Surprising results from the ‘seafloor’

The experiment to find out how biodegradable plastics break down in Norwegian seawater has been running since Summer 2021 and is already revealing some surprises.

In the Q&A below, Gunhild Bødtker, the researcher leading the experiment at the Bergen Aquarium, tells us what they’ve seen so far.

Want to know more about the experiment and why it’s being run? Visit our ‘About the experiment’ page.


What have you observed so far in the tank?

“A lot has already happened with two of the samples in the tank! These are the biodegradable plastic Polyhydroxybutyrate (PHB) and the cellulose. Cellulose is part of trees and plants, it naturally and easily breaks down in the environment and is a useful comparison to assess biodegradability and to compare environmental biodegradation rates of plastic polymers.

“After only 75 days, almost all the PHB plastic and cellulose has disappeared from the test frames placed on the ‘seafloor’ of the experiment tank.

“This is surprising, as we had expected that biodegradation would be slower in cold, Nordic seawater compared to warmer, Asian seawater. However, after just 34 days we could see clear signs of biodegradation on both PHB and cellulose, and after 75 days most of it was vanished – that’s as fast or maybe even faster than some warmer test sites!

“The plastic samples that float freely in the tank water are biodegrading more slowly, but these samples also show clear signs of biodegradation by mass loss after 91 days, and even more so after 125 days.”

Picture of PHB plastic (left) and cellulose (right) after 34 days on the ‘seafloor’. Photo: Andreas R. Graven, NORCE



Which microorganisms are making the plastic biodegrade?

“This we shall find out! We have sampled the residual PHB plastic and cellulose from the ‘seafloor’ samples. By using DNA analysing techniques, we will be able to read the DNA code of all bacteria and fungi present on the samples and find out which microorganisms were a part of the biodegradation process.”

Was anything else interesting discovered?

“Yes! While the biodegradation of PHB plastic was in progress we observed a black spot on the ‘seafloor’ around the PHB samples (picture to the left below). After the PHB plastic was almost fully biodegraded, these spots disappeared. What had happened? Why the discolouration?

“We took samples from the black sediment on the ‘seafloor’ and will analyse them to find out which chemical and biological processes are likely to have caused the discolouring. It is important to find out whether the discolouring is linked to any harmful effects of PHB plastic, and if so, how long it will take before the environment will recover after the PHB plastic has been completely biodegraded.”

Discolouring of the sediment below PHB samples during the biodegradation process (left picture). Photo: Andreas R. Graven, NORCE Picture of samples taken from the discoloured sediment (left tube in picture to the right) and the sediment below the cellulose. Photo: Gunhild Bødtker, NORCE.



What next?

“We need to analyse the Bergen Aquarium experiment samples, calculate the percentage of plastic that has disappeared, and then compare these results with a study carried out in South East Asia. For now, we can tell by the photographs we’ve taken that some samples of biodegradable plastic on the ‘seafloor’ in our experiment biodegrade at a similar rate to the biodegradable plastics on the seafloor of warmer seawaters in South East Asia.”

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