Bacterial Supplements Significantly Improve the Growth Rate of Cultured Asparagopsis armata

Seaweed aquaculture is an expanding industry with innovative applications beyond the traditional uses as human foods and phycocolloids. Asparagopsis armata, a red seaweed, is cultivated as a feed supplement to reduce methane emission from ruminants. The manipulation of microbiota with seaweed beneficial microorganisms (SBMs) has shown promise in enhancing disease resistance and growth in seaweeds and has potential to aid the cultivation of A. armata. In this study, we developed a growth assay for the rapid selection of bacteria that promote the growth of A. armata tetrasporophytes. We tested bacterial strains from the genera Phaeobacter and Pseudoalteromonas for their impact on the growth of A. armata, as these bacteria have been recognized for their beneficial traits in other seaweeds. All strains significantly enhanced the specific growth rate (SGR) of A. armata tetrasporophytes compared to controls without bacterial treatment. Bacterial 16S rRNA gene amplicon sequencing confirmed the presence of the inoculated growth-promoting SBMs (SBM-Gs) in A. armata cultures with no significant impacts on the resident microbial community. Co-occurrence network analysis of the resulting communities demonstrated that the inoculated Phaeobacter spp. formed distinct modules, exclusively interacting with resident Phaeobacter species, while the Pseudoalteromonas sp. was absent from the network. These results demonstrate that microbial inoculation is an effective strategy for incorporating SBM-Gs into the A. armata microbiota to promote growth. The tested SBM-Gs may exert their influence by interacting with specific resident species or by directly affecting host physiology, resulting in minimal undesired effects on the microbiome.