Role of Plant Secondary Metabolites in Enteric Methane Mitigation

Neelam Kumari¹ and Rukkiya Siddiqui²

¹Assistant Professor at Mahatma Ghandhi Veterinary College Bharatpur, Rajasthan

²Ph.D. Scholar at Indian Veterinary Research Institute Izatnagar Bareilly U.P.

A symbiotic relationship exists between ruminants and rumen microorganisms. Microorganisms utilize lignocellulosic feed to produce organic acids and synthesize microbial protein as energy and protein sources for the host animal and in turn, the animal offers nutrients and appropriate environmental conditions for microbial habitation in the rumen. During the whole process of feed fermentation in the rumen, CO₂ and H₂ gases are produced as the end by-products and rumen methanogenic archaea use H₂ to convert CO₂ into methane which is eructed out through mouth in the environment. Methanogenesis occurs both in the rumen and the hindgut of ruminants, but the majority of the CH₄ originates from the rumen, nearly 90% of the total CH₄ production (Kumar et al., 2014). Ruminants are responsible for 16 – 25% of world greenhouse gas emissions and 33% of global anthropogenic methane emissions. Enteric methane emissions, primarily from rumen feed fermentation, account for around 30% of total anthropogenic methane emissions which is equivalent to 5.9×10⁹ metric tonnes CO₂ (Steinfeld et al., 2006).

What is the need of reduction of methane emission?  

1. Methanogenesis metabolic pathway wastes 2-12% of the ingested energy and lowers the potential conversion of dietary energy into metabolizable energy, thus reducing animals’ feed utilization efficiency (Lozano et al., 2017).
2. Apart from wasting dietary energy, methane traps atmospheric heat 23 times more effectively than CO₂ and is classified as a greenhouse gas (GHG) that contributes to global warming and has detrimental effects on the global ecosystem (Bodas et al., 2012).

Plant secondary metabolites/Phytogenics

Phytogenics are plant secondary metabolites (PSM), comprised of diverse groups of chemical substances produced by plants but not engaged in their core metabolic activities of growth and reproduction. Bioactive compounds of PSM have antibacterial properties and are intended to defend the host plant from invading particles such as harmful microorganisms (Kamra et al., 2012). There are about 200,000 PSM structures that have been recognized (Patra and Saxena, 2010). The majority of PSMs are divided into three major categories: saponins, tannins and essential oils (EOs).

Mechanism of action

Saponins

Saponins are high molecular weight glycosides, with a sugar linked to a hydrophobic aglycone. They usually occur as glycosides of steroids or as polycyclic triterpenes. Saponins can be generally classified as steroidal and triterpenoid. It is generally considered that their main biological effect of saponin is on cell membranes. Saponins have been described to be toxic to protozoa (Patra and Saxena, 2009) and it has been suggested that methanogenic archaea are symbiotically associated to rumen protozoa. Saponins have the capacity to form complexes with the lipid membrane of bacteria, which increases their permeability, generating an imbalance and consequently lysis of the microorganism, most of the saponins influence protozoa (Makkar et al., 1995). Wallace et al. (2002) proposed that saponins may disrupt protozoa by forming complexes with sterols in the protozoal membrane surface which then becomes impaired and disintegrate. In addition, some saponins have influence on different types of membrane proteins such as Ca²⁺ channels and Na⁺-K⁺ ATPases (Chen et al., 2009).

Essential oil

EOs are blend of secondary metabolites produced from the plant volatile fraction by solvent extraction and steam distillation and have antimicrobial properties (Rira et al., 2015).

Several theories have been proposed to explain how EO works as an antibacterial agent. The antimicrobial effect of EO is not due to a single mechanism of action, but rather to multiple targets within the bacterial cell due to the presence of the high number of bioactive components in EO (Burt, 2004).

Most EO exhibits their antimicrobial activity by interfering with bacterial cell membrane functions such as electron transport, protein translocation, ion gradients, phosphorylation and other enzyme-dependent events (Rira et al., 2015). Terpenoids and phenylpropanoids show antibacterial effects by interacting with cellular components (Dorman and Deans, 2000). Due to this interaction membrane structure undergoes conformational changes, resulting in fluidification and expansion (Griffin et al., 1999). Further, the loss of membrane stability causes ion leakage across the cell membrane, cause a decrease in the transmembrane ionic gradient. In most circumstances, bacteria can counteract these effects by using ionic pumps so a significant amount of energy is spent on this function that slows bacterial development (Ultee et al., 1999).

Tannin

Tannins are water-soluble phenolic compounds (M. wt. 500-3,000 D) and contain many hydroxyl and other functional groups (1-2 per 100 D). They can establish cross-links with proteins and other macromolecules reducing methane production (Bate-Smith, 1972).

The mechanism by which condensed tannin (CT) affects methanogenesis in ruminants and lowers CH₄ production is not well understood. The mechanisms of action of tannins on enteric CH₄ mitigation can be explained by number of different hypotheses (Naumann et al., 2017). One of them proposes that condensed tannins have a direct influence on rumen methanogenic ache by binding proteinaceous adhesion or portions of the cell envelope, preventing the formation of the methanogen protozoa complex, limiting interspecies hydrogen transfer and inhibiting the growth of methanogen (Tavendale et al., 2005). Another explanation is the indirect inhibition of rumen microorganisms by reducing the nutrient availability (i.e., amino acids and carbohydrates). At rumen pH, the tannin-protein complex is formed and protein from this complex is released at abomasum pH (Mueller‐Harvey et al., 2006). The complex reduces the digestibility of feed results in reduction in the rumen microbial population. The last theory suggests that condensed tannins serve as hydrogen sinks, reducing the amount of hydrogen available for the conversion of carbon dioxide to methane (Naumann et al., 2017).