Optimizing degradation of PLA bioplastic by a thermophilic microbial consortium and downstream uses of spent medium

Abstract

Materials made of polylactic acid (PLA) bioplastic, including the disposable foodware on the Sonoma State campus, are marketed as compostable. However, they cannot currently be processed by the vast majority of commercial composting facilities due their slow rate of degradation. We have previously reported on the enrichment of a thermophilic PLA-degrading microbial community, termed 5A1, that was derived from a combined inoculum of PLA-containing composts and soils. 5A1 cultures were incubated at 63°C in a basal salts medium with PLA fragments, potassium nitrate as an electron acceptor, and gelatin as an inducer of extracellular enzymes that are known to degrade PLA (i.e. serine proteases). The degree of PLA degradation was determined by filtering cultures through a 2-mm screen, gently rinsing with distilled water, and determining the dry weight of the retentate. PLA degradation rate was positively correlated with nitrate levels up to the maximum concentration tested (i.e. 75 mM KNO3). The 5A1 metagenome was found to be dominated by endospore-forming members of the bacterial phylum Firmicutes, with the classes Bacilli, Tissierellia, and Clostridia comprising 67%, 14%, and 12% of the bacterial community, respectively. A total of 11 putative extracellular serine proteases were found in the metagenome. We are currently examining potential beneficial uses of spent 5A1 culture medium. Initial results demonstrate that it can be converted into biogas via anaerobic digestion. An ongoing experiment is testing soil application of the spent medium for use as a fertilizer. Widespread deployment of this technology could greatly improve the lifecycle costs of PLA products and thereby expand their use as sustainable replacements for petroleum-based plastics.

Also see previous related study, Biodegradation of Polylactate Bioplastic

Results

Key Terms: water pollution, sustainability