Amending coral soil using exopolysaccharide from salt-tolerant Bacillus velezensis TSD5 bacteria from an atoll in Vietnam

Salinity and drought are significant abiotic stresses affecting plant growth and productivity. To convert arid and saline coral sandy soil in the Vietnamese coral archipelago into arable land, this study isolated and evaluated the salinity tolerance and exopolysaccharide (EPS) production of Bacillus velezensis TSD5 strain from the coral archipelago in Vietnam. EPSTSD5 was estimated to have a molecular weight of 1.09 × 10⁵Da based on its intrinsic viscosity, with monosaccharide components consisting of glucose, rhamnose, and mannose (of which mannose is the majority). The results of FT-IR, EDX, and zeta potential analysis showed that EPSTSD5 contains many hydrophilic and negatively charged functional groups, in addition to having a very negative zeta potential, giving it good water-holding capacity (1793%) and the ability to absorb salts and cations. Under NaCl stress, EPS production was significantly stimulated with increased polysaccharide and protein content, thereby enhancing cellular stress tolerance. Adding EPS or the B. velezensis TSD5 strain to coral sand soil improved aggregate formation and water retention while reducing free salt concentration in the soil. The highlight of this study is the analysis from the structural characteristics of EPS to experiments that clearly demonstrate the role of the B. velezensis TSD5 strain in water retention and salinity reduction in coral sandy soil through its water-holding capacity, aggregation formation, and salt absorption ability of EPS. These findings provide an important premise for the development of the B. velezensis TSD5 strain to improve arid and saline soils into arable land, as well as expand environmental treatment directions.