This study aimed to quantity total numbers of bacteria, fungi and archaea in different types of commercial liquid anaerobic digestates, and to identify common patterns in their microbial numbers post-digestion and possible implications of their use as biofertiliser. Relationships between microbial numbers and physical-chemical traits of the digestates were also investigated. Quantification was performed using culturable and molecular (quantitative PCR) approaches. Bacterial and fungal CFUs ranged up to five orders of magnitude (105-1010; 0-105 g-1 DW, respectively) between different types of anaerobic digestates. Bacterial, archaeal and fungal gene copy numbers (GCN) varied by two orders of magnitude (108-1010; 107-109; 104-106 g-1 DW, respectively) between digestates. All microbial variables analysed showed significant differences between the different types of anaerobic digestate investigated (p < 0.05). Culturable microbial numbers for fungi (6.43 × 104 CFU g-1 DW) were much lower than for bacteria (2.23 × 109 CFU g-1 DW). Gene copy numbers were highest for bacteria (16S) (1.09 × 1010 g-1 DW), followed by archaea (16S) (5.87 × 108 g-1 DW), and fungi (18S) (1.77 × 106 g-1 DW). Liquid anaerobic digestates were predominantly dominated by bacteria, followed by archaeal and fungal populations. At 50% similarity level, the microbial profiles of the eleven anaerobic digestates tested separated into just two groups, indicating a broad relative degree of similarity in terms of microbial numbers. Higher bacterial (16S) GCN was associated with low OM and C/N ratio in digestates.
Keywords: Anaerobic digestates; archaea; bacteria; fungi; qPCR.