A Novel Discovery of Microorganisms Associated with Certain Seaweed Species from the Cat Ba Archipelago Area, Vietnam: Isolation, Identification and Exploration of Their Antimicrobial Activities

Marine macroalgae, or seaweed, are an attractive source of novel natural products, which are in many cases not only produced by the algae themselves but also by their associated microorganisms. Therefore, the search for novel products should not only be aimed at the algae but also their symbionts. In this study, we attempted to investigate culturable microorganisms associated with nine marine macroalgal hosts collected from the Cat Ba Archipelago area (Vietnam), which is known to have significant biodiversity value. More specifically, such microorganisms were isolated by using standard plating methods with eight media under varied nutrient and salinity conditions, screened for antimicrobial activity via agar diffusion assays and MIC/MBC determination methods; those with the most remarkable activities were identified based on their morphology, 16S rRNA gene sequences, and housekeeping gene rpoB sequences if necessary. We obtained a total of 331 isolates, among which 15 bacterial isolates exhibited detectable antimicrobial activity against at least one of the four test microorganisms. The isolates displaying notable antimicrobial activities were affiliated predominantly with Bacillus species, together with representatives of Virgibacillus, Exiguobacterium, Alcaligenes and Serratia. Interestingly, almost all of the associations of those bacteria with the seaweed hosts have not yet been reported. Among those isolates, Bacillus velezensis S.002, found to be associated with the red alga host Gelidium crinale for the first time, exhibited the most remarkable antibacterial activity, and the ethylacetate extract of its culture showed the lowest MIC values against both B. subtilis and X. oryzae. The extract was therefore fractionated and the fraction exhibiting pronounced antibacterial activities was analyzed by LC–HRMS to putatively annotate its metabolites. The results revealed that, in addition to canonical lipopeptides, S.002 could produce several other metabolites that have merely been reported for Bacillus velezensis, including some alkaloids and a unique bioactive amino acid, which might synergistically contribute to the antimicrobial efficacy of the strain. Collectively, the results highlight Cat Ba macroalgae as promising sources of bioactive microorganisms, which in turn can be a promising source of bioactive compounds, produced as part of the host–microbe interactions.