In this experiment, scientists are studying how long it takes for some specific kinds of biodegradable plastics to break down fully in a marine environment similar to the sea near Bergen in Norway.

They are also studying which microbes are involved, and whether the location of the samples makes any difference: some samples are in the water column, while others are on a sediment simulating the seafloor.

The experiment has been running since summer 2021. 

By conducting the experiment as part of a public exhibition at Bergen Aquarium, and providing a livestream on the web, anyone can watch these plastics breaking down and understand the kind of timescale involved.

This page explains the experimental methodology and the science behind it.

 

Research questions

  • What is a relevant time span for degradation of the biodegradable plastics PHB (Mirel) and starch-based Mater-Bi in cold seawater conditions typical for Nordic environments?
  • How is the degradation rate in the water column compared to a sediment surface?
  • Which microbes are involved in the (bio)degradation in the current environment?

Experimental design

 

The tank contains 400–500 litres seawater from the Bergen fjord (140 m depth). The water is exchanged three times per day. The temperature in the tank is thus close to the temperature in the Bergen fjord, and relatively stable at 11 – 12 degrees Celsius due to all year stable temperature at the depth of water intake (9 degrees Celsius).

Four different types of plastics are submerged in the water column and in the sediments :

  • Polyhydroxybutyrate (PHB, MIREL® P5001, Metabolix, Cambridge, USA). This is a biodegradable plastic made from natural polymers produced by bacteria. Bacteria produce these polymers at conditions when carbon-rich nutrients are available in excess, and the polymers are stored inside the cells as energy reserve for nutrient-poor days.
  • High-density polyethylene (HDPE). This is an example of conventional plastic polymer made from the hydrocarbon monomer ethylene. HDPE is commonly used to produce plastic bags and containers. Although some hydrocarbon-degrading bacteria have been shown to biodegrade polyethylene in controlled laboratory conditions, the biodegradation rate is so slow that it is considered non-biodegradable. High-density polyethylene (HD-PE) fruit and vegetable bags with a thickness of 10 µm were obtained from a local market (Marina di Campo, Italy).
  • MaterBi. This is a blend of thermoplastic starch and biodegradable polyesters. The material is commercially available as fruit and vegetable bags of 12 µm thickness. The bags for the samples displayed here was bought in a supermarket in Italy,
  • Cellulose. This is a polymer which naturally biodegrades in the open environment. Cellulose is a natural polymer found in plants and other organisms. It is the most abundant organic polymer on earth. In this experiment, cellulose is the so-called ‘positive control’, which means scientists can check if active microbes are in the tank, and also compare its biodegradation rate to the plastic polymers that are tested.

Three parallel samples of each of the plastics are placed in the water column, which gives 12 frames in total. Additionally, another three parallel samples are placed on the sediment, thus another 12 frames.

In the tank, you can also see fishes that are typical for the Bergen fjord. This will allow us to observe how typical local wildlife interacts with the sample frames.

Monitoring and sampling

 

  • A camera placed outside of the tank is permanently recording and streaming what happens with the plastics in the tank.
  • We are monitoring water temperature, pH and oxygen.

We regularly sample for nutrient analysis and assessment of microbial composition in water and sediment. The samples will be processed and frozen upon analysis. All samples will be analyzed after the completion of the experiment.

  • We will document polymer films by taking pictures for comparison as the experiment progresses. However, each time we take out the samples from the tank, we risk disturbing the biodegradation process, thus the number of retractions will be limited. How often we will take additional photographs will be decided by the researchers, based on how the experiment develops.
  • Post analysis will include sampling of biofilm from plastic film surfaces for microbiome analysis (DNA analysis). So we can identify the microbes involved in the biodegradation process. We will make sure to take samples before the entire plastic films are degraded – so we should keep the incubation going up to when no more than 90 % of the plastic is disintegrated. That leaves 10% for post analysis.

An experiment conducted in South-East Asia revealed the biodegradation rate of a PHB sample in benthic conditions to be 54 days (half-life t0.5). The image below shows degree of material loss with increasing exposure time of the sample. The material used in our experiment in the Bergen Aquarium, was stored on the shelf before use, so there is a theoretical possibility of weathering of the samples.

We expect that the (bio)degradation time in cold water of Bergen fjord will be significantly slower, but we will monitor the development closely and be prepared to document the polymer samples tested from 1 month incubation time and onwards by photography.

Online monitoring by streaming will ensure that any eventual physical interference caused by fishes in the tank will be monitored and documented.

PHB sample after increasing exposure time to conditions occurring at a sandy seafloor in a protected bay in South-East Asia (Lott et al).

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