Duckweed (Lemnaceae) as a Functional Protein Ingredient in Koi Carp Diets: Species-Dependent Effects on Growth, Pigmentation, Antioxidant Status, and Gut Health

Abstract

Duckweed (Lemnaceae) has emerged as a promising aquatic plant protein for aquafeeds, yet its functional impacts on ornamental fish remain insufficiently characterized. This study evaluated five duckweed species, Lemna minor (LM), Wolffia globosa (WO), Landoltia punctata (LP), Spirodela polyrhiza (SP), and rootless L. punctata (LR), as partial fishmeal replacers in koi carp (Cyprinus carpio). Phase I determined optimal inclusion levels based on growth performance using isonitrogenous and isolipidic diets (0–40% weight-for-weight [w/w]). Phase II further assessed selected inclusions (LM₂₀, WO₂₀, LP₃₀, SP₁₀, and LR₃₀) for pigmentation, antioxidant status, intestinal immunity, and gut microbiota. Moderate replacement with LM and WO (20%) significantly improved weight gain (WG), feed conversion ratio (FCR), protein digestibility, and essential amino acid retention compared with the control. These treatments also enhanced skin redness and yellowness, accompanied by higher muscle astaxanthin, zeaxanthin, and lutein deposition. Antioxidant defenses (superoxide dismutase [SOD] and catalase [CAT]) were elevated and lipid peroxidation (malondialdehyde [MDA]) reduced, while intestinal lysozyme and phenoloxidase (PO) activities increased. Proinflammatory cytokines (tumor necrosis factor-α [TNF-α] and interleukin-1β [IL-1β]) were downregulated, whereas IL-10 and tight-junction genes (claudin, occludin, and ZO-1) were upregulated, indicating improved mucosal immunity and barrier function. Gut microbiota profiling revealed enriched Lactobacillus and Bacillus and higher α-diversity in LM₂₀ and WO₂₀, whereas SP (SP₁₀) showed limited benefits. These findings demonstrate that specific duckweed species, particularly LM and WO, act as functional ingredients beyond protein supply, improving pigmentation, antioxidant capacity, mucosal immunity, and beneficial gut microbial profiles. These results identify LM and WO as nutritionally feasible and functionally active protein ingredients for koi carp, supporting their application in sustainable ornamental aquafeeds.