Earthworms and the degradation of lactic acid-based stereocopolymers

Two lactic acid-based stereocopolymers, namely 50/50 and 96/4 L/D poly(L-lactic-co-D-lactic acids) and corresponding oligomers, were allowed to age under different conditions in order to investigate their toxicity and that of some potential degradation by-products, namely lactic acid and sodium and calcium lactates, to earthworms. Degradation characteristics in various worm-free and worm-containing media were also investigated under various conditions including direct feeding using impregnated paper or coated tree-leaves, model composting, and vermi composting. Data were compared with abiotic degradation in sterile neutral phosphate buffer. Last but not least, a novel method aimed at assessing the bioassimilation of degradable polymers and oligomers was utilized, which is based on the monitoring of weight changes of a population of starved worms when the worms are given the polymeric or oligomeric compounds as potential nutrients. The work shows that high molar mass poly(lactic acids) can be ingested by earthworms provided they are disintegrated first. However, they cannot be bioassimilated before hydrolytic degradation generates oligomers. The involvement of microorganisms in the bioassimilation is discussed.