Valorization of Lemna minor Leaves via Solid-State Fermentation by a Fish-Gut Bacillus subtilis for Aquafeed Application

Abstract— This study aimed to enhance the nutritional value of the freshwater macrophyte Lemna minor (duckweed) for use as an aquafeed ingredient through Solid-State Fermentation (SSF). A phytase-producing bacterium, Bacillus subtilis (HM352551), isolated from the gut of the teleost fish Labeo bata, was used as the fermenting agent. Key SSF parameters were optimized for maximum phytase yield. The highest phytase activity of 15.26 ± 0.09 U/g was achieved after a 10-day incubation at 35°C, with an initial substrate moisture content of 50% and a moistening media pH of 7.0. An inoculum size of 4% (v/w) also yielded high activity (14.28 ± 0.11 U/g). Proximate composition analysis of the fermented leaf meal revealed a significant increase (p < 0.05) in crude protein, lipid, ash, and mineral content (Na, K, Ca, Mg, P, Zn, Fe, Cu, Mn). The levels of all essential amino acids increased. Concurrently, there was a significant reduction (p < 0.05) in antinutritional factors, including crude fibre, phytic acid, trypsin inhibitor, and tannin. Concentrations of heavy metals (Pb, Cd, Cr, Ni) were also reduced. The results demonstrate that SSF using a host-derived gut bacterium is an effective strategy for the bioconversion of low-cost aquatic weeds into a nutritionally enhanced, safer, and sustainable component for aquafeed formulations.