARTICLE CAPTURE
Corals may feed in a number of ways including particle capture (such as the tentacles "grabbing" items from the water column), and the uptake of dissolved materials including mucus, sugars, amino acids, and microbes. Heterotrophy and particle capture is very important for coral health and is a key component of coral nutrition, especially in corals recovering from bleaching. Therefore, factors interfering with coral feeding may have serious and unforeseen consequences on coral fitness.
THE SCIENTIFIC REPORT
In this study, Hall and co-authors collected fragments of the coral Dipsastrea pallida from reefs of Orpheus Island on the Great Barrier Reef, Australia, and exposed them to feeding treatments of natural zooplankton, brine shrimp, and microplastic debris (10 um - 2 mm). The authors observed microplastics adhering to the coral's external mucus layer, and ingested plastics were found to became lodged within the coral's internal tissues (called the mesentery filaments). Further, feeding on plastic was comparable to rates of feeding on natural zooplankton, suggesting that plastic debris may antagonize natural feeding processes in corals.
The authors suggest two important points. First, microplastics settling on the coral may cause tissue damage and greater production of mucus from the coral. Mucus production is normal for corals, but more mucus may lead to more plastics adhering to the coral and a greater risk of ingestion. Secondly, plastic ingestion in any form may interfere with the digestion of captured particles, interfere with coral growth, and cause internal damage to coral polyps. Not to mention, there may be other unknown biological consequences associated with having petroleum products wedged inside the coral's digestive cavity! However, this is among the first studies to report corals feeding on microplastics, and it is unclear what impacts such cases of "mistaken feeding identity" might have on corals. More research is required to better understand how coral growth, metabolism and fitness are affected by plastic pollution over short and long periods.
THE PLASTIC PLANET
What are the implications of this study? Well, first off, very little is understood about the impacts of microplastics on marine organisms outside of vertebrates--and even then the effects are mostly noted postmortem. Corals are very important organisms in the ocean, and they build massive structures critical to the culture and livelihoods of millions of people. But this extends beyond reefs: plastic pollution is affecting ecosystems across the world, and not just by muddying up your vacation photos on a sandy beach (#bummer). Plastic pollution is killing migratory birds, marine mammals, and the breakdown of plastics releases toxic chemicals known to cause cancer. Further, these compounds may enter the food chain and bioaccumulate with the potential to affect whole trophic systems, including humans.
In less than 100 years, humanity's uninhibited use of plastic has led to the contamination of nearly every corner of the earth with plastic garbage--including remote beaches and coastlines far from humans (such as those found in the Papahanaumokuekea Marine National Monument). But how much plastic is REALLY entering the ocean, you ask? Well science has an answer! In 1975, it was estimated (back of the envelope perhaps...) that 0.1% of plastic made it into the ocean. Not bad! Well, don't get excited... that number is grossly underestimated. In actuality, 15 - 40% of plastic from dumps, landfills, and litter enters the ocean each year, totaling 4 - 12 million metric tons of plastic ANNUALLY! What's worse is that we still don't know where all this debris is ending up--only a fraction actually makes it back to shore. The remainder may sink as it becomes fractured or colonized by biofoulers, begin a new and perpetual journey as part of the ocean's plankton, or sink to the ocean floor creating submarine garbage dumps. Clearly, something needs to change...
The implications of our plastic planet for the health and function of ecosystems and organisms is uncertain, but it doesn't take a rocket scientist (or a marine scientist) to see the potential (and realized) threat of plastic debris on marine ecosystems. It is urgent for humanity to reverse our dependency on petroleum (i.e., climate change) and petroleum products (i.e., plastics) because we don't have another 100 years to waste.