Characterization of an Amphiphilic Exopolymer from Rhodothermus marinus DSM 16675 Reveals Its Potential as a Natural Bioemulsifier

The thermophilic bacterium Rhodothermus marinus DSM 16675 represents a promising yet underexplored source of natural polymers with potential industrial applications. The aim of the current study was to screen for emulsifying activity in R. marinus cultures, identify conditions for production of the potential emulsifier, characterize the compound, and analyze the functional properties. Production trials in shake flasks combined with emulsification screening of the cell-free cultures showed that R. marinus produced an emulsifying compound, with fed-batch cultivation yielding up to 20 mg L−1 of crude emulsifying exopolymer. The purified exopolymer contained carbohydrates, proteins (19.22 ± 0.3 wt%), and lipids (approximately 10 wt%), with rhamnose, arabinose, and mannose as predominant carbohydrate moieties in the produced polymer. Amino acid analysis and fatty acid analyses supported its amphiphilic nature. Dynamic light scattering revealed a bimodal particle-size distribution (Z-average 288.8 ± 63.9 nm) and a moderately negative ζ-potential (−26.9 mV). The exopolymer exhibited highest emulsifying activity at 5 g L−1, with a critical micelle concentration (CMC) of 0.12 ± 0.01 g L−1 and retained good performance over pH 3–9 and 4–25 °C, whereas elevated salinity and strongly alkaline conditions reduced its activity. These findings advance knowledge on microbial exopolymers and highlight their potential as natural bioemulsifiers.