A theoretical systems-biology framework for the degradation of mixed plastics and conversion into fertilizer-grade compounds via engineered microbial consortia

Abstract

Plastic pollution is a global environmental challenge of increasing severity. Global plastic production reached approximately 413.8 million metric tonnes in 2023, yet global assessments, including those by Geyer et al., suggest that fewer than 10% of post-consumer plastics are effectively recycled. Here we present a theoretical Plastic-to-Fertilizer (P2F) framework that proposes an engineered four-member microbial consortium capable of partially depolymerizing mixed plastic waste, including polyethylene terephthalate (PET), high-density polyethylene (HDPE), polystyrene (PS), and polypropylene (PP), and channeling selected plastic-derived carbon intermediates toward biosynthesis of agronomically beneficial compounds, including organic acids, amino acids, and humic-like macromolecules (HLMs). The proposed consortium comprises an engineered Pseudomonas putida KT2440 chassis expressing heterologous PETase and MHETase [mono(2-hydroxyethyl) terephthalate hydrolase] for PET depolymerization and harboring a native styrene catabolic pathway for PS intermediates; Bacillus subtilis 168 providing CotA laccase-mediated polyolefin surface oxidation; Aspergillus niger serving as a biofilm scaffold and oxidative enzyme source; and a biocontained Azotobacter vinelandii strain with a conditionally active, speculative nitrogen-fixation module. A theoretical mathematical framework encompassing Langmuir-adsorption-based surface degradation kinetics, substrate-specific Haldane-Andrews growth models, and enzyme synergy quantification is presented alongside a corrected stoichiometric mass balance for the P2F metabolic funnel. Techno-economic projections are presented as illustrative scenarios only, given the current technology readiness level (TRL 1–2). Key biological constraints are explicitly acknowledged throughout: HDPE and PP are highly crystalline polymers requiring mandatory abiotic pre-treatment before enzymatic action is feasible; PS depolymerization to metabolisable intermediates requires abiotic pre-treatment as no biological route has been demonstrated for bulk PS; heterologous nitrogen fixation is technically challenging and is framed as a speculative high-risk long-term aspiration rather than a functional module; and all stoichiometric yields are theoretical upper bounds. This paper provides a conceptual foundation, a corrected mathematical framework, and a five-phase experimental roadmap intended to guide empirical validation.