By Katie Doonan, Center for Sustaining Agriculture and Natural Resources, Washington State University
Okay, okay- while baby kangaroos singlehandedly solving climate change is out of the question, the potential for baby kangaroo droppings to help decrease methane emissions is an exciting prospect!
Methane from ruminant digestion is a significant contributor to greenhouse gas emissions in agriculture- we’ve all heard the concern over cow burps, otherwise known as enteric emissions. The relative potency of greenhouse gases is assessed through both their warming potential and with their longevity in the atmosphere. Methane persists in the atmosphere for roughly 12 years while carbon dioxide warms for centuries. While methane’s longevity is significantly shorter than carbon dioxide, methane is about 20-30 times more potent of a greenhouse gas in terms of warming potential (IPCC 2007). Reducing methane emissions, whether from enteric digestion or other sources, provides a significant reduction in warming over a more immediate timeframe than carbon dioxide. It has thus been a priority for policymakers, researchers, and agriculturalists in the effort to mitigate climate change impacts. Proposed solutions for reducing enteric emissions from ruminants range from dietary changes to altering the rumen microbiota through vaccines, though each of these possible solutions is still in development.
Ruminants are a critical component of the food system, as they take in fiber-rich biomass (that is, stuff like grass that we can’t digest!) and convert it into an available source of protein for humans. In the process of ruminant digestion, feed is broken down through anaerobic (oxygen-free) fermentation. Cellulose, the major component of grassy feedstuffs, has thick cell walls that are difficult to break apart in most digestive processes. Ruminants recruit specialized bacteria called methanogens to help break down the cellulose into usable sugars and take up hydrogen generated in the fermentation process. Hydrogen uptake is key, as this balances the entire fermentation process within the rumen. Methane is generated as the methanogenesis and hydrogen byproduct and is subsequently released back into the atmosphere when the ruminants burp to release the pressure in their rumen.
So, what does this have to do with baby kangaroos? Kangaroos, like other marsupials, are also able to convert cellulose into usable dietary components. In marsupials, anaerobic digestion ends at the acetogenesis pathway rather than entering methanogenesis. Acetogens, the microorganisms responsible for acetogenesis, channel hydrogen to form acetic acid as the end product of marsupial digestion (see Figure 1). Instead of releasing as a greenhouse gas like methane, research shows that the acetic acid generated can actually be utilized by the animal for increased energy and growth promotion.
Young ruminants (such as cows) actually utilize acetogens as the primary microorganisms involved in their early digestion. Methanogens eventually take over as primary digesters when ruminants mature, though acetogens still reside in their digestive system. By investigating the potential for utilizing the baby kangaroo’s alternative digestion pathway, researchers aim to understand how a portion of methanogenesis could be converted to acetogenesis by increasing the presence of (bioaugmenting) acetogens. Converting methanogenesis to acetogenesis has the potential to reduce enteric methane emissions without harm to the animal, and research aims to establish under which conditions this conversion can occur.
Through work supported by WSU and WSDA Applied Bioenergy Research Program (Appendix A), WSU Researchers Supriya Karekar and Dr. Birgitte Ahring explored how two options of acetogen bacterial cultures, Acetobacterium woodii and those cultured from baby kangaroo droppings, interact with methanogens in a simulated rumen environment (called a rumen bioreactor). Both of the cultures and control were assessed for the effects on hydrogen uptake and methane production, and the relative efficiencies of each. The study resulted in three primary findings:
- The acetogens from the baby kangaroo dropping culture metabolized H2 in the rumen bioreactors in a similar way to methanogenesis.
- Without the addition of a methanogen inhibitor, both A. woodii and baby kangaroo dropping acetogen cultures were outcompeted by methanogens over time. Baby kangaroo dropping cultures were able to suppress methanogenesis in favor of acetogenesis for seven days, at which point methanogenesis returned as the primary metabolic pathway.
- With the addition of a methanogen inhibitor, both A. woodii and baby kangaroo dropping acetogen cultures effectively replaced methanogenesis with the process of acetogenesis over the twelve-day to one-month experimental period.
While these findings require further study before being implemented into an industry-scale solution, the prospect of alternative pathways for ruminants to acquire energy from cellulosic foods without producing methane provides hope for efforts pursuing methane emissions reduction. Even beyond the methane reduction, this alternative pathway from methanogenesis to acetogenesis could potentially increase the animal’s rate of growth and feed utilization efficiency without causing harm to the animal through the acetic acid generation and uptake. If effective, I would call that a win-win situation.
Be on the lookout for more information as further rumen metabolism research develops. In the meantime, take a minute to appreciate the innovation of the kangaroo foregut!
Girard, M., H., J., Belzile, M., Godbout, S., & Pelletier, F. (2013). Biodegradation in Animal Manure Management. InTech. doi: 10.5772/56151.
Karekar, S.C., and B.K. Ahring (2023). Reducing methane production from rumen cultures by bioaugmentation with homoacetogenic bacteria. Biocatalysis and Agricultural Biotechnology 47, 102526. Online Access
IPPC (2007). Climate Change 2007: The Physical Science Basis. Contribution of working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon S., Qin D., Manning M., Chen Z., Marquis M., Averyt K.B., Tignor M., Miller H.L (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp.
This article is also posted on the WSU CSANR Perspectives in Sustainability blog.