Bioprospecting Bacteria From Psychrophilic Anaerobic Digestate for Potential Plant Growth-Promoting Attributes

The psychrophilic anaerobic digestion (PAD) system is a diverse and underexplored microbial ecosystem that typically harbors cold-adapted microorganisms with possible agronomic potential. The plant growth-promoting bacteria in the residue of PAD have the potential to enhance crop production, particularly during cold winter months. In this context, the characteristics of cultivable, cold-tolerant bacteria isolated from digestate obtained during PAD were investigated. Of the 20 isolates, 12 (60%) were able to solubilize phosphate from insoluble compounds at 15°C. Furthermore, nine (45%) and six (30%) isolates exhibited nitrogen fixation activity and produced indole acetic acid (IAA), respectively, while only two (10%) isolates were capable of producing siderophores. Hydrolytic enzyme production varied with cellulase production observed as a common trait since all isolates produced varying levels of cellulase ranging from 3.3 ± 0.5 to 15.3 ± 4 mm activity diameter. Isolates Comamonas sp._A3-1, Acinetobacter iwoffi_B5-1, and Pseudomonas sp._B5-5 displayed maximum cellulolytic activity with activity diameters of 13 ± 2, 13 ± 1.2, and 15.3 ± 4 mm, respectively. However, only two (10%) of the bacterial isolates produced protease with Pseudomonas sp._B5-5 demonstrating maximum proteolytic activity as depicted by an activity diameter of 11.3 ± 2.5 mm. Nucleotide sequence analysis of seven isolates, possessing multiple plant-beneficial traits, revealed their affiliation to three genera: Acinetobacter (57%), Comamonas (28.7%), and Pseudomonas (14%). Biolog Phenotype MicroArray plates revealed varied catabolic capability among bacterial strains, with isolate B5-5 demonstrating the highest metabolic diversity. The findings of this study revealed that cold-tolerant isolates from low-temperature AD possess promising plant growth-promoting characteristics, which indicates the potential of psychrophilic digestate for application in agriculture.