Marine algal polysaccharides have emerged as a promising research avenue because of their abundance and sustainability. Polysaccharides, such as alginate, laminaran, and fucoidan, which are extracted from marine seaweeds, have demonstrated the potential to reduce methane emissions by influencing the microbial populations in the rumen.
1. Introduction
Global warming is a major environmental challenge that poses serious threats to the well-being of our planet and its inhabitants. This warming is mainly driven by an increase in atmospheric greenhouse gas concentrations, which trap heat in the Earth’s atmosphere and lead to rising temperatures. Methane, a potent contributor to global warming, has warming potential approximately 28 times greater than that of CO
2 over a 100-year timescale
[1]. Methane emissions result from both natural and human activities, including organic matter decomposition in wetlands, rice cultivation, landfilling, and the digestive processes of ruminants such as cows, sheep, and goats
[2]. According to the Intergovernmental Panel on Climate Change, approximately 14.5% of global greenhouse gas emissions can be attributed to the agricultural sector, and enteric fermentation (i.e., methane emissions from livestock) is responsible for approximately 40% of these emissions
[3]. As the global demand for meat and dairy products is expected to increase in the coming years, the problem of methane emissions from ruminants will intensify, making it imperative to reduce these emissions for environmental and economic reasons
[4]. A range of strategies have been proposed, including dietary modifications, genetic selection, and improved herd management practices.
Methane emissions from ruminants result from enteric fermentation
[5], which occurs in the digestive systems of animals, specifically in the rumen, a vast fermentation chamber that harbours a large number of microorganisms
[6]. These microorganisms play a pivotal role in enabling ruminants to digest complex plant materials such as cellulose and hemicellulose, which cannot be broken down by monogastric animals such as pigs
[7]. During enteric fermentation, the microorganisms in the rumen decompose feed materials and generate methane as a byproduct. Methane is released into the atmosphere via belching, contributing to increases in the concentrations of greenhouse gases and exacerbating warming global temperatures (
Figure 1)
[8]. Apart from the environmental impact, reducing methane emissions is crucial for the economic sustainability of animal agriculture, as numerous countries have established targets for the reduction of greenhouse gas emissions to meet international agreements and address climate change
[9]. Additionally, reducing methane emissions from ruminants may lead to substantial improvements in animal health and productivity. Methane production results in energy depletion in animals, as it represents lost potential energy that could be used for growth or milk production
[10]. By reducing methane emissions, more energy can be made available for production purposes, which may result in increased efficiency and profitability in animal agriculture.
Figure 1. Pathway of methane emission in livestock. Methanogens use various organic compounds, including carbon dioxide and hydrogen, to produce methane gas as a byproduct, illustrating potential role of feed additives such as MAPs in mitigating methane emissions.
Macroalgae, popularly known as seaweed, are large multicellular algae primarily found in marine habitats. By contrast, microalgae are single-celled algae that inhabit both marine and freshwater environments
[11]. Both macroalgae and microalgae contain various intricate polysaccharides, such as laminaran
[12], fucoidan
[13], alginate
[14], carrageenan
[15], and porphyran
[16], which exhibit several biological properties, including immune modulation and antioxidant, antiviral, prebiotic, and antimicrobial effects
[17]. Marine algal polysaccharides (MAPs) mitigate the risk of inflammatory disorders in ruminants, improve food digestion, and augment nutrient absorption, thereby decreasing the probability of pathogen proliferation in the digestive system and enhancing overall animal health
[18][19].
As shown in Table 1, some feed additives have been used to reduce methane production. As feed supplements, MAPs are generally deemed ecologically sound and sustainable as they are sourced from renewable sources and do not pose the same hazards or create the same concerns as other feed additives, such as antibiotics. These compounds hold promise in modifying the microbial populations in the rumen and/or inhibiting specific enzymatic pathways involved in methane production, so they are plausible candidates for the reduction of methane emissions from ruminants.
Table 1. Estimates of methane reduction through the use of feed additives.
Feed Additive |
Animal/In Vitro |
Treatment |
Methane Reduction (%) |
Reference |
3-Nitrooxypropanol |
Cattle |
10 mg/kg dry matter |
39 |
[20] |
Corn oil, wheat starch, marine algae |
Dairy cows and goats |
1.5% inclusion |
28 |
[21] |
Asparagopsis armata |
Cows |
1% inclusion level |
47.2 |
[22] |
origanum oil, hydrolysable tannins, and tea saponin |
Sheep |
40 mL/kg origanum oil |
30 |
[23] |
Grape marc |
Dairy cows |
5.0 kg dry matter of grape marc and 10.0 kg dry matter of ryegrass |
15 |
[24] |
Nannochloropsis oceanica (polysaccharide) |
In vitro |
2.5% incubation |
10 |
[25] |
Macrocystis pyrifera (polysaccharide) |
In vitro |
0.25 g of each diet |
47.3 |
[26] |
Fucus vesiculosus (polyphenol and polysaccharides) |
In vitro |
inclusion rate of 20% in dry matter |
62.6 |
[27] |
Laminaria japonica |
In vitro |
inclusion rate of 20% in dry matter |
18.3 |
[28] |
Sunflower and marine oils |
In vitro |
2.0% inclusion |
16 |
[29] |
Ulva sp. (ulvan) |
In vitro |
25% incubation |
55 |
[30] |
Zonaria farlowii (high starch and protein) |
In vitro |
5% inclusion |
11 |
[31] |
2. Effect of MAPs and VFAs in Rumen Fermentation and Methane Production
In the field of ruminant nutrition, an essential component playing a pivotal role in rumen health and overall productivity is VFAs. VFAs are the main end products of microbial fermentation in the rumens of ruminants. They are primarily produced via the anaerobic breakdown of carbohydrates by ruminal microorganisms. These acids include C2 to C6 carboxylic acids, including acetic, propionic, butyric, isobutyric, valeric, isovaleric, and caproic acids
[32][33]. These VFAs act as energy sources for host animals and are essential for rumen fermentation and digestion. Fibre digestion in ruminants is facilitated by the interplay between VFAs and the rumen microbial population
[34]. The microorganisms involved in VFA production are summarised in
Table 2.
Table 2. Microorganisms involved in the volatile fatty acid process.
VFAs are vital components of ruminal ecosystems. A balance among VFAs is essential for optimal ruminal function and animal performance. The generation of VFAs in the rumen is closely linked to methane production
[43]. The proper allocation of VFAs for different physiological processes ensures efficient energy use and supports growth, reproduction, milk production, and methane emissions
[44]. Different factors, such as the diet composition, rumen microbial population, rumen pH, and management practices, can influence the production and composition of VFAs and methane emissions
[45]. Understanding these factors allows nutritionists and producers to formulate diets and management strategies that promote the production of VFAs that are favourable for rumen health and animal productivity.
Methanogens use a diverse range of substrates during methane production, including formate, hydrogen, methanol, butanol, 2-propanol, 2-butanol, propanol, dimethyl sulphide, dimethylamine, and trimethylamine
[46]. Anaerobic digesters predominantly harbour hydrogenotrophic methanogens, indicating that hydrogen acts as the primary substrate for methane generation
[47]. However, competition for substrates occurs between VFAs and methanogens because higher concentrations of VFAs can compete with methanogens for available hydrogen
[48]. MAPs generally either have no marked effect on or decrease the levels of VFAs in the rumen. For instance, when 4% brown seaweed byproducts were incorporated as feed additives, only minimal alterations in volatile fatty acid concentrations were observed after 24 h
[49]. The inclusion of
Sargassum horneri did not influence the overall production of VFAs, whereas the 4% incorporation of
Ulva sp. resulted in a decline in total rumen VFA production. Notably, both marine algal species led to a reduction in the methane content in the rumen
[50]. When
Laminaria ochroleuca,
Gigartina sp., and
Gracilaria vermiculophylla were combined with corn silage, a negative effect on total VFA production was evident, which was accompanied by a decline in methane production
[30].
The effects of marine algae on the content and composition of VFAs vary depending on the substrate used. The breakdown of intricate polysaccharides derived from marine algae and the subsequent generation of fatty acids rely on the involvement of diverse bacterial species
[51]. Notably, the phylum Bacteroidetes is recognised for its wide range of carbohydrate-active enzymes (CAZymes), which are responsible for this process. CAZymes can be categorised as glycoside hydrolases (GHs), polysaccharide lyases (PLs), and carbohydrate esterases (CEs) based on their distinct enzymatic catalytic mechanisms. These CAZymes specifically target bonds within MAPs, leading to their cleavage into smaller sugar units that can then undergo further metabolism. For instance, the CAZyme-encoding genes associated with fucoidan degradation and the breakdown of fucoidan linkages are found in the families CE4, GH29, GH107, S1_17, and S1_25
[13][52]. Kalyani et al. discovered mrbExg5, an enzyme that demonstrates exo-β-1,3-glucanase activity toward β-1,3-linked glucooligosaccharides and laminaran. This glycoside bears a structural resemblance to a member of the GH5_44 family, which is prominently present in
Pseudobutyrivibrio sp. ACV-2 is an isolate derived from the rumen of cows
[53]. The presence of abundant VFAs reduces the accessibility of hydrogen to methanogens, consequently hindering their activity and ultimately leading to a decline in methane production.
In vitro fermentation and in vivo studies have demonstrated the multiple beneficial effects of VFAs on host health. For instance, the use of porphyran and its partially acid-hydrolysed derivatives derived from
Porphyra haitanensis can increase the concentrations of acetate, propionate, isobutyrate, butyrate, isovalerate, and valerate in the rumen
[54][55]. Acetate is a key VFA generated during the breakdown of fibrous materials in the rumen. Their primary function is to provide substantial energy to ruminants, thereby fulfilling their energy needs. Acetate is an easily accessible energy substrate for animals that facilitates the maintenance of essential physiological processes
[56]. However, acetate, as a substrate for methanogenesis, contributes to elevated methane emissions
[57]. Propionates are vital VFAs that play pivotal roles in ruminant nutrition and energy metabolism. They serve as key precursors for gluconeogenesis, a fundamental process in glucose synthesis
[58]. Glucose is essential for various metabolic functions, including milk production in dairy cows
[59]. Higher propionate levels are associated with enhanced milk yields, emphasising their importance as VFAs in lactating animals. Additionally, propionate production is negatively correlated with methane emissions because it competes with methanogens for available hydrogen
[60]. Choi et al. examined the effects of incorporating dried
Sargassum fusiforme on ruminal fermentation in vitro. The experiment involved testing four different doses of
Sargassum fusiforme (1%, 3%, 5%, and 10% of the total ratio). The findings indicated that supplementation with
Sargassum fusiforme resulted in elevated propionate production with a simultaneous reduction in methane production
[61]. Therefore, increasing propionate production relative to acetate production has the potential to mitigate methane emissions in ruminants. Although butyrate is produced in smaller quantities than acetate and propionate, its importance remains: it acts as a valuable energy source for the rumen epithelium and contributes to rumen health and integrity
[62]. Furthermore, butyrate actively participates in microbial protein synthesis, facilitating the production of essential amino acids that are necessary for the overall growth and development of animals
[63].
VFAs also provide the benefit of lowering the fermentation pH to below 6.0, thereby inhibiting the proliferation of methanogenic microorganisms. The inhibitory growth of the ruminal methanogen
Methanobrevibacter ruminantium was demonstrated when suspended with lauric acid and myristic acid in low-pH conditions (approximately pH 5–6). The results showed that the decline in methane formation may have been related to the decreased survival of
Methanobrevibacter ruminantium via increased ATP efflux, potassium leakage, and an increasing degree of protonation
[64]. VFAs may have direct or indirect toxic effects on protozoa-associated methanogens, which are the microorganisms responsible for methane production. Macroalgae and their secondary metabolites are effective in reducing methane production based on in vitro results. For example,
Asparagopsis taxiformis reduces methane production by suppressing methanogenesis
[65].
In summary, understanding the interplay between MAPs, VFAs, rumen fermentation, and methane emissions is crucial in developing sustainable strategies to reduce the environmental footprint of ruminant livestock. By manipulating the production and use of VFAs through dietary interventions, people can potentially reduce methane emissions without compromising animal health or productivity. Future research should focus on further elucidating the mechanisms by which VFAs influence methane production and exploring novel dietary strategies and additives that can optimise VFA profiles and mitigate methane emissions.
This entry is adapted from the peer-reviewed paper 10.3390/polym15153153