Microplastics are small (less than 5 mm) pieces of plastic litter. They mainly originate from the breakdown of plastic debris in rivers and seas, but are also flushed out from other sources such as personal hygiene products that contain microbeads. Once in the ocean, the fate of microplastics is largely unknown. To address that question, PhD students Zandra Gerdes and Sophia Reichelt are currently on an international scientific cruise omboard the German research vessel Sonne (RV Sonne) and will spend five weeks in total (May 30 – July 5, 2019) with 30 other scientists and 31 crew members on a journey that will take them from Vancouver, Canada, to Singapore.
The aim of this unique project is to understand how microplastics behave in the water and in marine food webs. How do they get transported to the bottom, what microorganisms contribute to their degradation and what animals facilitate their transfer through the food webs? Zandra and Sophia will take water samples at several stations along the route, most of which are located within the Great Pacific Garbage Patch. They will then look for so-called marine snow aggregates, which may potentially play a major role in the downward transport of microplastics.
In search of marine snow
Marine snow is a continuous “shower” of mostly organic material falling from the upper layers of the water column to the sea floor. “Marine snow is what we call a vector of transport and biodegradation of microplastics. Microplastics are so light that it would take ages for them to sink to the bottom by themselves. When they are bound to an aggregate, these particles sink faster but also become an energy source for various microbes that contribute to the plastic degradation,” says Elena Gorokhova, Professor at ACES and a project leader for the Swedish team.
During the cruise, Zandra and Sophia will collect water samples using a unique instrument called a Microplastics Sampler, which is a bespoke version of a commercially available instrument that is built in the UK and modified with a help of the ACES team. The samples will be stored and transported to ACES for analyses.
“The reason why we became interested in marine snow in the first place was that this is an important food for organisms living in deep ocean. Any environmental contaminants contributing to abundance, accumulation of toxic substances and nutritional value of these aggregates may impact productivity and functioning of marine food webs,” says Elena Gorokhova.
Bacteria degrade microplastics in the water
In addition to the downward transport of microplastics in the ocean, the research team is keen to explore which of the microorganisms found in marine snow can break down microplastics. The sun can only degrade plastic floating on the surface. With increasing depth, however, the UV-degradation quickly becomes ineffective, and microorganisms take over the degradation process.
“The specific microbial communities that grow on microplastic-containing marine snow are of particular interest because they are likely to become more common as more and more plastic litter ends up in the oceans,” says Elena Gorokhova. Once back at ACES, the team is planning to decipher the chemical and bacterial composition of the marine snow samples using infrared spectroscopy and DNA sequencing.
This cruise is a spin-off project of WEATHER-MIC, a JPI Oceans –funded multidisciplinary project carried out by a consortium of European experts from Germany, Sweden, Norway, and Belgium, with the broader aim of developing novel tools to study weathering and degradation of microplastics.