Bacterial protein meal digestibility, digestive enzyme activity and kinetics in barramundi (Lates calcarifer) and tilapia (Oreochromis mossambicus) juveniles

Single-cell protein meals, particularly bacterial protein meals (BPMs), show promise as alternative protein sources for fish diets. However, their digestibility in non-salmonid species remains underexplored. This study evaluated the apparent digestibility coefficients (ADCs) of a BPM derived from brewery by-products when fed to barramundi ( Lates calcarifer ) and tilapia ( Oreochromis mossambicus ). Two cohorts of 150 barramundi (48.9 ± 4.2 g, 95 dph) and 150 tilapia (48.7 ± 11.4 g, 219 dph) were raised in recirculated systems with brackish water (28°C, 5 ppt salinity) over two trials. Fish performance, ADCs for 42 fatty acids and 17 amino acids, and gut enzyme activity, including the Michaelis–Menten constant (Km) for trypsin, chymotrypsin, and leucine aminopeptidase, were assessed. Both species exhibited high thermal-unit growth coefficients (1.7–1.9) and low feed-conversion ratios (1.1) on control diets. Barramundi had a slightly but significantly higher diet ADC for protein (95.8 ± 0.5%) compared to tilapia (94.8 ± 0.6%) (P < 0.05), consistent across most essential amino acids except lysine. Conversely, barramundi showed lower digestibility for PUFA (92.0%) and omega-6 (85.7%) than tilapia. The ADCs of BPM for dry matter and protein were higher in barramundi (66.3 ± 5.8% and 87.5 ± 0.9%) than in tilapia (54.1 ± 5.1% and 71.8 ± 5.3%). Tilapia exhibited higher gut enzyme activities than barramundi, and these activities were unaffected by BPM incorporation (P > 0.05), indicating no presence of digestive inhibitors in the tested BPM. The affinity of digestive trypsin in both species (Km = 11.8–15.1) was 3 to 30 times higher than that measured in other fish species. High affinities for chymotrypsin, and leucine aminopeptidase were observed in both tilapia (240.0 ± 58.6; 132.2 ± 3.2, respectively) and barramundi (425.7 ± 50.7; 70.0 ± 2.2, respectively) and significant interaction suggest that the relationship between diet and enzyme kinetics varies between species. This study demonstrates that both barramundi and tilapia can effectively digest the tested BPM, with barramundi showing superior efficiency. It underscores the importance of tailoring aquafeed ingredients to specific species based on nutrient digestibilities and enzyme kinetics.