Marine Rhodobacterales as Drivers of Ulva Growth: From Macroalgal–Bacterial Interactions to Bioactive Factor Enrichment

Abstract Marine bacteria significantly influence the development and productivity of algal communities. The green seaweed Ulva ( Chlorophyta) relies on bacteria that secrete algal growth and morphogenesis-promoting factors (AGMPF). In a reductionist model system of Ulva compressa (cultivar U. mutabilis ), the diverse microbiome can be substituted by two key bacteria, Roseovarius sp. MS2 and Maribacter sp. MS6, both of which release AGMPFs, thereby inducing cell division and cell differentiation. Our study examined various algal growth-promoting substances produced by marine Rhodobacterales ( Alphaproteobacteria ). The exploration survey indicated that 74 of the 97 examined strains demonstrated growth-promoting, cytokine-like action. These findings underscore the extensive impact of marine Rhodobacterales on Ulva growth. Bioactive fractions containing AGMPF activity were enriched from the culture supernatant of Roseovarius sp. strain MS2. Bioactive raw extracts were obtained through bioassay-guided fractionation and semi-preparative high-performance liquid chromatography; cell division-promoting properties were observed, but did not reach the activity level of the harvested supernatant. This study provides new insights that underscore the importance of bacterial–macroalgal interactions for marine ecosystems. Replacing bacteria with AGMPFs, including thallusin, enables the complete thallus formation of axenic Ulva cultures, facilitating various applications in biotechnology and aquaculture.