Abstract
Copious amounts of methane, a major constituent of greenhouse gases currently driving climate change, are emitted by livestock, and efficient methods that curb such emissions are urgently needed to reduce global warming. When fed to cows, the red seaweed Asparagopsis taxiformis (AT) can reduce enteric methane emissions by up to 80%, but the achieved results can vary widely. Livestock produce methane as a byproduct of methanogenesis, which occurs during the breakdown of feed by microbes in the rumen. The ruminant microbiome is a diverse ecosystem comprising bacteria, protozoa, fungi, and archaea, and methanogenic archaea work synergistically with bacteria to produce methane. Here, we find that an effective reduction in methane emission by high-dose AT (0.5% dry matter intake) was associated with a reduction in methanol-utilizing Methanosphaera within the rumen, suggesting that they may play a greater role in methane formation than previously thought. However, a later spike in Methanosphaera suggested an acquired resistance, possibly via the reductive dehalogenation of bromoform. While we found that AT inhibition of methanogenesis indirectly impacted ruminal bacteria and fermentation pathways due to an increase in spared H2, we also found that an increase in butyrate synthesis was due to a direct effect of AT on butyrate-producing bacteria such as Butyrivibrio, Moryella, and Eubacterium. Together, our findings provide several novel insights into the impact of AT on both methane emissions and the microbiome, thereby elucidating additional pathways that may need to be targeted to maintain its inhibitory effects while preserving microbiome health and animal productivity.
Generated Summary
This research article investigates the mechanistic basis of methane inhibition by Asparagopsis taxiformis (AT) in dairy cattle, focusing on the rumen microbiome and its response to AT supplementation. The study employs a multi-faceted approach, combining 16S rRNA sequencing, real-time PCR, and shotgun metagenomics analyses to identify methanogenic species and pathways. The research aims to elucidate the impact of AT on methane emissions, rumen fermentation, and the potential for developing strategies to maintain its inhibitory effects while preserving microbiome health and animal productivity. The study explores the transient nature of AT’s effects, seeking to understand the mechanisms behind potential resistance and the role of specific microbial populations in methane production and mitigation. The research design involves a replicated 4 x 4 Latin square design with twenty Holstein cows assigned to four different treatments: control, low-dose AT (LAT), high-dose AT (HAT), and oregano. The data were analyzed to determine the changes in methanogenic archaeal communities, the impact of AT on methanogenesis pathways, and the effects of AT on bacterial populations and fermentation pathways, contributing to a comprehensive understanding of AT’s role in mitigating methane emissions.
Key Findings & Statistics
- Methane Reduction: The red seaweed Asparagopsis taxiformis (AT) can reduce enteric methane emissions by up to 80% in cows.
- Variable Results: The effectiveness of AT in reducing methane emissions can vary widely.
- HAT Effects: High-dose AT (0.5% dry matter intake) reduced methane emissions by 55% in periods 1 and 2, but the effect declined in periods 3 and 4.
- Methanol-Utilizing Methanosphaera: Effective methane reduction by AT was associated with near-total elimination of methanol-utilizing Methanosphaera in the rumen.
- Methanosphaera Rebound: Methanosphaera populations rebounded over time, potentially due to bromoform inactivation.
- MCR Inhibition: The gene copy number for the enzyme EC: 2.8.4.1, involved in methane production, was significantly reduced in HAT compared to control in periods 1 and 2 (61% and 65% reduction), but increased by 19% in period 3.
- MCR Subunits: In period 1, all three subunits of MCR were inhibited by AT by approximately 60%.
- Hydrogenase Activity: AT resulted in a sevenfold increase in gaseous H2 concentrations compared to the control.
- Butyrate Increase: Butyrate molar proportions increased in HAT compared to control across all periods.
- Acetate Reduction: The molar proportion of acetate was reduced in HAT compared to control across all periods.
- VFA Changes: Concentrations of total VFA were consistently lower in HAT compared to all other treatments.
- Enzyme Reduction: The gene copy number for the enzyme methanol-corrinoid protein co-methyl-transferase (EC: 2.1.1.90) was significantly reduced in HAT compared to control in period 1.
- CO2-reducing pathway: In periods 1 and 2, the copy number of the gene encoding EC: 1.2.7.12 was significantly lower in AT- and oregano-supplemented cows compared to the control cows (P < 0.05 for both periods).
- Bromoform Concentrations: Methanobrevibacter ruminantium M1 reached its maximum CH4-emitting potential within 12 h with bromoform, but increasing bromoform to 130 and 260 µg/mL resulted in 16% and 34% inhibition, respectively, at 12 h.
- Methane Increase: Methanosphaera stadtmanae methane emissions were inhibited with bromoform, but increasing dose from 7% to 22% at 12 h.
Other Important Findings
- The study revealed that AT inhibition of methanogenesis indirectly impacted ruminal bacteria and fermentation pathways due to an increase in spared H2.
- An increase in butyrate synthesis was found due to a direct effect of AT on butyrate-producing bacteria (Butyrivibrio, Moryella, and Eubacterium).
- In the methanol-utilizing pathway, the gene copy number of all three enzymes exhibited a significant reduction (P < 0.05) in HAT compared to control in period 1.
- The most abundant methanogenic species were Methanobrevibacter and Methanosphaera.
- Methanobrevibacter species were more sensitive to the MCR inhibitor 3-NOP than methylotrophic methanogens, resulting in a 30% reduction in total CH4 emissions.
- The study found that Methanosphaera may have a greater share in total CH4 formation than previously thought.
- The study determined that the increase in butyrate does not depend on increased acetyl CoA in the context of inhibited methanogenesis but instead occurs via the direct stimulation of certain bacterial populations such as Clostridia.
Limitations Noted in the Document
- The effects of AT appear unstable, and the achieved results can vary widely.
- The study noted that the measurement of dissolved H2 correlates with hydrogenases rather than free H2 gas concentrations.
- The study was performed with 20 animals including first and 2+ lactation cows, and the cow-to-cow variation was notable.
- The study noted that the effects of AT on CH4 emissions were transient, and the bromoform may lose its functionality over time.
- The measurement of bromoform concentrations in the rumen was not done and requires more frequent sampling to help understand methanogen-bromoform interactions in the rumen.
- The study also noted the difficulty to detect patterns in hydrogenases, making it more difficult to detect patterns.
Conclusion
The study highlights the significant impact of Asparagopsis taxiformis (AT) on mitigating methane emissions in dairy cattle and provides valuable insights into the complex interactions within the rumen microbiome. The findings demonstrate that while AT effectively reduces methane production, its impact is transient, emphasizing the need to understand the mechanisms driving this variability. The research reveals that the initial success of AT in reducing methane is associated with the near-elimination of Methanosphaera, suggesting a critical role for this methanogen in methane formation. However, the subsequent rebound of Methanosphaera populations points to the development of resistance mechanisms, potentially through the dehalogenation of bromoform, the active compound in AT. This resistance underscores the need for further research into the long-term efficacy of AT and the potential for developing strategies to maintain its inhibitory effects. Moreover, the study identifies both direct and indirect effects of AT on the rumen bacteria and fermentation pathways, highlighting the importance of considering the broader impact on the microbiome. The increase in butyrate synthesis, for example, suggests that AT may influence fermentation patterns, which has implications for animal health and productivity. The study underscores the need for a comprehensive assessment of the effects of AT on the rumen microbiome, including potential disruptions in symbiotic relationships and impacts on feed intake and animal productivity. In conclusion, this research provides valuable information for the development of effective and sustainable methane mitigation strategies in livestock, paving the way for future studies that address the complexities of AT-microbiome interactions and the challenges of maintaining its efficacy over time. The study also calls for the need to find different compounds in AT that may further suppress Methanosphaera and methane production.