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/gwas 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.
Keywords
Download Options
Introduction
In recent years, diverse terrestrial and aquatic macrophytes have been incorporated into carp diets as partial replacements for costly fishmeal [1]. As protein is a critical component for fish growth, alternative plant protein sources have been explored from the early days of freshwater aquaculture [2]. Duckweed, Lemna minor, is considered a promising natural feed for carps [3,4] due to its relatively high protein content, favourable amino acid profile, and small size [5]. Its leaves contain low fibre, and the cell walls have low lignin content [6]. Duckweed is also a source of trace minerals like potassium and phosphorus, as well as pigments such as carotenes and xanthophylls [7].
However, plant-based ingredients contain antinutritional factors (ANFs) like tannins, phytic acid, trypsin inhibitors, and saponins, which reduce nutrient digestibility and bioavailability [8]. Solid-State Fermentation (SSF) using exo-enzyme-producing microorganisms is an effective method to reduce ANFs [8]. SSF is often preferred over submerged fermentation due to its higher product concentration, lower energy and wastewater output, simpler operation, and reduced space requirements [9].
Indigenous phytase-producing bacteria from fish guts are advantageous for fermenting plant materials. Their enzymes can offer precise activity, protease resistance, and high catalytic efficiency compared to fungal alternatives [9]. Using invasive aquatic weeds as an SSF substrate provides economic benefits via low-cost biomass and enables sustainable nutrient recovery. The primary objective of this research was to ferment Lemna minor leaves using an autochthonous fish-gut bacterium, Bacillus subtilis (HM352551), to degrade ANFs and improve nutrient bioavailability, thereby evaluating its potential as a beneficial ingredient in aquaculture feeds.
Conclusion
This study confirms that solid-state fermentation of Lemna minor leaf meal using a phytase-producing Bacillus subtilis strain isolated from fish gut is a viable bioprocessing strategy. The process significantly degraded antinutritional factors, improved the bioavailability of nutrients and minerals, and reduced heavy metal content. This approach valorizes a low-cost aquatic weed into a sustainable, nutrient-enhanced ingredient suitable for incorporation into aquafeeds, offering a pathway to reduce feed costs and environmental footprint in aquaculture.
