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Aquaculture provides a rich resource of high-quality protein; however, the production is challenged by emerging pathogens such as Vibrio crassostreae. While probiotic bacteria have been proposed as a sustainable solution to reduce pathogen load in aquaculture, their application requires a comprehensive assessment across the aquaculture food chain. The purpose of this study was to determine the antagonistic effect of the potential probiotic bacterium Phaeobacter piscinae against the emerging fish pathogen V. crassostreae in aquaculture feed algae that can be an entry point for pathogens in fish and shellfish aquaculture. P. piscinae strain S26 produces the antibacterial compound tropodithietic acid (TDA). In a plate-based assay, P. piscinae S26 was equally to more effective than the well-studied Phaeobacter inhibens DSM17395 in its inhibition of the fish pathogens Vibrio anguillarum 90-11-286 and V. crassostreae DMC-1. When co-cultured with the microalgae Tetraselmis suecica and Isochrysis galbana, P. piscinae S26 reduced the maximum cell density of V. crassostreae DMC-1 by 2 log and 3-4 log fold, respectively. A TDA-deficient mutant of P. piscinae S26 inhibited V. crassostreae DMC-1 to a lesser extent than the wild type, suggesting that the antagonistic effect involves TDA and other factors. TDA is the prime antagonistic agent of the inhibition of V. anguillarum 90-11-286. Comparative genomics of V. anguillarum 90-11-286 and V. crassostreae DMC-1 revealed that V. crassostreae DMC-1 carries a greater arsenal of antibiotic resistance genes potentially contributing to the reduced effect of TDA. In conclusion, P. piscinae S26 is a promising new candidate for inhibition of emerging pathogens such as V. crassostreae DMC-1 in algal feed systems and could contribute to a more sustainable aquaculture industry. IMPORTANCE The globally importan

Microalgae are very important as primary producers in the ocean, but also as forthcoming sustainable producers of biotechnologically interesting compounds. Accordingly, the bacterial microbiomes associated with microalgae are attracting increasing attention due to their effects on the growth and health of microalgae. The bacterial communities associated with microalgae are vital for the growth and health of the host, and engineering algal microbiomes can enhance the fitness of the algae. Characterization of these microbiomes mostly relies on sequencing of DNA, which can be extracted with an array of protocols that potentially impact DNA quantity and quality and thus potentially affect subsequent analyses of microbiome composition. Here, we extracted DNA from Isochrysis galbana, Tetraselmis suecica, and Conticribra weissflogii microbiomes using four different protocols. DNA yield and quality was greatly impacted by the choice of extraction protocol, whereas microbiome composition determined by 16S rRNA gene amplicon sequencing was only impacted to a minor degree, with microalgal host species being the main determinant of microbiome composition. The I. galbana microbiome was dominated by the genus Alteromonas, whereas the microbiome associated with T. suecica was dominated by Marinobacteraceae and Rhodobacteraceae family members. While these two families were also prevalent in the microbiome associated with C. weissflogii, Flavobacteriaceae and Cryomorphaceae were also highly dominant. Phenol-chloroform extraction resulted in higher DNA quality and quantity compared to commercial kits; however, because they have other advantages such as high throughput and low toxicity, commercial kits can be employed to great benefit for the characterization of microalgal microbiomes. IMPORTANCE Microalgae are very important as primary producers in the ocean, but also as forthcoming sustainable producers of biotechnologically

With the rising plastic pollution in the oceans, research on the plastisphere-the microorganisms interacting with marine plastic debris-has emerged. Microbial communities colonizing plastic have been characterized from several ocean regions and they are distinct from the communities of the surrounding waters, and a few plastic-degrading microorganisms have been isolated from other environments. Therefore, we propose that marine microorganisms have adapted to plastic as a surface for colonization and potentially degradation. When comparing the taxonomic patterns of plastic-associated, marine bacteria, recurring groups and families such as the families Erythrobacteraceae and Rhodobacteraceae (Alphaproteobacteria), Flavobacteriaceae (Bacteriodetes), and the phylum of cyanobacteria (such as the Phormidium genus) can be identified. Thereby, we provide a perspective on which bacterial candidates could play a role in the colonization and possible degradation of plastic in the oceans due to their occurrence on marine plastic debris. We emphasize the need for extended and reproducible collection of data to assess the existence of a core microbiome or core functionalities of the plastisphere and confirm the capability of these bacterial candidates for biodegradation of plastic. Furthermore, we suggest the next steps in research to elucidate the level of natural bioremediation and the exploitation of bacterial degradative mechanisms of plastic.