Microplastics from agricultural mulch films: a threat to growth promoting abilities of bacteria?

This thesis investigated the impact of polyethylene (PE) and biodegradable (BIO) microplastics derived from mulching films on growth-promoting bacteria (PGPB) and their influence on the growth of lettuce plants (Lactuca sativa L.). Plastic mulching in agriculture is a process by which the soil is covered with plastic films, with the advantages of better water retention and less erosion, but some synthetic mulches pose potential threats to environmental safety over time due to the release of microplastics. The work focused on five bacterial strains from the genera Bacillus, Enterobacter, Kosakonia, Rhizobium and Pseudomonas, and evaluated the tolerance of the strains to exposure to the two types of plastics and their possible involvement in relieving stress in plants growing in soil contaminated with plastic microparticles. The first part of this study contextualizes the use of plastic films as an agricultural methodology, its advantages and disadvantages. The second part was dedicated to investigating the effect of PE microplastics of different sizes on bacterial growth, revealing that smaller particles tend to cause more pronounced negative effects on most strains. However, one strain (Enterobacter sp. C11) demonstrated tolerance to PE microplastics and showed its potential as a bioinoculant to protect against oxidative damage caused by microplastic contamination in plants. The inoculated plants showed greater tolerance to PE microplastics, attenuating the phytotoxicity that would otherwise affect plant growth. The third part of the study was dedicated to determining the impact of biodegradable microplastics (BIO) on PGPBs and plants. Most of the bacterial strains showed growth inhibition in the presence of BIO microplastics. However, Kosakonia sp. O21 showed greater tolerance, suggesting its potential role as a bioinoculant in agricultural environments polluted with BIO materials. This research also highlights that BIO microplastics have damaging effects on plant biochemistry, largely through oxidative stress, impact on photosynthetic activity and growth disturbance, and bacterial inoculation is useful for mitigating toxic effects, especially at lower concentrations of microplastics. This thesis addresses the tripartite interactions between microplastics, soil bacteria and plants, arguing that bacterial inoculants have enormous potential in protecting crops against microplastic-induced stress. These results contribute to the development of sustainable agricultural practices, indicating strategies based on microorganisms to mitigate the risks of environmental contamination by microplastics.