A comprehensive literature search was conducted using major electronic databases, including PubMed, ScienceDirect, and Google Scholar. Relevant studies were identified using combinations of the following keywords: tropical seaweed alginate, alginate oligosaccharides, poultry feed additives, and alternatives to AGPs. Reference lists of selected articles were also screened to identify additional relevant publications.

Peer-reviewed articles published within the last 10 years were considered eligible for inclusion. Studies were selected if they addressed one or more of the following aspects: chemical characteristics of alginate and AOS, extraction and depolymerization methods, physicochemical properties, biological activities, and applications of AOS in poultry nutrition. Only articles published in English and available as full texts were included. Conference abstracts, non-peer-reviewed reports, and studies lacking sufficient methodological detail were excluded.

Data were systematically extracted from eligible studies, including information on seaweed species used, alginate extraction techniques, AOS production methods, molecular and physicochemical properties, and reported biological effects in poultry. Particular emphasis was placed on studies evaluating growth performance, gut health, immune modulation, and nutrient utilization. Where applicable, findings from studies comparing AOS supplementation with conventional AGPs were highlighted to assess their relative efficacy.

The review critically examined the chemical composition of alginate obtained from tropical seaweeds, with specific focus on the mannuronic-to-guluronic acid (M/G) ratio, DP, and their influence on the bioactivity and functional performance of AOS. The prebiotic potential of AOS, including effects on gut microbiota composition and nutrient absorption, was also evaluated based on available experimental evidence.

Poultry performance parameters assessed in this review included average daily gain (ADG), feed conversion ratio (FCR), egg production, immune response indicators, and measures of gut health. Outcomes from AOS-supplemented diets were compared with those reported for AGP-based feeding strategies to determine whether AOS could provide comparable or superior benefits under different production conditions.

The quality of included studies was assessed based on methodological rigor, sample size, experimental design, and consistency of reporting. Meta-analyses and systematic studies were prioritized when available. Studies exhibiting major methodological limitations or insufficient data were excluded to ensure the reliability and scientific robustness of the synthesized evidence.

AOS are derived from alginate, a naturally occurring polysaccharide primarily located in the cell walls of brown seaweeds. AOS represent depolymerized fragments of alginate composed of linear chains containing 2–25 monomer units. These monomers consist of two uronic acids, α-L-guluronic acid (G) and β-D-mannuronic acid (M)—linked by 1,4-glycosidic bonds, forming linear polymers whose arrangement governs their physicochemical and biological properties [15].

Structurally, alginate and its oligosaccharides are characterized by homopolymeric blocks of mannuronate (polyM), guluronate (polyG), or heteropolymeric sequences (polyMG or polyGM) [39]. This diversity arises from biological synthesis in algae or enzymatic depolymerization mediated by alginate lyases, which selectively cleave glycosidic bonds within alginate chains [40]. AOS retain key functional properties of alginate, including high solubility and low viscosity, while lacking the strong gelling capacity typical of high-molecular-weight alginates [15].

Integrating alginate production with aquaculture and poultry systems supports multiple Sustainable Development Goals, including SDG 2 (zero hunger), SDG 12 (responsible consumption), and SDG 14 (life below water) [66–69]. Circular bioeconomy models enable by-products from one sector to support another, enhancing resource efficiency and reducing waste [70, 71]. While tropical seaweeds offer high renewability and carbon sequestration potential, responsible harvesting and farming practices are essential to prevent ecosystem disruption [72–74]. Incorporation of seaweed-derived AOS into poultry diets further enhances sustainability by improving feed efficiency, reducing reliance on synthetic additives, and lowering greenhouse gas emissions [75].

Seasonal variation and geographical location strongly influence the biomass yield and alginate composition of tropical brown seaweeds. Environmental parameters such as temperature, salinity, light intensity, and nutrient availability regulate metabolic activity and polysaccharide biosynthesis in these algae. For example, Turbinaria conoides has been reported to exhibit higher alginate yields during pre-monsoon periods, reflecting favorable growth and metabolic conditions during this season [76]. Such environmental fluctuations result in measurable changes in both total biomass production and the physicochemical characteristics of extracted alginate.

Quantitative data summarizing alginate content, mannuronic-to-guluronic acid (M/G) ratios, and seasonal variability among different seaweed species are presented in Table 1 [15, 16, 77–79]. While temperate species such as Laminaria and Ascophyllum generally exhibit higher alginate content with pronounced seasonal variation, tropical species, including Sargassum, consistently provide moderate yet valuable alginate yields, supporting their suitability as sustainable biomass sources in tropical regions.

In addition to macroalgal sources, microbial production of alginate represents an alternative pathway, particularly for applications requiring high purity and precise compositional control. Bacterial species such as Azotobacter vinelandii and Pseudomonas balinensis, including strains isolated from tropical environments, are capable of synthesizing alginate via regulated biosynthetic pathways [92, 93].

Microbial alginates share structural similarity with macroalgal alginates, consisting of linear chains of β-D-mannuronic and α-L-guluronic acids. However, they often exhibit higher degrees of acetylation and more uniform molecular weight distributions due to tightly controlled fermentation conditions [94]. Factors such as oxygen availability during A. vinelandii cultivation significantly influence polymer length and functional group composition, enabling fine-tuning of alginate properties [94].

Microbial alginate extraction is typically conducted under mild processing conditions, offering advantages in reproducibility, environmental sustainability, and product consistency [95, 96]. Unlike macroalgal extraction, which depends on seasonal biomass availability and may involve harsh chemical treatments, microbial fermentation allows year-round production and precise control over alginate characteristics. Although total yields from microbial sources are generally lower than those from macroalgae, this approach enables the production of alginate tailored for specialized applications, including biomedical uses and precursors for AOS with defined bioactivities.

Targeted enzymatic hydrolysis using alginate lyases, particularly enzymes belonging to the polysaccharide lyase family 7 (PL7), facilitates controlled depolymerization of both macroalgal and microbial alginate into low-degree-of-polymerization oligosaccharides [97]. PL7 lyases exhibit activity against both polyM and polyG blocks, generating AOS with diverse structural profiles. Enzymes such as PsMan8A display substrate specificity toward polyM regions, enabling tailored AOS production [98].

Importantly, enzymatically generated AOS preserve unsaturated uronic acid termini and functional groups associated with enhanced immunomodulatory and prebiotic activities [17]. This mild and environmentally friendly approach supports sustainable AOS production for feed applications [14].

Recent research has focused on developing cold-adapted and engineered alginate lyases capable of operating efficiently at lower temperatures, thereby reducing energy requirements and reliance on high-temperature processing [99]. These innovations improve biomass utilization efficiency and align with biorefinery principles.

Fermentative production of AOS using specific microbial strains has also emerged as a promising alternative. Pathways involving indigenous gut microbiota or selected fermentative organisms have demonstrated potential for optimizing AOS yields, particularly from tropical alginate sources [100]. Such microbial-based synthesis offers a sustainable alternative to chemical depolymerization methods within tropical agricultural systems.

Production pathways for alginate differ markedly between macroalgal extraction and microbial fermentation. Macroalgal extraction from brown seaweeds such as Laminaria typically yields approximately 1% alginate on a dry-weight basis under sustainable harvesting conditions. However, costs associated with collection, processing, and seasonal variability can limit economic feasibility, with raw alginate prices ranging from approximately USD 3–5 per kg [78].

In contrast, microbial production systems using Pseudomonas and Azotobacter species can yield alginate equivalent to 10%–15% of microbial biomass under controlled conditions. Estimated production costs of USD 2–4 per kg reflect advantages of scalability, continuous production, and reduced environmental impact [15].

In summary, tropical macroalgae and microbial systems represent complementary platforms for alginate and AOS production. Macroalgae provide abundant, renewable biomass, while microbial systems offer precision, consistency, and process control. Integration of these approaches may optimize yield, quality, and application diversity, supporting broader adoption of AOS as natural alternatives in poultry nutrition.

Key characteristics of tropical brown seaweeds suitable for alginate production are summarized in Table 3 [12, 28, 29, 44, 45, 53], including morphological traits, geographical distribution, alginate content, M/G ratios, and ecological roles of Sargassum, Turbinaria, Padina, and Hormophysa. The overall workflow for tropical alginate extraction and AOS production is illustrated in Figure 1, depicting sequential steps from seaweed harvesting and drying through alginate extraction, purification, depolymerization, and final product processing.

The successful transition of AOS derived from tropical seaweeds from laboratory research to commercial poultry feed applications requires a clearly defined and integrated development pathway. A structured roadmap is essential to ensure technical feasibility, economic viability, and sustainability.

Alginate extraction from tropical brown seaweeds has gained increasing scientific attention due to its applications across food, pharmaceutical, and agricultural sectors. Conventional extraction methods include acid, alkaline, and calcium-based techniques. Among these, acid extraction has demonstrated high efficiency in Turbinaria murayana, yielding up to 26.93% alginate with favorable physicochemical properties, including moderate viscosity and molecular weight [104]. Variability in alginate yield is influenced by processing techniques; for instance, salt-reduced T. murayana contains approximately 13.51% alginate [105], while fermentation using fruit-based local microorganisms can increase alginate content to 40.93% [106].

Standard alkaline extraction typically involves acid pre-treatment, alkaline solubilization using NaOH or Na₂CO₂ solid–liquid separation, and precipitation [107, 108]. Although widely applied, this approach may lead to polysaccharide degradation and impurity contamination under high temperatures or prolonged processing [109, 110]. Hypochlorite treatment is commonly used to remove pigments and polyphenols [111].

Recent refinements include microwave-assisted extraction (MAE) and ultrasound-assisted extraction (UAE), which enhance alginate yield and purity while reducing processing time. MAE has been shown to improve alginate recovery from Sargassum species [112], whereas UAE accelerates extraction but may compromise polymer integrity if excessive energy is applied [113]. These techniques represent promising alternatives to conventional extraction when properly optimized.

Enzyme-assisted extraction offers a greener and more sustainable alternative to chemical-based methods. Enzymes such as cellulases, proteases, and alginate lyases facilitate cell wall degradation and reduce chemical consumption [114, 115]. This results in alginates with more favorable molecular characteristics and improved purity [116]. Alginate lyases from genera such as Microbulbifer and Pseudoalteromonas selectively cleave 1,4-glycosidic bonds, improving extraction efficiency while preserving molecular integrity [117–120]. These methods reduce chemical usage and maintain structural integrity [121, 122]. Cold-adapted alginate lyases function effectively at ambient temperatures, significantly lowering energy requirements [123, 124]. Combined enzymatic strategies and immobilized enzymes further enhance extraction efficiency and cost-effectiveness [125–127].

Low-energy extraction methods such as MAE, UAE, and enzyme-assisted processes are particularly suitable for decentralized alginate production in tropical coastal communities [128]. Chelating agents like sodium citrate can replace harsh alkalis while maintaining extraction efficiency [129]. Smallholder feasibility has been demonstrated in studies involving local Sargassum and Laminaria species, optimized using response surface methodology [130, 131]. These approaches reduce costs, promote sustainability, and support local biotechnology development [110].

At pilot-scale, alkaline extraction remains a critical step, with extraction parameters such as temperature, solvent concentration, and duration requiring careful optimization to balance yield and resource efficiency [132]. Advanced techniques such as reactive extrusion further enhance extraction performance by improving biomass–solvent interaction [88]. Environmental factors during seaweed growth, including light intensity, wave exposure, nutrient availability, and harvesting age, significantly influence postharvest alginate yield [133].

Economic feasibility remains a key consideration, with biomass cost and supply stability affecting large-scale production [134]. Techno-economic analyses indicate that integrated biorefinery frameworks can improve the financial viability of alginate extraction and AOS production [135].

Extraction conditions strongly influence the molecular characteristics and bioactivity of feed additives. Studies on plant-derived bioactives, such as Peronema canescens, demonstrate that optimized extraction parameters yield compounds that enhance poultry growth performance, gut health, and immune regulation without adverse effects [136]. These molecular traits influence gene expression related to metabolism and disease resistance, highlighting the importance of integrating extraction technology with nutritional and genetic strategies to optimize poultry productivity [137, 138].

The depolymerization of alginate into AOS is a critical step in enhancing its functionality and applicability in poultry feed. Among the available approaches, enzymatic depolymerization using alginate lyases is particularly effective due to its specificity and controllability. Enzymes belonging to the polysaccharide lyase family 7 (PL7) play a dominant role in AOS production and include both endo- and exo-type lyases, which determine the DP of the resulting oligosaccharides [124, 139].

Endo-type PL7 lyases cleave internal glycosidic bonds within alginate chains, producing AOS with a wide range of DP values, typically from disaccharides to decasaccharides. These oligosaccharides often contain unsaturated terminal residues that contribute to enhanced biological activity [140]. In contrast, exo-type lyases act on terminal residues and predominantly generate monosaccharides, as observed for enzymes such as VxAly7D [141, 142]. Bifunctional alginate lyases combine both endo- and exo-cleavage activities, allowing fine-tuning of DP and oligosaccharide profiles [139]. Mechanistically, alginate lyases operate through β-elimination reactions that introduce C4–C5 double bonds into uronic acid residues, thereby influencing enzyme specificity and DP distribution [143, 144].

Physical depolymerization techniques, including microwave irradiation and UAE, offer rapid processing and high extraction efficiency. Microwave treatment of Sargassum species, particularly when combined with mild acidic conditions, has been reported to increase alginate recovery by up to 36% [112]. UAE enhances cell wall disruption and mass transfer, improving extraction efficiency and reducing processing time [128]. However, these physical methods often result in heterogeneous molecular weight distributions and may compromise bioactivity due to excessive polymer degradation if not carefully controlled [145]. Consequently, while physical approaches are efficient, they lack the precision of enzymatic methods in producing AOS with predictable functional properties [146, 147].

Compared with physical and chemical depolymerization, enzymatic methods are inherently more sustainable. They operate under mild reaction conditions, require minimal energy input, and significantly reduce the need for harsh chemicals [148]. The high substrate specificity of alginate lyases minimizes by-product formation and waste generation, aligning enzymatic depolymerization with green chemistry principles [127]. These advantages make enzyme-based AOS production particularly suitable for environmentally responsible feed-grade applications.

Incorporation of AOS into poultry feed presents challenges related to thermal and mechanical stresses encountered during pelleting and extrusion. Elevated temperatures, shear forces, moisture, and pressure can induce hydrolysis or oxidation of AOS, leading to reduced bioactivity and shelf-life [15, 151, 152]. Interactions with other feed components may further affect AOS stability and bioavailability.

Encapsulation technologies, particularly microencapsulation using alginate matrixes, have been developed to preserve AOS integrity during feed processing and storage. Calcium-mediated crosslinking produces thermally stable microcapsules that protect AOS from degradation and enable controlled release in the GIT [153, 154]. Targeted delivery enhances gut health, immune modulation, and feed efficiency [155].

High encapsulation efficiencies have been reported for alginate-based systems. For example, Oninku et al. [156] documented encapsulation efficiencies of 97 ± 2.63% using alginate as a carrier, highlighting its effectiveness in stabilizing bioactive compounds [157].

Recent advances in spray-drying and hybrid biopolymer matrixes have further improved AOS delivery in poultry feeds. Crosslinked alginate systems combined with carrier agents such as maltodextrin and whey protein enhance microencapsulation efficiency and protect co-delivered bioactives, including probiotics, during gastrointestinal transit [158, 159]. Co-spray-drying produces stable microparticles that safeguard AOS against moisture and temperature fluctuations while enabling incorporation of synergistic additives such as enzymes and probiotics [153, 160].

Comprehensive summaries of alginate extraction, depolymerization, and feed-compatibility techniques are presented in Table 4 [15, 109, 110, 112, 114–117, 125, 126, 128, 129, 141, 142, 153, 160], while comparative data on extraction yield, energy consumption, and purity are provided in Table 5 [95, 109, 112, 132, 149, 161].

AOS possess distinct structural characteristics, including DP, mannuronic-to-guluronic acid (M/G) ratio, and limited branching, which collectively govern their physicochemical behavior. These parameters critically influence solubility, viscosity, gelation potential, and bioavailability within the poultry GIT, thereby determining their functional efficacy as feed additives.

Oligosaccharides with a low DP (typically <5) exhibit enhanced intestinal accessibility and bioavailability, facilitating efficient fermentation by beneficial gut microbiota [165]. In poultry, this promotes increased production of short-chain fatty acids (SCFAs), which support intestinal health. In contrast, AOS with higher DP values are less likely to be absorbed intact and are preferentially fermented in distal intestinal segments rather than the small intestine [166].

The physicochemical properties of AOS translate into several key physiological effects in poultry:

Prebiotic activity: AOS selectively stimulate beneficial gut microbiota, serving as fermentable substrates that promote SCFA production, thereby enhancing gut integrity, nutrient absorption, and immune function [33, 180].

From an economic perspective, local production of AOS offers a cost-effective alternative to imported AGPs. Incorporation of seaweed-derived AOS has been associated with improved poultry performance and egg quality, translating into tangible economic benefits for producers [182]. Furthermore, localized seaweed farming can reduce dependence on volatile global feed supply chains [178].

The poultry GIT presents a dynamic pH gradient, ranging from near-neutral conditions in the crop (pH 6–7), highly acidic environments in the proventriculus and gizzard (pH 2–4), and a return to neutral-pH in the intestine (pH 6–7). AOS must retain structural integrity across these conditions to exert their functional effects.

In the crop, AOS remain stable, preserving viscosity and bioactivity [183]. Although acidic conditions in the proventriculus and gizzard pose a risk of degradation, hydrogen bonding within AOS structures confers partial resistance to acid hydrolysis, particularly when encapsulated [184]. Upon entry into the intestine, neutral-pH conditions restore solubility, promoting swelling and release of bioactive components [185, 186]. These transitions enhance targeted delivery and support the proliferation of beneficial bacteria such as Lactobacillus and Bifidobacterium, while suppressing pathogenic populations and optimizing nutrient absorption [183, 185, 186].

Guluronic acid-rich regions within alginate exhibit a strong affinity for divalent cations, particularly Ca²⁺ and Mg²+ forming gel networks through ionic crosslinking described by the “egg-box” model [15]. Calcium ions produce stronger and more stable gels than magnesium due to superior coordination with G-blocks [103]. Calcium alginate gels can destabilize under neutral intestinal pH conditions, enhancing solubility and bioavailability of co-delivered bioactive compounds [187].

Microbial fermentation of AOS in the intestine generates SCFAs such as butyrate, which strengthen gut integrity, modulate immune responses, and improve energy utilization [16, 188]. Gelation also supports mucosal protection by facilitating sustained nutrient release and enhancing bioadhesion to the intestinal lining [189, 190]. Collectively, these ion-mediated properties enable controlled delivery, immune modulation, and microbial homeostasis, positioning AOS as effective and sustainable alternatives to AGPs in poultry production systems [191–193].

A schematic representation of alginate metabolism can be seen in Figure 3. Alginate and its depolymerized derivatives, AOS, exhibit a high degree of resistance to enzymatic digestion throughout the poultry GIT, a property that underpins their role as functional prebiotics. This resistance is primarily attributed to the absence of endogenous alginate-degrading enzymes in poultry, particularly within the crop, gizzard, and small intestine [194]. Consequently, alginate polymers and AOS largely retain their structural integrity during early digestive transit.

Within the crop (pH 5–7) and the highly acidic environment of the gizzard (pH 2–4), alginates are subjected to mechanical agitation and chemical stress. Nevertheless, higher-molecular-weight alginates and structurally stable AOS are largely resistant to acid hydrolysis under these conditions [195, 196]. As digesta progresses into the small intestine, where pH conditions are closer to neutral, partial degradation may occur through microbial activity, particularly for shorter-chain AOS with favorable mannuronic-to-guluronic acid (M/G) ratios that enhance microbial accessibility [125, 197].

In addition, G-block-rich alginates interact with divalent cations such as Ca²⁺ and Mg²⁺, promoting gelation and bioadhesion to the intestinal epithelium. This interaction prolongs residence time within the gut, enabling gradual release of bioactive components and facilitating improved nutrient absorption and immune modulation [190–192]. Collectively, the digestive resistance and controlled intestinal persistence of AOS support their functional efficacy as prebiotic feed additives in poultry.

AOS have demonstrated considerable translational potential across multiple animal production systems. In aquaculture, AOS function as effective prebiotic agents that positively modulate gut microbiota and enhance overall health in aquatic species. Studies have reported improvements in growth performance, feed efficiency, and immune responses in marine shrimp, particularly Penaeus vannamei, following dietary supplementation with algae-derived AOS [225]. Similarly, favorable alterations in intestinal microbial composition and enhanced growth have been observed in Fenneropenaeus indicus [226].

Beyond aquaculture, AOS applications extend to terrestrial livestock species, including swine and ruminants. In pigs, microbial fermentation of AOS leads to increased production of SCFAs, which play key roles in maintaining gastrointestinal integrity and metabolic health [227]. Moreover, AOS supplementation has been associated with modulation of systemic immune responses in livestock, highlighting their dual role in promoting gut health and improving overall productivity [68].

Realizing the full potential of tropical seaweed-derived AOS requires integrated, interdisciplinary research approaches. Collaboration between marine biotechnology, animal nutrition, and environmental engineering can facilitate innovative and sustainable applications of AOS in animal agriculture. Marine biotechnology offers tools to optimize extraction, depolymerization, and scale-up processes, ensuring consistent production of high-quality AOS from tropical seaweeds [15]. Concurrently, advances in animal nutrition can guide the formulation of species-specific AOS supplements designed to target defined gut health and performance outcomes, improving feed palatability and digestibility [228]. Environmental engineering perspectives are essential for evaluating ecosystem impacts, resource efficiency, and life-cycle sustainability, ensuring that AOS integration aligns with environ-mentally responsible farming practices.

AOS have shown substantial functional benefits in poultry nutrition, particularly in improving ADG, FCR, and overall nutrient utilization. Acting as prebiotics, AOS selectively promote beneficial gut microbiota such as Lactobacillus and Bifidobacterium, thereby enhancing gut health, nutrient absorption, and growth performance [229].

Several studies report improvements in FCR following AOS supplementation, reflecting enhanced feed efficiency. However, reported magnitudes of weight gain vary considerably among studies, and consistent increases of 5%–10% compared with controls are not uniformly supported in the literature. Instead, growth responses appear dependent on diet composition, AOS structure, and inclusion level [230, 231]. Meta-analytical assessments suggest that AOS contribute to improved ADG and FCR, although the extent of improvement differs across experimental conditions [232].

AOS supplementation enhances nutrient digestibility by modulating gut microbial activity and supporting intestinal function. Improved protein digestibility is associated with microbial enzyme production, while enhanced energy and fat utilization are linked to SCFA-mediated improvements in gut morphology and lipid metabolism [23, 176]. Mineral absorption, particularly of calcium and magnesium, is also improved, as AOS gel-forming properties facilitate ion solubilization and transport within the intestine [23, 176].

In addition to digestive benefits, AOS exhibit antioxidant properties that mitigate oxidative stress and inflammation in poultry. Increased activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px) has been reported in broilers receiving AOS-supplemented diets, although variability exists across studies [231]. These effects contribute to improved metabolic health and resilience under production stress.

Broilers fed AOS-supplemented diets often show statistically significant increases in ADG relative to control groups. These effects are attributed to improved nutrient absorption, reduced pathogenic load, and enhanced epithelial integrity supported by SCFAs generated during microbial fermentation [23, 176]. AOS have been shown to play a role in modulating gut health, enhancing microbiota balance, and exhibiting prebiotic activity, which collectively contribute to improved nutrient digestibility and utilization [231]. Improved FCR reflects reduced feed intake per unit of weight gain, as sustained SCFA production lowers digestive energy demands and redirects energy toward growth [234].

AOS also positively influence blood biochemical parameters and cecal microbial populations, further supporting nutrient bioavailability and gut health. Osman et al. [235] demonstrated that prebiotic supplementation improved metabolic indices and microbial balance, contributing to enhanced growth performance in broiler chickens.

The effects of AOS on poultry performance are strongly dose-dependent [236]. Higher inclusion levels have generally been associated with greater improvements in gut health and immune function [237]. Continuous supplementation may enhance nutrient digestibility and microbiota stability, contributing to sustained improvements in feed efficiency [238]. However, prolonged use without dietary rotation may result in diminishing returns, underscoring the importance of optimized dosing strategies [239].

Threshold levels refer to the minimum effective concentration required to elicit beneficial biological responses without adverse effects [240]. Several studies suggest that inclusion levels around 1.0 g/kg diet effectively enhance immune parameters and growth performance, providing a practical benchmark for poultry diets [240]. Reported responses vary with dosage: supplementation at 0.05% has been associated with improvements in ADG and FCR; 0.1% enhances immune markers and modestly increases egg production; 0.2% produces more pronounced gains in ADG and FCR; and inclusion levels between 0.5% and 1.0% yield the most consistent improvements in growth and productivity, particularly under environmental or management stress [241].

Despite substantial evidence supporting the growth-promoting effects of AOS, several limitations remain. Long-term studies under commercial production conditions are limited, and few investigations have evaluated stage-specific responses across different growth phases [236]. Variability in diet composition, gut microbiota, and environmental factors also contributes to inconsistent findings regarding optimal inclusion levels and performance outcomes [217]. Future research should focus on long-term field trials, dose optimization, and interactions with other functional feed additives to fully elucidate the role of AOS in sustainable poultry production.

In laying hens, dietary supplementation with AOS has been shown to improve laying rate, egg mass, and overall egg quality. These benefits are largely attributed to enhanced intestinal morphology, including increased villus height and a more favorable gut microbial balance, which together improve nutrient absorption and utilization [242]. Such effects are particularly important during later stages of the laying cycle, when productivity and shell quality typically decline.

Improvements in eggshell quality are consistently reported following AOS supplementation, with increased shell thickness and breaking strength. Enhanced bioavailability and intestinal absorption of minerals, particularly calcium, support more efficient shell mineralization and structural integrity [243, 244]. These attributes contribute directly to reduced egg breakage and improved economic returns for producers.

AOS supplementation has also been associated with enhanced yolk color, an important quality attribute that influences consumer preference. AOS facilitates the intestinal absorption and stabilization of carotenoids such as lutein and zeaxanthin derived from carotenoid-rich feed ingredients. Improved carotenoid uptake results in deeper yellow yolk pigmentation [245–247].

Furthermore, AOS protects carotenoids from oxidative degradation during digestion and storage, allowing greater deposition into the yolk [248, 249]. This antioxidant-mediated stabilization not only improves yolk color but may also enhance the nutritional value of eggs.

AOS have emerged as promising alternatives to AGPs in poultry production due to their multifactorial modes of action that enhance growth performance, immune competence, and gut health. Similar to AGPs, AOS modulate intestinal microbiota, suppress pathogenic bacteria, and support host immunity through both direct and indirect mechanisms. Figure 4 illustrates a comparative overview of the mechanisms of action of AOS relative to other bio-based feed additives, highlighting their influence on gut health, immune modulation, antioxidant activity, and nutrient absorption.

A primary mechanism of AOS is their prebiotic effect, whereby they selectively stimulate the growth of beneficial gut microorganisms such as Lactobacillus and Bifidobacterium [275, 276]. This selective enrichment competitively excludes pathogenic bacteria and enhances the production of SCFAs, creating an intestinal environment unfavorable to pathogens such as Salmonella spp. and Eimeria spp. [218].

In terms of immune modulation, AOS has been shown to stimulate cytokine production, including IL-6, which enhances macrophage activation and T-cell-mediated immune responses [277, 146]. Concurrently, AOS regulates inflammatory processes by downregulating excessive pro-inflammatory cytokine expression, contributing to immune homeostasis [218]. Enhanced mucosal immunity is further supported by increased secretion of immunoglobulin A (sIgA), strengthening the intestinal barrier against pathogen invasion [99, 278].

AOS contributes to intestinal barrier integrity by promoting goblet cell proliferation and reinforcing tight junction protein expression, thereby improving mucus secretion and epithelial resilience [256, 261]. These structural improvements reduce pathogen translocation and systemic infection risk [158].

Beyond indirect microbial modulation, AOS also exhibits direct antimicrobial properties. Notably, AOS interferes with quorum-sensing (QS) signaling pathways by inhibiting acyl homoserine lactones, thereby disrupting bacterial communication and biofilm formation [279, 280]. Since biofilms often confer resistance to antibiotics, their disruption by AOS enhances microbial susceptibility and reduces persistence within the gut ecosystem [281]. Collectively, these mechanisms closely parallel those of AGPs, positioning AOS as functionally comparable feed additives.

Several in vivo studies have directly compared the efficacy of AOS with commonly used AGPs such as bacitracin and virginiamycin. In broiler chickens, AOS supplementation has been associated with significant improvements in ADG and FCR. Chen et al. [282] reported increased ADG in broilers receiving AOS, while Qiu et al. [283] observed FCR improvements comparable to those achieved with virginiamycin supplementation.

Similar to organic acids and essential oils, other recognized AGP alternatives, AOS supports digestive health and growth performance during both starter and finisher phases [284]. These performance gains are attributed to enhanced nutrient absorption and improved microbial balance in the GIT, mediated by the prebiotic effects of AOS [285].

Mechanistically, AOS supplementation promotes increased villus height and crypt depth, expanding the absorptive surface area of the intestine and improving nutrient uptake efficiency [286, 287]. In addition, AOS reduces oxidative stress and intestinal inflammation by modulating antioxidant and oxidative stress markers such as SOD, glutathione peroxidase (GSH-Px), and MDA [288]. These physiological effects mirror those induced by AGPs but without contributing to antimicrobial resistance.

Despite encouraging findings, direct comparisons between AOS and AGPs across studies remain challenging due to methodological variability. Differences in experimental design—including bird age, dietary composition, housing systems, and environmental conditions—substantially influence outcomes [289, 290]. Moreover, inconsistencies in defining and measuring performance indicators such as ADG and FCR complicate cross-study comparisons and meta-analytical assessments [291].

Variations in AOS and AGP dosages, formulations, and feed matrixes further confound interpretations [292]. Importantly, fundamental mechanistic differences exist between AOS and AGPs: AOS primarily function through targeted microbiota modulation and immune enhancement, whereas AGPs exert broad-spectrum antimicrobial effects [293]. These distinctions can lead to different physiological and ecological outcomes within the gut.

Increasing global regulatory scrutiny over AGP use, coupled with rising concerns regarding antimicrobial resistance, has accelerated the transition toward antibiotic-free poultry production systems [294]. In this context, AOS offers a safe and effective alternative capable of improving growth performance and health without promoting resistance development [295, 296].

Comparative performance data summarized in Table 7 [289, 297–303] demonstrate that AOS performs comparably to, or better than, common AGPs such as bacitracin, virginiamycin, and zinc bacitracin in terms of FCR, ADG, and immune modulation. Notably, AOS enhances immune responses while preserving beneficial gut microbiota, an advantage not consistently observed with AGPs.

Bacitracin primarily targets gram-positive bacteria by inhibiting peptidoglycan synthesis and compromising bacterial cell wall integrity [297]. While effective in enhancing growth performance and nutrient absorption, bacitracin use has been associated with shifts in microbial resistance profiles [304]. Virginiamycin operates through a different mechanism, disrupting bacterial protein synthesis via ribosomal interference [305]. However, similar to bacitracin, its use has been linked to increased antimicrobial resistance in gut microbial populations [306].

In contrast, AOS provides broader biological benefits, including antioxidant activity, gut barrier enhancement, and immune modulation, without exerting selective pressure that promotes resistance [307, 308]. As a renewable, marine-derived biopolymer, AOS also aligns with sustainability goals and the global shift toward environmentally responsible feed additives [309, 310]. Nevertheless, challenges remain for AOS and other bio-based adhesives, particularly in achieving mechanical properties comparable to synthetics and maintaining performance under variable environmental conditions, while lignin-based systems continue to struggle with water resistance and formulation consistency [311].

Advances in metagenomics provide powerful tools for deciphering the genetic composition and functional potential of poultry gut microbiota. The chicken cecum harbors a highly diverse microbial ecosystem dominated by members of the phyla Firmicutes and Bacteroidetes, which play central roles in nutrient digestion, immune modulation, and resistance to enteric pathogens such as Campylobacter jejuni [319]. Complementary to this, metabolomics enables the characterization of microbial-derived metabolites, offering insight into functional outcomes of microbial activity. Notably, the fermentation of oligosaccharides leads to the production of SCFAs, which are critical for gut integrity, host energy metabolism, and regulation of inflammatory responses [320].

The integrated application of metagenomics and metabolomics offers a comprehensive framework for understanding how dietary interventions reshape gut microbial communities and metabolic pathways. This multi-omics approach can identify key microbial taxa, functional genes, and metabolic biomarkers associated with beneficial outcomes, thereby enabling the design of precision nutrition strategies to enhance gut health and growth performance in poultry [321]. Recent evidence suggests that AOS supplementation increases SCFA production, which in turn favorably modulates gut microbiota composition and promotes beneficial bacterial populations [322, 323]. Such insights are instrumental in unraveling the mechanisms underlying AOS functionality and optimizing their use in poultry nutrition.

The synergistic integration of AOS with other functional feed additives represents a promising avenue for advancing poultry nutrition. AOS can enhance gut health and nutrient absorption by selectively stimulating beneficial microbial populations, particularly when combined with prebiotics and probiotics such as Lactobacillus spp., which produce organic acids and antimicrobial compounds that inhibit pathogenic bacteria [298].

Similarly, dietary enzymes can complement AOS by improving the digestibility of complex feed components, thereby enhancing nutrient utilization and improving performance indicators such as ADG and FCR [324]. Phytogenic feed additives, recognized for their antioxidant, antimicrobial, and immunomodulatory properties, may further augment the benefits of AOS by supporting microbial balance and host immune function [325]. Collectively, these combinatorial strategies offer a viable pathway for improving broiler performance while reducing reliance on AGPs.

Despite encouraging findings, several critical research gaps must be addressed to support the broader adoption of AOS, particularly those derived from tropical seaweeds. Long-term, large-scale trials evaluating the sustained effects of tropical AOS on poultry health, productivity, and welfare remain limited. This lack of extended evaluations constrains understanding of their long-term efficacy and consistency under commercial production conditions.

In addition, there is a notable absence of standardized extraction and depolymerization protocols for feed-grade AOS. Variability in production methods, including hydrolytic and enzymatic processes, can significantly influence the physicochemical properties and biological efficacy of AOS [29]. Establishing standardized, reproducible processing techniques is essential to ensure consistent quality, safety, and regulatory compliance.

Comprehensive toxicological and residue studies are required to confirm the safety of AOS for human consumption through poultry products. Given their marine origin, AOS may be subject to bioaccumulation of environmental contaminants, necessitating rigorous safety assessments to protect public health [221].

Furthermore, systematic dose–response and bioavailability studies are needed, particularly in tropical poultry breeds. Current literature often extrapolates findings from temperate breeds, overlooking potential differences in metabolic rates, environmental stress tolerance, and nutritional requirements [259]. Targeted research focusing on breed-specific responses would enable more precise dosing strategies and maximize functional benefits.

Finally, comprehensive economic and sustainability analyses comparing AOS with imported AGPs are largely lacking. While AOS present a promising natural alternative, their widespread adoption depends on clear evidence of cost-effectiveness and scalability [326]. Sustainability assessments should extend beyond environmental metrics to include economic feasibility, supply chain resilience, and socio-economic benefits, particularly in regions with abundant tropical seaweed resources.

Addressing these research priorities will be essential for translating the promising biological potential of AOS into practical, economically viable, and sustainable solutions for modern poultry production systems.