Aquaculture has witnessed rapid growth in recent years to meet the escalating demand for seafood. However, the aquaculture industry’s utilization of over 70% of the world’s fishmeal, despite aquafeeds comprising only 4% of the total production of industrial feeds (which amounted to approximately 900–1000 million t in 2018), raises concerns about the long-term sustainability and impact on wild fish stocks due to its reliance on conventional feed ingredients including fish oil derived from wild-caught fish [83,84]. Biomass and extracts derived from macroalgae or seaweeds offer a promising alternative or replacement for these ingredients due to their high growth rates, abundance, and diverse chemical composition [85,86]. However, the nutritional composition of macroalgae can be variable, requiring careful selection and processing to optimize the inclusion level in their use in aquafeeds. Scientific investigations have focused on evaluating the impact of incorporating macroalgae or macroalgae extracts into aquafeed on growth performance, feed utilization, and health indicators. Macroalgae-derived feed supplements have positively impacted aquatic organisms’ growth, survival rates, digestibility, and immune responses in aquatic species [87]. The bioactive compounds in macroalgae, such as polysaccharides, polyphenols, and antioxidants, contribute to these beneficial effects by improving nutrient absorption, gut health, and immune function [1,88]. While macroalgae offer nutritional benefits, it is important to consider the presence of antinutrients in their biochemical composition when using them as feed [89]. Antinutrients are naturally occurring compounds that can interfere with nutrient absorption or utilization, potentially affecting animal health and performance [90]. Some common antinutrients in macroalgae include phytates, tannins, oxalates, and lectins [91]. Various processing methods can be employed to mitigate the adverse effects of antinutrients. These include heat treatments (e.g., blanching, steaming), fermentation, enzymatic treatments, and enzymatic liquefaction, which can reduce the levels of antinutrients and improve the overall nutritional value of macroalgae-based feeds [92,93,94]. On the other hand, the utilization of macroalgae extracts presents an alternative option as they are more concentrated, potentially offering higher concentrations of specific bioactive compounds such as antioxidants or immunostimulants [19,95,96]. This approach can effectively counteract the negative impacts of anti-nutrients in the macroalgae, ensuring their harmful effects are minimized.

The inclusion of macroalgae in aquafeeds can enhance the fatty acid profile of fish, increasing the levels of desirable omega-3 fatty acids. Legarda et al. (2021) conducted a study in which they examined the dietary incorporation of Ulva fasciata at three different inclusion levels (5%, 10%, and 20%) in the diets of Seriola dorsalis [97]. The study’s results indicated a significant increase in the levels of docosahexaenoic acid (DHA) in the muscle tissue of the fish. Sultana et al., in 2023, conducted a study to evaluate the effects of the dietary inclusion of the macroalgae Hypnea sp. on Nile Tilapia (Oreochromis niloticus). The study revealed that incorporating Hypnea sp. at a 10% inclusion level in the diets resulted in a significant increase in the levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the muscle tissue of the tilapia. This finding suggests that an appropriate inclusion of Hypnea sp. in aquafeed can serve as a crucial strategy to enhance the quality of meat in aquaculture [98]. Metabolites and bioactive compounds derived from seaweed have been evaluated for their beneficial effects on cultivated fish, shrimps, and oysters, including improved growth performance, enhanced digestibility, immunostimulatory and antioxidant activities, up-regulation of immune-related genes, resistance against viruses and bacteria, and tolerance to thermal and salinity stress [96,99,100,101,102,103,104,105,106,107]. Table 2 provides up-to-date information and references to significant research studies focusing on the effects of seaweeds and their extracts as bioactive ingredients in shrimp feeds. The discussed studies explore dosages and the effects on growth performance, gut health, antioxidant activity, gene regulation, immune responses, pathogen resistance, and stress in various shrimp species.

Poultry is crucial in fulfilling the worldwide need for animal protein and supplying necessary nutrients to an ever-increasing population. However, the poultry industry faces numerous challenges, including the need for sustainable practices, disease management strategies, improved productivity, and the development of alternative feed ingredients [129]. Recent advancements in poultry nutrition have focused on alternative feed ingredients, including plant-based proteins and the incorporation of macroalgae extracts, which are alternative sources that offer potential benefits such as reduced environmental impact, improved animal health, enhanced meat quality, and meat shelf-life indicators [130,131]. Achieving optimal weight gain is a crucial objective for producers. Macroalgae is an alternative feed additive that can improve weight gain and promote sustainability. A study showed that supplementing broilers with 3% Laminaria japonica and cecropin led to better broiler growth performance and increased levels of antibodies against Newcastle disease [132]. Moreover, in the cecum, the growth of Escherichia coli was inhibited, while the growth of Lactobacillus was increased. Adding 2 to 3.5% of Ulva spp. to Boschveld chicken feed increased their weight gain and feed intake but did not affect the nutrient digestibility or feed utilization efficiency, according to a study by Nhlane et al. (2020) [133]. Balasubramanian et al. (2021) studied the addition of Halymenia palmata to broiler diets to enhance meat quality; they evaluated concentrations of 0.05%, 0.10%, 0.15%, and 0.25%. With increasing the inclusion levels of the red seaweed, the results indicated a linear decrease in the water-holding capacity, a crucial meat quality parameter. Additionally, incorporating red seaweed has proven to have a beneficial impact on broiler growth, nutrient absorption, microbial levels in feces, gas emissions from feces, blood composition, and tissue structure analysis [134]. On the other hand, macroalgae present a sustainable approach by offering natural compounds with antimicrobial characteristics, including tannins, which are polyphenolic compounds with antinutritional attributes [92]. Nonetheless, tannins have demonstrated antimicrobial, antioxidant, and anti-inflammatory properties, making them an intriguing prospect as bioactive agents to address the concerns associated with removing antimicrobials in the poultry sector [135]. Furthermore, according to Kulshreshtha et al. (2020), red algae-derived polysaccharides demonstrate antimicrobial properties due to their affinity for bacterial surface appendages, affecting Salmonella enteritidis virulence factors [136]. Dietary supplementation with macroalgae has been observed to increase the population of beneficial probiotic bacteria while reducing harmful enteric bacteria in poultry, as well as improving egg production and quality by reducing lipid and cholesterol levels [137,138,139].

Ruminant cattle production is essential for meeting the global protein demand (both meat and dairy products), but conventional feeding methods heavily rely on resource-intensive feed production, leading to environmental degradation [140]; additionally, ruminant livestock contributes 14.5% of greenhouse gas emissions [11,141,142]. Cattle release methane into the atmosphere by exhaling primarily through their mouths and nostrils. Methane is produced during enteric fermentation in the foregut of ruminants, where 95% is expelled through belching and the remaining 5% through the hindgut, expelled through the anus. It is important to highlight that the production of enteric methane represents a loss of dietary energy and constitutes an inefficiency in livestock feeding. Therefore, it is crucial to consider livestock feed ingredients to reduce methanogenesis, improve nutrition, and mitigate CH4 emissions [143]. Macroalgae may be a suitable alternative feed source due to their nutritional value; studies show that including macroalgae in ruminant diets improves animal performance and reduces methane emissions [144]. Sofyan et al. (2022) found varied effects of brown, green, and red macroalgae on methane production in ruminants [145] (Figure 2).

Notably, Ascophyllum nodosum and Asparagopsis taxiformis demonstrated significant potential for methane (CH₄) reduction, although their effects on dairy cows and small ruminants were minimal. The study suggests that incorporating macroalgae into animal feed can effectively mitigate CH₄ emissions without compromising animal performance [145]. Nevertheless, caution should be exercised regarding the elevated levels of bromoform and iodine residues in milk when utilizing high levels of A. taxiformis. Macroalgae contain bioactive compounds that can suppress methanogenesis and improve livestock health. A. nodosum promotes weight gain and enhances meat quality while reducing saturated fatty acids [126,148]. Including A. taxiformis and Asparagopsis armata in cattle and sheep feed can reduce methane production by up to 98% [146]. Roque et al. (2019) found that Holstein cows consuming A. armata showed a significant decrease in CH₄ production of 26% at 0.5% inclusion and 67% at 1% inclusion, improving feed utilization efficiency [32]. Feed consumption was reduced by 10.8% and 38.0% for the respective inclusion levels, and CH₄ yield was reduced by 20.3% and 42.7%. Also, Kinley et al. (2020) examined A. taxiformis as a feed component for the primary reduction of enteric CH₄ in cattle. Feeding cows with Asparagopsis spp. reduces CH₄ emissions by up to 98% and improves weight gain by up to 53%, with no adverse effects on feed consumption or meat quality [149]. In addition to the controversial efforts to reduce emissions by meat consumption, dairy products come from the same ruminal cattle.

Limited scientific evidence currently exists regarding the supplementation of ruminant diets with macroalgae and its impact on milk yield and composition. However, several studies have provided valuable insights on this topic. Studies have shown that feeding Lithothamnion calcareum [150] to dairy cows and supplementing lactating ewes’ diets with a combination of A. nodosum and flaxseed resulted in a higher milk production [151] and improved oxidative stability of fats. Additionally, cows supplemented with A. nodosum showed increased levels of δ-tocopherol in their milk [152]. A dietary supplement of pineapple oil, garlic, and brown algae in Holstein cows also demonstrated antioxidant activity, reduced COX-2 expression, and increased milk production; additionally, cows in the supplement group displayed a higher milk production and a tendency to engage in increased rumination when experiencing heat stress compared to the control group. Despite the potential benefits, challenges must be addressed to successfully integrate macroalgae into ruminant diets [104]. These include identifying suitable macroalgae species, optimizing the processing techniques, assessing the long-term effects on animal health and productivity, and the economic viability [12,153].

Pork production plays a vital role in meeting the global demand for protein [154]. However, pig farming poses challenges related to environmental sustainability and animal health [155]. Since the middle of the last century, studies have been reported using macroalgae as a sustainable dietary supplement for pigs. However, a significant increase in interest in this area has been found since the year 2000. In addition to their role as nutritional supplements, macroalgae are known to be rich in beneficial bioactive compounds for pigs [156]. They offer a potential solution to reduce dependency on antimicrobials and antibiotics while serving as prebiotics to prevent gastrointestinal diseases, especially in weaned piglets [157,158]. All three types of macroalgae have been extensively studied as feed and nutraceuticals in pigs due to their observed ability to enhance the immune system, improve the oxidative stability of meat, and promote intestinal health [159,160,161]. Polysaccharides found in red algae, such as laminarin, fucoidan, ulvans, and carrageenan, are responsible for this. Among these, brown algae supplementation in pigs is particularly preferred as laminarin demonstrates anti-inflammatory properties, thereby mitigating the proinflammatory cytokine response [162,163]. Moreover, brown algae supplements may boost the immune system by stimulating immunoglobulin production and modulating cytokine production [28].

Macroalgae cultivation and commercialization are essential in fisheries and aquaculture economic sectors. Despite the potential benefits of macroalgae-based foods as sustainable and nutritious components in animal feed, a lack of scientific literature has hindered their widespread adoption by domesticated animals. Macroalgae can provide essential nutrients to domestic animals, including chelated micro-minerals that are more readily absorbed than inorganic minerals [164]. Complex carbohydrates with prebiotic properties, pigments, and polyunsaturated fatty acids found in macroalgae can also positively impact consumer health [165]. Therefore, this section provides a comprehensive scientific overview of recent studies conducted within the past five years. These studies investigate the potential advantages of incorporating macroalgae-based feeds to promote the health and growth of various domestic animal species, such as cats, dogs, and rabbits.

Despite the widespread use of Ascophyllum nodosum, a brown alga (Phaeophyceae) from the Fucaceae family, in animal products, there is a lack of studies assessing its impact on pet feed palatability. Isidori et al., in 2019, studied the effect of A. nodosum (0.3 & 1% of inclusion) on the palatability of extruded dog foods using the split plate test, and reported a negative impact [166]. Furthermore, a study by Pinna et al. (2021) examined the effects of supplementation with Ascophyllum nodosum, Undaria pinnatifida, Saccharina japonica (brown seaweed), and Palmaria palmata (red seaweed) in healthy adult dogs [167]. The study administered 15 g/kg per 28 days and evaluated parameters such as the fecal bacterial metabolites, fecal IgA, and tract digestibility of nutrients. However, no significant effects were observed. These findings suggest that further investigation is necessary regarding the formulation and processing of dog feed. While there are limited reports indicating that macroalgae may not be recommended for canine feeding, conducting more comprehensive tests is necessary to make informed decisions. Regarding cat feed, only one study has investigated the effects of dietary supplementation with enzymolysis algae powder (20 g/kg of food). The results showed that the supplemented group displayed an increase in Bacteroidetes, Lachnospiraceae, Prevotellaceae, and Faecalibacterium. This suggests that the addition of macroalgae powder to the diet may enhance gut health and gut microbiota [168]. On the other hand, a study by Abu Hafsa et al. (2021) reported that the inclusion of 1% U. lactuca in rabbit diets positively affected gut health and growth [169]. Additionally, other studies have indicated that including extracts of Laminaria digitate, such as polysaccharides and polyphenols, at concentrations of 0.3–0.6%, can enhance the antioxidant status and metabolism of fat while providing essential minerals and nutrients, thereby promoting animal growth [170,171,172]. Two similar studies employing the same percentage inclusion of macroalgae extracts reported improved growth, reduced cholesterol, enhanced oxidative stability, and an improved sensory quality of the meat [173,174]. Finally, in accordance with sustainable agriculture programs, specifically the European Green Deal Plan, incorporating seaweed in rabbit nutrition has been highlighted in a review paper as a promising approach to improve animal health. The integration of seaweed into rabbit nutrition serves as a nutraceutical, a viable alternative to antibiotics, and it can potentially enhance gut health [175].