The Role of CO₂ in Ruminants’ Adaptations to Fermentation
Most organisms on Earth have evolved to live in environments with relatively low carbon dioxide (CO₂) levels (Cummins et al., 2020). Ruminants, however, have developed a specialized foregut for microbial fermentation and have consequently adapted to environments with much higher CO₂ concentrations (Turner & Hodgetts, 1955; Laporte-Uribe, 2023).
Several physiological and metabolic adaptations enable ruminants to tolerate elevated CO₂ concentrations in the rumen:
Adult hemoglobin: Ruminant hemoglobin exhibits a lower affinity for oxygen than that of many other mammals (Takano & Nishikura, 1976). This facilitates efficient oxygen release to tissues even under conditions of high CO₂ (Ferguson & Roughton, 1934; Bunn, 1980).
Low levels of 2,3-diphosphoglycerate (DPG) in red blood cells: DPG is an allosteric regulator that enhances oxygen release from hemoglobin. Ruminants have comparatively lower DPG levels, which further supports oxygen delivery in CO₂-rich environments (Benesch et al., 1969; Gustin et al., 1997).
The chloride shift: Active exchange of bicarbonate (HCO₃⁻) and chloride ions across the red blood cell membrane helps maintain blood acid–base balance, facilitating CO₂ transport and excretion (Westen & Prange, 2003; Kurbel, 2011).
Ruminal CO₂ recycling: A substantial portion of ruminal CO₂ is directly reused for nutrient absorption and recycling, reducing the burden of CO₂ transport to the lungs for exhalation (Whitelaw et al., 1972; Veenhuizen et al., 1988; Rackwitz & Gäbel, 2018).
Increased saliva secretion: Under certain dietary conditions, ruminants increase saliva production. Saliva provides buffering capacity via bicarbonate, which helps neutralize excess acid and returns HCO₃⁻ to the rumen (McDougall, 1948; Beauchemin et al., 2008; Ricci et al., 2021).
Urea formation, recycling, and disposal: Ruminants possess highly efficient systems for forming urea from ammonia (NH₃) and CO₂, recycling it through the rumen, and excreting it as needed. This process mitigates CO₂ accumulation in the body (Thorlacius et al., 1971; Taylor & Curthoys, 2004; Abdoun et al., 2010; McCoard & Pacheco, 2023).
Cellular cholesterol deposition: The rumen epithelium contains the highest intracellular cholesterol concentrations in the gastrointestinal tract (Arias-Hidalgo et al., 2018; Jiang & Loor, 2023). Elevated membrane cholesterol decreases CO₂ permeability across epithelial cells, helping to stabilize acid–base balance.
The glyoxylate shunt in the liver: The glyoxylate pathway bypasses CO₂-generating steps of the tricarboxylic acid (TCA) cycle by producing succinate and malate directly from acetyl-CoA. While considered a non-functional gene in most mammals (Kondrashov et al., 2006), its presence in ruminants is linked to conditions such as ketosis, acidosis, and negative energy balance (Soares et al., 2021). In ruminal bacteria, the glyoxylate pathway enables succinate and propionate production even under high CO₂ levels. Thus, expression of this pathway in ruminant liver during metabolic stress may indicate a direct connection between CO₂ holdup and metabolic disorders.
The Significance of Ruminant Adaptations to High CO₂ Levels
Ruminants have evolved remarkable physiological and metabolic adaptations that allow them to tolerate the high CO₂ concentrations generated by their specialized forestomach fermentation. These adaptations enable them to thrive in conditions that would otherwise be detrimental to health and productivity.
One example is the deposition of cholesterol in the rumen epithelium, which reduces CO₂ transport across cell membranes and helps maintain a stable acid–base balance. This adaptation is particularly important during the transition period, when ruminants shift to diets richer in fermentable carbohydrates (Loor et al., 2006; Steele et al., 2011).
Monitoring ruminal dCO₂ concentrations during this transition phase provides a valuable tool for ensuring proper adaptation to the altered physicochemical environment created by high-production diets. By proactively detecting and managing CO₂ holdup, producers can help maintain optimal dCO₂ levels and reduce the risk of metabolic disorders such as subacute ruminal acidosis (SARA), ketosis, abomasal dilatation, and other syndromes linked to fermentable diets.
In this way, sustaining adequate ruminal dCO₂ concentrations during dietary transitions is essential for protecting animal health, enhancing productivity, and preventing diseases associated with CO₂ holdup.
