The dual role of Actinobacteria in aquaculture: a systematic review of metabolic benefits and detrimental effects

Introduction Actinobacteria are ubiquitous and metabolically versatile members of aquatic microbiomes, yet their roles in aquaculture remain under-characterized compared with other microbial groups. This systematic review synthesizes current evidence on their functional duality in farmed fish systems, integrating both metabolic benefits and detrimental effects. Methods Studies describing Actinobacterial isolates with at least one laboratory validation were systematically collected and critically evaluated. Methodological approaches were examined to distinguish in vitro bioactivity from in vivo effects and to assess taxonomic, genomic, and functional characterization. Results Only a minority of studies progressed beyond in vitro screening, and a critical gap in genomic resolution was identified: almost half lacked molecular identification, and most relied solely on partial 16S rRNA sequencing. Streptomyces emerged as the predominant genus, accounting for most reports of both favorable and detrimental effects. Selected Streptomyces strains and their postbiotic fractions showed consistent antagonism against major fish pathogens, including Vibrio harveyi , Aeromonas hydrophila , and Streptococcus agalactiae , through quorum-sensing disruption, antibiofilm activity, nutrient competition, and targeted immunometabolic modulation. Conversely, Streptomyces , together with Nocardia and Mycobacterium , have been associated with geosmin and 2-methylisoborneol production, sporadic opportunistic infections, and putative involvement in tetrodotoxin cycling. Discussion These outcomes appear highly context-dependent and reflect ecological adaptations rather than intrinsic hazards. By consolidating evidence on both beneficial and detrimental traits, this review highlights the need for genome-resolved taxonomy and ecologically informed screening pipelines to guide the safe and effective integration of Actinobacteria into aquaculture.