Researchers from North Carolina investigate how many microbes can thrive inside a plastic bottle. Their findings may surprise you!
Citation: Grogan A.E., Mallin M.A., Cahoon L.B. Investigation of polyethylene terephthalate (PET) drinking bottles as marine reservoirs for fecal bacteria and phytoplankton. Marine Pollution Bulletin. 2021 Oct; 173(A), 113052. doi.org/10.1016/j.marpolbul.2021.113052. ISSN 0025-326X.
We are all too familiar with images of plastic entangling unsuspecting fish and mammals or mimicking food. But in addition to well-documented dangers for marine life, researchers have found plastic surfaces to be ideal homes for micro-organisms such as algae and bacteria. As soon as a piece of plastic enters the sea, it is rapidly colonized and becomes a diverse biofilm in a matter of days.
Plastic: The perfect biofilm
A biofilm is made from a community of microbes that adhere to a surface. You have likely seen algal biofilms as bright green scum floating in a pond. Fecal bacteria also create biofilms by secreting complexes of proteins and sugars and growing closely together. Previous studies have found that algae and disease-causing bacteria make up a high portion of microbial communities living on plastic in the marine environment, especially in coastal waters.
Coastlines are home to densely populated cities, and in the summer public beaches around the world are packed with people trying to escape the heat. As a result, coastal zones are the most impacted by plastic pollution, both from local sources and communities further inland, whose plastic is carried down-river and dumped in estuaries and bays. Researchers from the University of North Carolina were curious about just how much algae and bacteria plastics can contain and they devised a clever experiment to study this question at Wrightsville Beach, a popular summertime tourist destination in North Carolina.
Researchers crafted floats with pipes, buoys, and rope to anchor plastic bottles in an estuary near a stormwater outflow. After an eight week study period, the scientists retrieved the bottles to measure the amount of algal and bacterial growth inside the plastic bottles versus outside.
The team found that concentrations of the fecal bacterium, Enterococcus, inside the plastic bottles were four times greater than the seawater outside. While this bacteria lives in the lower intestine of people and mammals, it causes illness if ingested and its presence in water is closely watched by health organizations. The chlorophyll concentration inside the plastic bottles was five times greater than the surrounding seawater, which implies that discarded bottles could seed harmful algal blooms, which are already increasing in frequency around the world. Harmful algae can produce toxins or create dead zones by depleting oxygen in the water.
The researchers measured the amount of light that can penetrate the plastic bottles to see how habitable the bottles were for photosynthesizing algae. The bottles let in plenty of visible light but blocked a substantial amount of UV radiation, which would normally destroy microbial cells like bacteria and algae. This UV protection is unique in the marine environment, allowing algae and bacteria a rare respite from sun exposure in the surface ocean.
The researchers hypothesized that the algae and bacteria had a mutualistic relationship inside the plastic micro-environment. Algae produce sugars through photosynthesis, which bacteria feed on. In turn, the bacteria benefit algae by eating sugars and respiring CO2, which the algae need to photosynthesize. In this manner, high concentrations of bacteria and algae can cohabitate inside a plastic water bottler. Plastic can then act as floating rafts for harmful bacteria and algae, carrying harmful algae and disease-causing bacteria to remote corners of the oceans while shielding them from UV light.
A dangerous refuge
Algae blooms and high amounts of fecal bacteria from human waste are major threats to coastal ecosystems and public health, which is why the U.S. Environmental Protection Agency has set legal limits on the concentrations of both in coastal waters. Plastic bottles act as tiny, self-sustaining ecosystems for algae and bacteria, emphasizing the danger of plastics to the environment and human health.
Fortunately, everyone can take part in reducing plastic pollution. Voting, making informed choices as consumers, and participating in beach clean-ups are all great options to help keep our beaches safe and plastic free. And be sure to wear gloves while handling any plastic waste!
Cover photo credit: Bryan Yurasits, Unsplash
I am a masters student at the University of Rhode Island, studying the ecology of the harmful algal bloom genus, Pseudo-nitzschia. My work is interdisciplinary and revolves around coastal health, water quality, and public engagement with science. In my free time I enjoy doing crafts, writing, cooking, and exploring the outdoors on my feet or a pair of skis.