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Plastic debris are accumulating in the marine environment and aggregate microorganisms that form a new ecosystem called the plastisphere. Better understanding the plastisphere is crucial as it has self-sufficient organization and carries pathogens or organisms that may be involved in the pollutant adsorption and/or plastic degradation. To date, the plastisphere is mainly described at the taxonomic level and the functioning of its microbial communities still remains poorly documented. In this work, metagenomic and metaproteomic analyzes were performed on the plastisphere of polypropylene and polyethylene plastic debris sampled on a pebble beach from the Mediterranean Sea. Our results confirmed that the plastisphere was organized as self-sufficient ecosystems containing highly active primary producers, heterotrophs and predators such as nematode. Interestingly, the chemical composition of the polymer did not impact the structure of the microbial communities but rather influenced the functions expressed. Despite the fact that the presence of hydrocarbon-degrading bacteria was observed in the metagenomes, polymer degradation metabolisms were not detected at the protein level. Finally, hydrocarbon degrader (i.e., Alcanivorax) and pathogenic bacteria (i.e., Vibrionaceae) were observed in the plastispheres but were not very active as no proteins involved in polymer degradation or pathogeny were detected. This work brings new insights into the functioning of the microbial plastisphere developed on plastic marine debris.☆ This paper has been recommended for acceptance by Maria Cristina Fossi.

Different plastic types considered as compostable are found on the market such as petro-based (e.g., polybutylene adipate terephthalate (PBAT)) or bio-based plastics (e.g., polylactic acid, (PLA)). Even if their degradation has been confirmed in industrial compost conditions, investigation of their degradation in natural marine environment has been limited. To better understand biodegradation into natural marine environment, commercial compostable (PBAT, semi-crystalline and amorphous PLA) and non-compostable polymers (low density polyethylene, polystyrene, polyethylene terephthalate, polyvinyl chloride) were submerged in situ on the sediment and in the water column in the Mediterranean Sea. These samples were studied by chemical and microbiological approaches. After 82 days of immersion, no significant bacterial degradation of the different polymers was observed, except some abiotic alterations of PBAT and LDPE probably due to a photooxidation process. However, after 80 days in an enrichment culture containing plastic films as a main carbon source, Marinomonas genus was specifically selected on the PBAT and a weight loss of 12% was highlighted. A better understanding of the bacterial community colonizing these plastics is essential for an eco-design of new biodegradable polymers to allow a rapid degradation in aquatic environment.

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Cocultures have been widely explored for their use in deciphering microbial interaction and its impact on the metabolisms of the interacting microorganisms. In this work, we investigate, in different liquid coculture conditions, the compatibility of two microorganisms with the potential for the biocontrol of plant diseases: the fungus Trichoderma harzianum IHEM5437 and the bacterium Bacillus velezensis GA1 (a strong antifungal lipopeptide producing strain). While the Bacillus overgrew the Trichoderma in a rich medium due to its antifungal lipopeptide production, a drastically different trend was observed in a medium in which a nitrogen nutritional dependency was imposed. Indeed, in this minimum medium containing nitrate as the sole nitrogen source, cooperation between the bacterium and the fungus was established. This is reflected by the growth of both species as well as the inhibition of the expression of Bacillus genes encoding lipopeptide synthetases. Interestingly, the growth of the bacterium in the minimum medium was enabled by the amendment of the culture by the fungal supernatant, which, in this case, ensures a high production yield of lipopeptides. These results highlight, for the first time, that Trichoderma harzianum and Bacillus velezensis are able, in specific environmental conditions, to adapt their metabolisms in order to grow together.