Mineralization rate and biochemical fate of natural polymers and microplastics in freshwaters. Mineralization rate and biochemical fate of natural polymers and microplastics in freshwaters

Microbial mineralization of organic compounds is important for recycling carbon in the food webs. Microbes play an important role in decomposition of terrestrial recalcitrant and semi-recalcitrant polymers such as lignin and cellulose which are precursors for humus formation. In addition to naturally occurring recalcitrant substrates, microplastics have been found in different aquatic environments. However, microbial utilization of lignin, cellulose and microplastics as carbon sources in freshwaters is poorly understood and their biochemical fate and mineralization speed in freshwaters is still unknown. To fix this knowledge gap, we tracked biochemical fate and mineralization rates of several natural and synthetic polymer-derived carbon in the clear lake and the humic lake waters. We used isotope analysis to unravel mineralization process of different 13C-labeled substrates (polyethylene, polypropylene, polystyrene, lignin/hemicellulose, cellulose and leaves (Fagus sylvatica)). We also used compound specific isotope analysis and molecular biology to identify microbes associated to used substrates. Leaves, cellulose and hemicellulose were fastly mineralized in comparison to microplastics which were mineralized slowly or below detection level. Aromatic polystyrene was mineralized at faster rate than aliphatic polyethylene and polypropylene. Most of the decomposed substrate carbon was assimilated into microbial biomass. Gram-positive bacteria were the main decomposers for natural substrates except in the humic lignin/hemicellulose treatment. Microplastics were equally decomposed by fungi, gram-negative and gram-positive bacteria except in the clear lake water polystyrene treatment where gram-negative biomarkers were highly labeled. Microbial community compositions were shown to be mainly dependent on used lake water, but also temporal changes and substrate-specific communities were found.