Bacterial-fungal interactions are important in the functioning of natural ecosystems. We examined possible synergistic or antagonistic effects during the degradation of polycyclic aromatic hydrocarbons (PAHs) by a fungal–bacterial co-culture. Bacteria and fungi were grown in a liquid nutrient medium supplemented with PAH substrates. The degradation of PAHs and the identification of metabolites were checked by HPLC. Enzyme activities were spectrophotometrically measured with test substrates. Compared to monocultures, the co-culture yielded higher mycelium dry weights and higher numbers of bacterial colony-forming units (CFUs). Both organisms and their co-culture transformed three- and four-ring PAHs into the corresponding quinones. The degradation of PAHs was accompanied by the production of fungal extracellular laccase and versatile peroxidase, whose activities were higher in the co-culture than they were in the monocultures. The presence of exogenous indole-3-acetic acid (IAA) boosted PAH degradation and enzyme production. The xylotrophic basidiomycete Pleurotus ostreatus Florida and the plant-growth-promoting rhizobacterium Azospirillum brasilense exerted a positive mutual effect, including increases in mycelium dry weight, number of CFUs, degradation of PAHs, and production of fungal extracellular enzymes. IAA may be a factor in the interactions of P. ostreatus Florida with A. brasilense.

Background: Bacteria and fungi are the most important soil organisms owing to their abundance and the key roles they play in the functioning of ecosystems. We examined possible synergistic and antagonistic effects during the degradation of polycyclic aromatic hydrocarbons (PAHs) by co-cultures of ascomycetes and a plant-growth-promoting bacterium. Methods: Bacteria and fungi were grown in a liquid nutrient medium supplemented with PAHs. The PAH degradations and the identification of metabolites were checked by high-performance liquid chromatography (HPLC). Enzymatic activities were measured spectrophotometrically using test substrates. All experimental treatments were analyzed using Excel 2019 (Microsoft Office 2019, USA). Results: The model system included the plant-growth-promoting rhizobacterium (PGPR) Azospirillum brasilense and one of the following ascomycetes: Fusarium oxysporum (plant pathogen), Talaromyces sayulitensis (rhizospheric fungus), Trichoderma viride (plant-growth-promoting fungus, PGPF), and Trichoderma harzianum (PGPF). The notable results are: (1) synergistic effects consisted of more active utilization of the PAH mixture compared to individual compounds, while the PAH mixture was more actively degraded by co-cultures than monocultures; (2) three effects of mutual influence by the studied organisms were also revealed: depressing (F. oxysporum and A. brasilense), partially depressing (T. sayulitensis suppressed the growth of A. brasilense but increased the degradation of anthracene, pyrene, and fluoranthene), and positive effects (A. brasilense and T. viride or T. harzianum); (3) for the first time quinone metabolites of PAH degradation and extracellular oxidase and peroxidase were produced during PAH degradation by T. sayulitensis.Conclusions: The results of the study contribute to the understanding of bacterial–fungal interactions in polluted settings.