Biodegradable plastics in the marine environment: enzymatic degradation and impact on invertebrates

In the framework of this dissertation, selected biodegradable plastic compounds were investigated for their enzymatic degradability under marine conditions and their impacts on marine invertebrates. The tested compounds are issued from the Horizon 2020 project "Bio-Plastics Europe" and are based on common biodegradable polymers.
In-vitro assays showed, that the enzymatic degradability of these compounds is impaired by low temperatures prevailing in the ocean. Additionally, almost no degradation was observed with exposure experiments in seawater and mud for six months, indicating no or only very slow degradation of biodegradable plastics in most marine environments.
Feeding experiments with different marine invertebrates showed, that biodegradable microplastics are ingested in the same way as conventional microplastics. Once ingested, they might be exposed to digestive enzymes. Gastric fluids from the edible crab Cancer pagurus and the American lobster Homarus americanus were incubated with microplastics and the hydrolysis was measured, showing that both species can potentially hydrolyze one of the biodegradable plastics. Further analyses of the gastric fluid of the edible crab identified high activities of carboxylesterases, which are thought to be responsible for the hydrolysis of the biodegradable plastic.
The implications following an ingestion of biodegradable microplastics were investigated using the rockpool shrimp Palaemon elegans. Acute feeding trials and also continuous feeding of microplastics did not induce oxidative stress in the shrimp. However, ingestion of one of the materials led to increased carboxylesterase activity in all experiments. Furthermore, the effects of substances that potentially leach out of biodegradable plastics were investigated with marine zooplankton. Leachates from one of the compounds induced high mortalities in the rotifer Brachionus plicatilis and in nauplii from the brine shrimp Artemia persimilis, indicating the presence of toxic additives.