Assessing the Biodegradation of Vulcanised Rubber Particles by Fungi Using Genetic, Molecular and Surface Analysis

Biodegradation of vulcanized rubber by a gut bacterium from plastic-eating mealworms

The disposal of vulcanized rubber waste is difficult due to the presence of three-dimensional crosslinking network structure. Here, we report that a bacterium Acinetobacter sp. BIT-H3, isolated from the gut of plastic-eating mealworm, can grow on and degrade vulcanized poly(cis-1,4-isoprene) rubber (vPR). Scanning electronic microscopy (SEM) shows that strain BIT-H3 can penetrate into the vPR and produce craters and cracks. The tensile strength and the crosslink density of vPR decreased by 53.2% and 29.3% after ten weeks' incubation, respectively. The results of Horikx analysis, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray absorption near-edge structure (XANES) spectroscopy reveal that strain BIT-H3 can break down both sulfide bridges and double bonds of polymeric backbone within vPR. Sulfate and oligo(cis-1,4 isoprene) with terminal aldehyde and keto groups were identified as metabolic products released during vPR degradation. Through genomic and transcriptional analyses, five enzymes of dszA, dszC1, dszC2, Laccase2147, and Peroxidase1232 were found to be responsible for vPR degradation. Based on the chemical structure characterizations and molecular analyses, a vPR biodegradation pathway was proposed for strain BIT-H3. These findings pave a way for exploiting vulcanized rubber-degrading microorganisms from insect gut and contribute to establish a biodegradation method for vulcanized rubber waste disposal.

Biodegradation of rubber in cultures of Rhodococcus rhodochrous and by its enzyme latex clearing protein

The biodegradation of rubber materials is considered as a sustainable recycling alternative, highlighting the use of microorganisms and enzymes in oxidative processes of natural rubber. Currently, the main challenge is the treatment of rubber materials such as waste tyres, where the mixture of rubber polymers with different additives and the cross-linked structure obtained due to the vulcanisation process positions them as highly persistent materials. This study characterises the degradation of different rubber-containing substrates in in vivo and in vitro processes using the bacterium Rhodococcus rhodochrous and the oxygenase latex clearing protein (Lcp) from the same strain. For the first time, the degradation of polyisoprene particles in liquid cultures of R. rhodochrous was analysed, obtaining up to 19.32% mass loss of the polymer when using it as the only carbon source. Scanning electron microscopy analysis demonstrated surface alteration of pure polyisoprene and vulcanised rubber particles after 2 weeks of incubation. The enzyme Lcp_RR was produced in bioreactors under rhamnose induction and its activity characterised in oxygen consumption assays at different enzyme concentrations. A maximum consumption of 28.38 µmol_O2/min was obtained by adding 100 µg/mL Lcp_RR to a 2% (v/v) latex emulsion as substrate. The bioconversion of natural rubber into reaction degradation products or oligoisoprenoids was calculated to be 32.54%. Furthermore, the mass distribution of the oligoisoprenoids was analysed by liquid chromatography coupled to mass spectrometry (LC-MS) and 17 degradation products, ranging from C20 to C100 oligoisoprenoids, were identified. The multi-enzymatic degradation capacity of R. rhodochrous positions it as a model microorganism in complex degradation processes such as in the case of tyre waste.