Intelligent bioagents: how Trichoderma spp. and Azospirillum brasilense modulate medium pH and its enzymatic activity.

This study aimed to evaluate the ability of three Trichoderma species (T. harzianum, T. asperelloides, and T. virens) and the bacterium Azospirillum brasilense to modify the pH of the growth medium in order to reach their optimal biological range. In vitro assays were conducted at five pH levels (4.0, 5.0, 6.0, 7.0, and 8.0) under controlled conditions (25 ± 1 °C; 12 h photoperiod). Additionally, a pot experiment was carried out with olive seedlings grown for 180 days in a greenhouse under three liming levels (pH 4.0, 5.0, and 6.0) and different microbial inoculation treatments. The results showed that all four microbial species were able to alter the initial pH of the culture medium to reach their optimal growth range. Trichoderma species acidified the medium to varying degrees (with species-specific patterns), while A. brasilense alkalinized it, indicating an optimal pH range between 4.5 and 5.0. This pH modulation directly influenced enzymatic activity: β-glucosidase and acid phosphatase reached maximum activity at the pH levels metabolically “regulated” by each microorganism. Under soil conditions, enzymatic activity also varied depending on the combination of microbial species and liming level, highlighting specific metabolic differences among bio-inputs. These findings emphasize the agronomic importance of understanding the pH preferences of each microorganism prior to application. Recognizing this capacity for active pH adjustment—and the resulting enhancement of β-glucosidase and phosphatase activity—suggests promising biotechnological applications in soils with variable acidity, improving nutrient mobilization and promoting greater productive efficiency.