Ulva lactuca Extract Increases the Growth of Lactuca sativa var. longifolia by Regulating Physiological States and Soil Microorganisms

In the context of carbon peaking and carbon neutrality goals, large-scale macroalgal cultivation provides a sustainable source of marine biomass, whereas epiphytic algae, although often regarded as a constraint in seaweed farming, remain an underutilized resource. In this study, biomass of the epiphytic green alga U. lactuca was collected from the surface of cultivated S. fusiforme, processed into a concentrated aqueous extract, and applied to lettuce (Lactuca sativa var. longifolia) at dilution ratios of 1:600, 1:400, and 1:200. The results showed that the algal extract affected lettuce growth and quality in a concentration-dependent manner. The 1:600 treatment increased soluble sugar, soluble protein, and vitamin C contents by 134.25%, 31.65%, and 20.62%, respectively, while the 1:400 treatment produced the greatest increase in shoot biomass. The extract was also associated with changes in selected soil properties, including pH. High-throughput sequencing indicated that the treatment altered the composition of the rhizosphere-associated bacterial community, including shifts in the relative abundance of several taxa. Overall, these results suggest that appropriate application of epiphytic algal extract may serve as a promising improvement to lettuce growth and quality, with potential value for sustainable agricultural production.