Humic Substances and Innate Immune Responses in Aquatic Organisms

Humic substances are complex, heterogeneous mixtures of partially decomposed organic matter that occur ubiquitously in freshwater and marine systems. Comprised principally of humic acids, fulvic acids and humins, these macromolecules interact with aquatic biota at multiple levels. In fish and invertebrates, humic substances modulate the innate immune system by influencing barrier defences, cellular phagocytosis and the production of antimicrobial enzymes. The electron‐rich phenolic and quinone moieties within fulvic acids can scavenge reactive oxygen species (ROS), while also acting as immunostimulants at subtoxic concentrations. Conversely, high doses may provoke proinflammatory pathways and oxidative stress. Understanding these dose‐dependent effects is critical for both environmental monitoring—particularly in browning waters driven by climate change—and for the optimisation of sustainable aquaculture practices. Humic substances thus expose an intricate nexus between water chemistry, organismal health and ecosystem resilience, with practical applications ranging from water treatment to functional feed additives that enhance disease resistance in cultured species.

Research from Nature Portfolio

Recent studies have demonstrated that bath application of phenol‐rich fulvic acid to juvenile trout significantly enhances growth and stress resilience via gill‐mediated immunostimulation. At concentrations of 5–50 mg C/L, treated fish exhibited elevated phagocytic activity in head‐kidney leukocytes, increased lysozyme activity in gill tissue and enhanced antioxidant capacity, leading to improved feed conversion and reduced handling stress responses. Complementing this, an advanced in vivo assay in zebrafish larvae revealed that a single fulvic acid preparation can exert both anti‐inflammatory and proinflammatory effects depending on dose. At low concentrations (5–50 mg C/L), the aromatic structure attenuated copper‐induced neutrophil recruitment, whereas at higher concentrations (≥ 300 mg C/L) persistent free radicals within the fulvic fraction exacerbated reactive oxygen species generation and inflammation. Together, these findings underscore the chemical heterogeneity of humic substances and the need for precise characterisation when considering their application as immunomodulators.

Humic Substances and Innate Immune Responses in Aquatic Organisms publication trend

The graph below shows the total number of articles in humic substances and innate immune responses in aquatic organisms across all publications each year (not limited to Nature Index journals).

Technical terms

Humic substances: Complex organic mixtures formed by the decay of plant and microbial matter, comprising humic acids, fulvic acids and humins.

Fulvic acid: The water‐soluble fraction of humic substances, rich in phenolic and quinone groups, capable of redox reactions and free‐radical scavenging.

Reactive oxygen species (ROS): Chemically reactive molecules derived from oxygen that play roles in cellular signalling and host defence, but can cause oxidative damage at high levels.

Phagocytosis: The engulfment of particles or pathogens by specialised immune cells, a fundamental mechanism of innate defence.

Lysozyme: An enzyme that cleaves bacterial cell walls, serving as an important component of mucosal and systemic innate immunity.

References

1. Phenol-rich fulvic acid as a water additive enhances growth, reduces stress, and stimulates the immune system of fish in aquaculture. Scientific Reports (2021).
2. Modification of the chemically induced inflammation assay reveals the Janus face of a phenol rich fulvic acid. Scientific Reports (2022).
3. Freshwater browning as a hidden threat – Transcriptomic responses in fish gills exposed to fulvic acid. Journal of Hazardous Materials (2025).
4. Proteomics Analysis in Japanese Medaka Oryzias latipes Exposed to Humic Acid Revealed Suppression of Innate Immunity and Coagulation Proteins. Biology (2022).
5. Optimized Feeding of a Humic Substance or Protease Complex in Channel Catfish Ictalurus punctatus (Rafinesque): Implications for Performance, Immunity, and Intestinal Microbiota. Aquaculture Research (2024).