Grass carp were an obvious choice for the investigation. As the most heavily farmed freshwater fish globally by production volume, the species carries enormous economic weight, and its herbivorous feeding habit made the team suspect it would be well suited to exploiting macroalgae as a feed ingredient. The researchers, led by Xingxing An and Qingyun Yan of Sun Yat-sen University’s Southern Marine Science and Engineering Guangdong Laboratory in Zhuhai, together with collaborators from industry and Hunan Agricultural University, designed a controlled feeding trial to test whether three chemically distinct seaweeds would produce different microbial and metabolic outcomes.

The experimental design was straightforward but rigorous. Young grass carp weighing roughly 91 grams were randomly assigned to one of four iso-nitrogenous and iso-lipidic diets: a control feed, or feeds supplemented with 5 percent powder of Sargassum hemiphyllum, Asparagopsis taxiformis, or Gracilaria lemaneiformis. The fish were reared for 52 days in a recirculating aquaculture system at 25 to 28 degrees Celsius, fed to apparent satiation twice daily, with no mortality recorded across the entire experiment. At the end of the trial, the team measured growth indices, collected serum and liver samples for immune and antioxidant assays, and aseptically sampled intestinal contents and feces for microbiome and metabolome analysis.

The physiological results immediately singled out one seaweed. Fish receiving the Sargassum diet showed significantly greater body weight, viscerosomatic index, and hepatosomatic index than controls, along with a marked boost in hepatic superoxide dismutase activity, a key antioxidant enzyme. The Asparagopsis group gained significantly in body length and viscerosomatic index, while the Gracilaria group produced more modest changes. Taken together, Sargassum hemiphyllum delivered the most comprehensive benefits for growth and antioxidant defense, hinting that its characteristic sulfated polysaccharides—compounds previously associated with immunomodulatory and anti-inflammatory activity—might be driving a distinctive microbial response.

Sequencing of the 16S rRNA gene from intestinal samples revealed a dramatic restructuring of the gut community. In control fish, the phylum Spirochaetota dominated at nearly 38 percent of the community, with the genus Brevinema as the standout member. In all three macroalgae groups, Spirochaetota collapsed to less than 0.03 percent, while Pseudomonadota surged—reaching over 61 percent in the Asparagopsis group. To pinpoint which genera truly distinguished the dietary groups, the researchers applied a Random Forest machine-learning classifier combined with differential abundance testing. One genus rose above all others: Shewanella, whose relative abundance increased significantly in the Sargassum group compared with controls. Other notable players included Vibrio, Bacteroides, and Butyricicoccus, the latter significantly enriched in the Gracilaria group.

To understand what these shifts meant functionally, the team turned to shotgun metagenomics, generating more than 392 gigabases of sequence data from fecal samples. From this they reconstructed 150 high-quality metagenome-assembled genomes, or MAGs—essentially draft genomes of the community’s constituent organisms. Functional annotation showed that Pseudomonadota carried the most comprehensive amino acid metabolic network, including a serine and threonine degradation pathway absent from other taxa. Crucially, one Shewanella MAG, designated C3_bin52, harbored 2,488 KEGG orthologs, with more than 10 percent of them tied to amino acid metabolism, including complete biosynthetic pathways for essential and branched-chain amino acids. Genes for branched-chain amino acid transferase (ilvE) and tryptophan synthesis (trpA, trpB) were identified within its repertoire.

The metabolomics data provided the physiological confirmation. Using ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry, the researchers profiled 947 metabolites in fecal samples and found clear separation between dietary groups. The Sargassum group stood out with 21 up-regulated metabolites, dominated by essential amino acids such as L-valine and L-threonine, the branched-chain amino acid leucine, and derivatives including 5-hydroxy-L-tryptophan and trans-cinnamic acid—a compound with documented antimicrobial activity against fish pathogens. Pathway enrichment confirmed that the Sargassum diet preferentially activated protein digestion and absorption and amino acid biosynthesis, whereas Asparagopsis skewed toward tyrosine and thyroid hormone pathways and Gracilaria toward nucleotide metabolism.

The statistical integration of all three data layers converged on Shewanella as the central hub. Spearman correlation analysis identified 78 significant microbiome-metabolite associations, and Shewanella displayed the highest connectivity, correlating positively with 22 metabolites, including L-valine and the aromatic amino acid derivative 2-hydroxycinnamic acid. A partial least squares path model, with a goodness-of-fit of 0.554, showed that Shewanella abundance was positively and significantly associated with amino acid metabolism (coefficient 0.655), which in turn correlated positively with growth performance (coefficient 0.596). In other words, the data support a causal-style chain: seaweed polysaccharides and free amino acids feed a microbial niche, Shewanella fills it, and its metabolic output enriches the host’s amino acid pool.

The biological plausibility of this axis is reinforced by what is already known about the genus. Shewanella species are metabolically versatile aquatic bacteria capable of dual pyruvate and lactate fermentation, degradation of complex polysaccharides such as alginate via thermostable lyases, and even biosynthesis of health-promoting omega-3 polyunsaturated fatty acids. Probiotic strains of Shewanella putrefaciens have previously been shown to improve growth and disease resistance in farmed fish. In fish-derived strains, roughly 8 percent of coding sequences are devoted to amino acid transport and metabolism—an extraordinary genomic commitment that aligns neatly with the metabolite patterns observed in the carp.

Beyond its mechanistic interest, the study carries practical weight for a rapidly growing industry under pressure to find sustainable alternatives to fishmeal and other conventional feed ingredients. Macroalgae cultivation requires no freshwater, arable land, or fertilizer, making seaweed-supplemented feeds an attractive sustainability proposition. The finding that different seaweeds steer the gut microbiome toward different functional outcomes—Sargassum toward amino acid metabolism and growth, Asparagopsis toward tyrosine pathways and suppression of the opportunistic pathogen Acinetobacter, Gracilaria toward butyrate-producing taxa and nucleotide metabolism—suggests that feed formulators could in principle select macroalgal species to elicit targeted microbial and metabolic effects. The authors caution that the specific functional roles of individual Shewanella species still require direct experimental validation, for example through isolation and gnotobiotic challenge studies. Even so, the work establishes a compelling diet-microbiome-metabolite framework for herbivorous aquaculture species and positions a once-obscure marine bacterium as a potential keystone of fish nutrition—one that farmers may one day cultivate deliberately, simply by choosing the right seaweed.

Subject of Research: Macroalgae supplementation modulates the gut microbiome and amino acid metabolism in grass carp

Article Title: Dietary macroalgae enhances amino acid metabolism via intestinal Shewanella in grass carp (Ctenopharyngodon idella)

Article References: An, X., Niu, S., Al, M. A., Su, E., Chen, L., He, H., Wang, Y., Zhang, S., Yang, Y., Wang, S., Wen, Z., Xu, B., Ming, Y., Zhu, W., Zhao, Z., Wu, K., Yang, Y., Xie, W., He, Z., & Yan, Q. (2025). Dietary macroalgae enhances amino acid metabolism via intestinal Shewanella in grass carp (Ctenopharyngodon idella). Advanced Biotechnology, 3(4), Article 36. https://doi.org/10.1007/s44307-025-00090-8

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00090-8

Keywords: grass carp, macroalgae, gut microbiome, Shewanella, amino acid metabolism, aquaculture, metagenomics, metabolomics, Sargassum hemiphyllum, feed additives, metagenome-assembled genomes, fish nutrition

Dry-cured grass carp, a staple of Chinese aquatic processing, owes its prized flavor to a dynamic web of biochemical reactions. The fish flesh is rich in unsaturated fatty acids, which undergo autoxidation, enzymatic breakdown, and photo-oxidation during curing. These processes generate a diverse array of volatile compounds that collectively define the product’s sensory identity. While salting begins the transformation, the drying stage is recognized as the critical window during which flavor accumulates and quality stabilizes, because the intensity of biochemical reactions peaks as moisture leaves the muscle.

Traditional sun-drying delivers the characteristic aroma, but natural sunlight can exceed 100,000 lux under direct outdoor exposure and remains hostage to geography, season, and weather, leading to long production cycles and inconsistent quality. Modern industrial methods such as cold-air, heat pump, and infrared drying are efficient, yet their products often lack the nuanced sun-cured notes consumers expect. The research team, led by Wangli Dai of Zhejiang Shuren University, set out to bridge this gap by testing controllable artificial light intensities of 0, 5,000, 10,000, and 15,000 lux on salted grass carp fillets dried for 24 hours at 50 degrees Celsius and 50 percent relative humidity under LED light spanning 380 to 800 nanometers.

The chemistry at play is elegant. Photo-oxidation proceeds through two pathways: a Type I mechanism in which light directly initiates free-radical chain reactions, and a Type II mechanism in which excited photosensitizers convert ordinary triplet oxygen into highly reactive singlet oxygen. Singlet oxygen attacks the double bonds of fatty acids directly, bypassing the slow induction period of autoxidation and dramatically accelerating the formation of flavor precursors. Myoglobin, the heme protein that gives fish flesh its color, is a particularly efficient photosensitizer. When illuminated, the porphyrin ring of its heme group undergoes conformational changes that convert oxymyoglobin to metmyoglobin, releasing reactive heme iron and reactive oxygen species that lower the activation energy for lipid peroxidation.

To test this synergy, the researchers salted fillets with 9 percent food-grade salt for five days, then supplemented minced samples with either ultrapure water or a 1.6 milligram per milliliter myoglobin solution before drying. They tracked myoglobin concentration and its redox state, measured thiobarbituric-acid-reactive substances (TBARS) as a marker of secondary lipid oxidation, quantified the reactive aldehydes 4-hydroxyhexenal and 4-hydroxynonenal by stable-isotope-dilution gas chromatography-mass spectrometry, and profiled total lipids, triglycerides, phospholipids, and free fatty acids. Volatile compounds were mapped using gas chromatography-ion mobility spectrometry, with each compound’s odor contribution estimated through relative odor activity values.

The results were striking. As light intensity climbed from 0 to 15,000 lux, myoglobin content fell significantly in both groups, from 3.81 to 3.06 milligrams per gram in controls and from 4.60 to 3.68 milligrams per gram in supplemented samples, reflecting light-induced oxidative degradation. Meanwhile, the proportion of oxymyoglobin dropped while metmyoglobin rose, from 23.11 to 26.53 percent in controls and from 24.72 to 28.03 percent in the myoglobin-treated fish. Because metmyoglobin and the heme and ferric iron it releases are potent pro-oxidants, this redox shift primes the muscle for accelerated lipid breakdown. Superoxide radicals generated during the conversion dismutate into hydrogen peroxide, which reacts with metmyoglobin to form a highly reactive complex that aggressively promotes lipid oxidation.

Oxidation markers told a consistent story. TBARS values rose by 29.65 percent in controls and 40.64 percent in myoglobin-supplemented samples across the light gradient, and at 15,000 lux the treated samples reached 2.63 milligrams of malondialdehyde equivalents per kilogram versus 2.23 in controls, the largest gap observed. Two-way analysis of variance confirmed a significant interaction between light and myoglobin, meaning the two factors amplify each other. The aldehydes 4-hydroxyhexenal and 4-hydroxynonenal, derived from omega-3 and omega-6 polyunsaturated fatty acids respectively, climbed by 20.04 and 27.13 percent in the supplemented group under increasing light, while total lipids, triglycerides, and phospholipids all declined more steeply with myoglobin present. Triglycerides proved especially sensitive, falling 35.69 percent in treated samples at the highest light intensity, and free fatty acids rose steadily to 0.47 grams per 100 grams, evidence of extensive lipid cleavage.

Crucially, this oxidation translated into flavor. The researchers identified 42 volatile peaks corresponding to 26 compounds, dominated by aldehydes, alcohols, and ketones, which together exceeded 90 percent of the profile. Total volatile signal intensity increased 2.47-fold in controls and 3.21-fold in myoglobin-supplemented samples as light intensified. Hexanal, octanal, and nonanal, products of unsaturated fatty acid degradation responsible for grassy and fatty aromas, rose sharply, with hexanal in treated samples at 15,000 lux reaching 1.41 times the control level. Branched-chain aldehydes from the Strecker reaction added malt-like and fruity notes, while esters such as ethyl acetate contributed fruity, wine-like scents. Although off-flavor compounds like heptanal and 1-octen-3-ol also increased, their relative odor activity values remained far below those of the desirable aroma compounds, suggesting the overall flavor balance shifted favorably.

Statistical correlation analysis tied the whole mechanism together. Mantel and Pearson tests revealed that metmyoglobin negatively correlated with total lipids, triglycerides, phospholipids, and polyunsaturated fatty acids, while positively correlating with free fatty acids, TBARS, and 4-hydroxynonenal. Volatile compound classes correlated strongly with myoglobin and lipid oxidation markers, confirming that the extent of oxidation directly shapes flavor intensity. The authors conclude that modulating light intensity and myoglobin-mediated photo-oxidation offers a practical strategy for regulating flavor formation, potentially allowing producers to dial in the sun-cured character of dry-cured grass carp with precision lighting, standardizing quality while preserving the taste that centuries of tradition created.

Subject of Research: Effects of light intensity and myoglobin supplementation on lipid oxidation and flavor formation in dry-cured grass carp

Article Title: Impact of light intensity and myoglobin supplementation on lipid oxidation and flavor development in dry-cured grass carp

Article References: Dai, W., Wang, Y., Cui, M., Bao, R., Zhou, X., Ding, Y., & Gu, S. (2026). Impact of light intensity and myoglobin supplementation on lipid oxidation and flavor development in dry-cured grass carp. Journal of Agriculture and Food Research, 31, Article 103319. https://doi.org/10.1016/j.jafr.2026.103319

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103319

Keywords: dry-cured fish, grass carp, myoglobin, lipid oxidation, photo-oxidation, singlet oxygen, volatile flavor compounds, TBARS, GC-IMS, food flavor chemistry, drying technology, aquatic products