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PubMed2026

Vibrio phycocola sp. nov. and Vibrio phycohabitans sp. nov., Isolated from the Phycosphere of Marine Algae.

Two Gram-stain-negative, facultatively aerobic, oxidase- and catalase-positive, motile (by means of a polar flagellum) rod-shaped bacterial strains, designated BS-M-Sm-2T and MA40-2T, were isolated from marine algae. Growth was optimal at pH 7.0-8.0 and 2.0-3.0% (w/v) NaCl, with temperature optima of 25°C for BS-M-Sm-2T and 25-30°C for MA40-2T. Ubiquinone-8 was the sole respiratory quinone. The major fatty acids common to both strains were C16:0, summed feature 3 (C16:1 ω7c and/or C16:1 ω6c), and summed feature 8 (C18:1 ω7c and/or C18:1 ω6c), while BS-M-Sm-2T additionally contained C12:0 and C14:0. The predominant polar lipids were phosphatidylethanolamine and phosphatidylglycerol, with diphosphatidylglycerol also detected in strain MA40-2T. The DNA G+C contents of strains BS-M-Sm-2T and MA40-2T were 44.2 and 39.8 mol%, respectively. The 16S rRNA gene sequence similarity, average nucleotide identity (ANI), and digital DNA-DNA hybridization (dDDH) values between the two strains were 93.8%, 71.4%, and 23.2%, respectively. Phylogenetic and phylogenomic analyses placed both strains within the genus Vibrio, forming distinct lineages. Comparisons with closely related Vibrio type strains yielded ANI and dDDH values below 91.6% and 44.3%, respectively, further supporting their classification as novel species. Genome analyses revealed genes potentially involved in algal symbiosis, including those for polysaccharide degradation and vitamin biosynthesis. Based on comprehensive genomic, phylogenetic, phenotypic, and chemotaxonomic evidence, strains BS-M-Sm-2T and MA40-2T represent two novel species, for which the names Vibrio phycocola sp. nov. (BS-M-Sm-2T =KACC 24066T =DSM 119941T) and Vibrio phycohabitans sp. nov. (MA40-2T =KACC 24064T = DSM 119942T) are proposed.

RNA, Ribosomal, 16S↗

PubMed2026

Complete genomes from a xenic Dolichospermum flosaquae FBCC-A233 culture reveal genome-inferred metabolic asymmetry with associated bacteria.

Cyanobacteria form phycosphere communities with associated bacteria, but genome-resolved resources are needed to formulate testable hypotheses about their metabolic interactions. Here, we reconstructed three complete circular genomes from a unialgal xenic culture, including Dolichospermum flosaquae FBCC-A233 and two associated alphaproteobacterial genomes assigned to Sphingorhabdus sp. and Brevundimonas sp. Genome-wide read mapping and genome-quality assessment supported the three recovered genomes as high-quality circular reconstructions. Comparative genome analysis placed the cyanobacterial genome within the Dolichospermum flosaquae species cluster under the GTDB framework, while the associated bacterial genomes represented Sphingorhabdus sp. and a putative undescribed Brevundimonas species-level lineage. Genome architecture analysis indicated reduced genome size and gene content in Brevundimonas relative to genus-level references although additional metrics did not support a strong conclusion of classical genome streamlining. Selected KEGG module and KO-level reconstructions indicated genome-inferred metabolic asymmetries across the consortium. FBCC-A233 encoded photosynthesis- and nitrogen-related modules and a BioU-mediated de novo biotin biosynthesis route, whereas the associated bacteria lacked complete de novo biotin biosynthesis but retained biotin-dependent carboxylase genes. FBCC-A233 also encoded extensive anaerobic corrinoid biosynthesis potential; however, canonical DMB-containing cobalamin completion, cobamide identity, and complete transporter systems were not resolved. Together, these complete genomes provide a genome-resolved resource for investigating genome-inferred metabolic differentiation and ecological interactions in cyanobacteria-associated bacterial consortia.IMPORTANCEPhycosphere interactions between cyanobacteria and associated bacteria can shape aquatic microbial communities, but many proposed interactions remain difficult to evaluate without genome-resolved resources. This study provides three complete circular genomes from a unialgal xenic Dolichospermum flosaquae culture, capturing the cyanobacterium and two co-maintained bacterial associates. Our analysis identifies genome-inferred metabolic asymmetries, particularly in biotin- and cobamide-related pathways. D. flosaquae FBCC-A233 encoded candidate de novo biotin and corrinoid biosynthesis capacity, whereas the associated bacteria lacked complete de novo pathways but retained cofactor-dependent enzymes. These findings nominate cofactor-related dependencies as experimentally testable hypotheses while emphasizing unresolved uptake, export, cobamide identity, and growth-dependence mechanisms. The complete genomes and KO-level reconstructions generated here provide a resource for future studies of cyanobacteria-associated consortia.

Genome, Bacterial↗

PubMed2026

Host-driven evolution shapes the polysaccharide utilization profiles of alga-associated Flavobacteriaceae.

BACKGROUND: Marine algae represent major producers of complex polysaccharides and serve as hosts for diverse microbial communities in the phycosphere. Flavobacteriaceae are among the key bacterial taxa involved in polysaccharide degradation and carbon remineralization in this environment. However, the extent to which algal hosts drive the divergence of polysaccharide utilization profiles in these bacteria remains unclear. RESULTS: We conducted a genome-resolved analysis of 103 cultured Flavobacteriaceae strains isolated from red, green, and brown macroalgae, as well as from diatoms and dinoflagellates. We found that macroalga-associated strains generally harbored more abundant and diverse CAZyme-encoding genes than their microalga-associated counterparts. Moreover, strains associated with different algal phyla showed distinct metabolic specializations that aligned with the typical polysaccharides of their respective hosts, strongly supporting host-specific adaptation. In four widely distributed genera (Maribacter, Flagellimonas, Polaribacter, Winogradskyella), CAZyme profile dissimilarity and key glycoside hydrolase gene divergence exhibited phylogenetic congruence with algal host phylogeny (Mantel r up to 0.76 and 0.85, respectively), indicative of host-associated functional adaptation. Using Maribacter as a model, cultivation experiments and transcriptome characterization demonstrated that polysaccharide utilization efficiency is not solely linked to the organization of genes into polysaccharide utilization loci (PULs), but also associated with the expression dynamics of key transcription factors (TFs), particularly those from AraC and DeoR families, whose expression patterns were coordinated with laminarin degradation. Notably, these two TF families also exhibited host-associated divergence patterns similar to those of CAZyme-encoding genes. Furthermore, analysis of the Tara Oceans metagenomic data indicated that, within the AraC and DeoR families, a higher proportion of genes were positively correlated with chlorophyll a content compared to other TF families, reinforcing their specialized roles in alga-associated bacterial lifestyles. CONCLUSIONS: Our integrative genomic and transcriptomic analyses reveal evolutionary and regulatory adaptation of marine Flavobacteriaceae to distinct algal hosts. These findings highlight algae-derived habitats as specialized niches that shape microbial metabolic potential, and suggest that carbohydrate metabolism plays a key role in host-driven bacterial evolution across global oceans. Video Abstract.

Flavobacteriaceae↗