The accumulation of persistent synthetic organic polymers in the environment has become a major environmental concern. Replacing these materials by biodegradable polymers in specific application areas may help to alleviate this problem. Among these areas are agricultural practices that heavily rely on the use of plastics (i.e., ‘plasticulture’). This contribution focuses on assessing the factors that govern the biodegradation of aliphatic polyesters, composed of alternating units of dialcohols and dicarboxylic acids, in agricultural soils. The contribution has three successive parts that target three key processes involved in polyester biodegradation. The first part focuses on enzymatic polyester hydrolysis, which is commonly considered the rate-limiting step in the overall biodegradation of these materials in soils. Two novel experimental approaches are presented and used to systematically study the hydrolysis of a series of structurally related aliphatic polyesters by two isolated esterases under well-controlled laboratory conditions. The enzymatic hydrolysis rates increased as the melting temperatures of the aliphatic polyesters decreased, strongly suggesting that the flexibility of the polyester backbone and hence its propensity to enter the active sites of the esterases governed hydrolysis rates. The second part focuses on the mineralization dynamics of a selected, 13C-labeled aliphatic polyester, polybutylene succinate, in an agricultural soil under laboratory conditions. While the two monomers that compose this polyester, 1,4-butanediol and succinic acid, mineralized over a relatively short time (hours to days) and in a position-specific manner, the mineralization of polybutylene succinate was slower (timeframe of weeks to months) and showed only a slight dependence on the monomer position at which the polymer was labeled. These findings are consistent with overall mineralization rates in soils being governed by the rates of enzymatic depolymerization of the bulk polyester and, hence, the rates at which mono- and oligomers are released from the polymer surface to become available to soil microorganisms. The third part addresses the colonization of polyester film surfaces by soil fungi and unicellular microorganisms as well as the uptake of polymeric carbon into microbial biomass using a combination of surface imaging techniques. The collected images unequivocally demonstrate that polymeric carbon is incorporated into microbial biomass. Furthermore, the images suggest that fungal hyphae play a key role in polyester degradation. The novel insights into polyester biodegradation will be summarized and will serve to provide a brief outlook to future work on the fate of biodegradable polymers in soils and other environmental systems.
Invited by Stefan Peiffer, Hydrology
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