Author: Zhengyi Fu
Fu, Zhengyi, 2025 Functions of plant-derived extracts in impacting environmental health, fish immunity and growth, Flinders University, College of Science and Engineering
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Aquaculture is the fastest-growing food production sector, but intensification has increased disease outbreaks, environmental stress, and reliance on antibiotics, raising concerns about sustainability and antimicrobial resistance. Plant-derived extracts are promising as natural feed additives, yet uncertainties remain regarding their safety, effective dosage, and long-term impacts.
This thesis investigates the suitability of four plant extracts: gallnuts (Rhus chinensis), green chiretta (Andrographis paniculata), white mustard (Sinapis alba), and betel nut (Areca catechu) as functional feed additives for juvenile barramundi (Lates calcarifer), a key aquaculture species and model for nutrition and health studies. The central research questions were: (i) can candidate extracts be applied safely without ecological or toxicological risk? (ii) do they remain physiologically safe in juvenile barramundi during short-term feeding, and how do they modulate systemic and immunological responses? and (iii) can long-term supplementation improve growth, immunity, and pathogen resistance under farming conditions?
A stepwise approach was employed to test the toxicity of four plant extracts in ecotoxicological acute assays. The safety was tested across multiple organisms, including bacteria (Aliivibrio fischeri), algae (Phaeodactylum tricornutum), rotifers (Branchionus plicatilis), and brine shrimp (Artemia salina). An acute oral exposure test in juvenile barramundi was conducted to evaluate the safety on a vertebrate. Based on these results, a short-term feeding trial (14 days) tested low-risk extracts on barramundi to assess feed intake, histology of mucosal and visceral organs, blood and serum immune parameters, and tissue-specific gene expression. Finally, a long-term feeding experiment (8 weeks) examined growth performance, feed efficiency, immune modulation, and survival, followed by Vibrio harveyi challenge.
The distinct impact on organisms at different trophic levels exists across various plant extracts. Gallnut displayed acute toxicity in A. fischeri and P. tricornutum, while all other extracts were within safe ranges for all the test organisms. Acute oral exposure in barramundi further confirmed the relative safety of all test extracts, with only moderate physiological perturbations observed at high doses (1-10%). The short-term feeding trial showed that betel nut supplementation had a narrow margin of benefit and a narrow safety margin for barramundi. Low extract levels stimulated immune markers, but higher levels suppressed serum albumin, complement activity, and mucosal integrity. By contrast, white mustard produced more stable responses, enhancing gut morphology and selectively boosting immune indicators without overt pathology, suggesting broader tolerance. In the long-term trial, white mustard supplementation (0.3–3%) improved growth and feed intake, with enhanced mucosal barriers, favorable liver and kidney histology, and elevated serum immune activities. Survival against Vibrio harveyi infection was significantly higher in groups supplemented with white mustard, particularly at the 3% level.
In conclusion, this thesis provides comprehensive evidence that plant-derived extracts can be integrated into aquafeeds to enhance fish health and resilience. By combining ecotoxicological screening with short- and long-term trials, the research establishes a framework for evaluating natural feed additives. The findings show that while gallnut extracts may pose environmental toxicity risks, betel nut, white mustard, and green chiretta did not exhibit toxicity in the screening assays. Among them, betel nut and white mustard offer clear benefits for growth, immunity, and disease resistance in barramundi, supporting their potential as sustainable alternatives to antibiotics and synthetic chemicals in aquaculture.
Keywords: Functional feed additives, Secondary metabolites, Ecotoxicological assessment, Mucosal barrier integrity, Visceral immune–metabolic responses, Pathogen resistance enhancement, Asian Seabass
Subject: Biology thesis
Thesis type: Doctor of Philosophy
Completed: 2025
School: College of Science and Engineering
Supervisor: Jian G. Qin