What a year in forest soil measured
The experiment is small and physical. Films of long-chain aliphatic polyester went about 10 cm down into the humus layer of the forest at the University of Konstanz's botanical garden, the layer where cellulose and plant polyesters are broken down anyway, and stayed there through a field trial of nearly 15 months. In parallel, powdered polymer was mixed into the same soil in the laboratory and the carbon dioxide coming off it was counted. By day 250 two of the three long-chain polyesters had converted roughly 50 per cent to 58 per cent of their substrate carbon; the third stopped near 22 per cent.[1]
The controls give the scale. Cellulose, the reference the forest floor handles every autumn, reached about 36 per cent by day 77. PCL, an older compostable polyester, reached about 53 per cent by day 318. High-density polyethylene, the material in a milk bottle, topped out at 3 per cent. Read together, these place the bioplastics between a leaf and a bottle, closer to the leaf, and put the distance on a scale of hundreds of days.[1]
The enzyme that decides it
Sequencing the whole soil community's DNA isolated one gene enriched only where the polyester was buried. It encodes a family-VIII esterase, and two structural details matter more than the family name. The active site sits in a large, wide-open groove rather than in a tunnel, which is where the pac-man nickname comes from. And the sequence carries a lipobox motif, marking the protein as a membrane-associated lipoprotein likely to be directed to the outer membrane, so the enzyme stays fixed to the bacterial cell surface.[1]
Anchoring is the part with ecological consequences, and the authors name them. A fixed enzyme stays with the cell instead of washing away in water, it concentrates at the interface where cell meets plastic, and it keeps catalysis next to membrane transport, which limits monomer loss to plastisphere community members that do not produce the enzyme. That last point is what makes the trait worth carrying: in open soil an enzyme released freely feeds whoever is nearest, and the plastisphere has no shortage of neighbours. Another reading is available — anchoring may be the ancestral arrangement for this protein family, with plastic depolymerization a later use of a trait that evolved for something else — and sequence analysis alone cannot separate the two.[1]
The same enzyme's second reaction
The esterase's closest structural relatives are type C beta-lactamases, the enzymes that make bacteria resistant to penicillin by cutting its beta-lactam ring. Docking calculations put both LCAP and beta-lactam antibiotics into that open groove, and heterologous expression with in vitro testing confirmed both reactions. The two rates are far apart: 0.5 units per milligram on the polyester in the first 4 hours, against 8.92 units per milligram on penicillin G, which disappeared nearly completely within 24 hours. On this enzyme, cutting the antibiotic is the easier job.[1]
The plastisphere already harbours an unusually high occurrence of antibiotic resistance genes, and this enzyme gives one concrete route by which the two capabilities sit on the same protein. Nothing here shows resistance arising in a field: the dual activity was demonstrated in vitro, and the soil work measured degradation; it did not measure a clinical outcome. What changes is the design question. Schleheck's own conclusion is that working with soil microbes means using polymers with biochemical breaking points, the hydrolysable ester bonds in polyesters like LCAP — and the polyester that stopped near 22 per cent shows that having such a bond does not by itself set the rate. If the same field-and-microcosm protocol is run on that stalled polyester with its chain length varied, the conversion figure at day 250 will rise above the about 22 per cent reported here; a procurement rule could be written against a measured conversion figure rather than against a compostable label.[1]