The Taxonomy of Cow Farts

The organism in the above picture (from Garrity & Holt 2001) may not look particularly remarkable, but this is a very important microbe. This is a thin section of Methanobrevibacter ruminantium, a member of the group of archaebacteria known as the Methanobacteriales (methanogens). Methanobacteriales get their name because they grow by reacting carbon and hydrogen to produce methane. They are found in a variety of habitats, though all are strict anaerobes (they die if exposed directly to oxygen). As suggested by its name, Methanobrevibacter ruminantium was described from the rumen (part of the digestive system) of cattle; it or closely related strains are also known from sheep, and even from the crop of the hoatzin, the South American bird that is the world's closest thing to a flying cow. These are the organisms that make cattle fart.

Climate change is a major issue in modern society, and a lot of questions have been raised about how to mitigate its effects. The most familiar factor in climate change is the burning of fossil fuels, but something else that has been subjected to scrutiny is the role of livestock. Anyone who has spent much time in the company of cattle will tell you that they belch and fart continuously. Methane emissions from livestock may not produce the same volume of greenhouse gases worldwide as fossil-fuel burning, but they still cause concern because methane is a significantly more potent greenhouse gas (litre for litre) than carbon dioxide. Methane production can also siphon off 10% or more of the energy contained in the livestock's feed, energy that might otherwise go towards putting on weight, making it a concern for the farmer as well as for the environment. As a result, the question has been raised of whether it would somehow be possible to get the world's cattle to effectively put a cork in it.

Unfortunately, it is not as simple as wiping the methanogens from the cattle's system, because they do have a very important role to play. The digestive system of cattle and other ruminants contains a whole ecosystem of micro-organisms that serve to break down the cellulose and other parts of the cattle's food that it can't digest alone. Ciliates in the rumen that break down cellulose excrete hydrogen gas, in the way that we breathe out carbon dioxide as a result of our own respiratory processes. But just as we would not be able to keep functioning if the concentration of carbon dioxide in a room got too high, these cellulose-digesting microbes become sluggish as the concentration of hydrogen in the gut increases. By converting the hydrogen to methane, the methanogens keep the rumen sweet-smelling and fresh (at least by ciliate standards). Without this process of hydrogen removal, the cattle themselves would not survive long.

Comparison of methanogen populations in low-methane (left) and high-methane (right) cattle, from Zhou et al. (2009).

And this is where taxonomy becomes important. I've been talking as if there is only a single methanogen species in cattle. This is not the case. Methanobrevibacter ruminantium is one of at least five Methanobrevibacter species known from cattle rumens, together with a number of methanogen species belonging to other genera (Zhou et al. 2009). And not all these methanogens are equal in the amount of methane they produce. Zhou et al. (2009) compared the methanogen flora in cattle that absorbed a high ratio of nutrients from their food, and produced relatively little methane, with that in cattle that were less efficient digesters and that produced more methane. They found that while members of both groups contained similar total numbers of methanogens, they differed in taxonomic composition. In the high-efficiency cattle, M. ruminantium made up a higher proportion of the gut flora, while other methanogens such as Methanobrevibacter sp. 'AbM4' increased in abundance in low-efficiency cattle.

Now obviously, there's a lot this doesn't tell us. We don't know whether the high-efficiency cattle produce less methane because of the differences in their methanogen flora, or whether the methanogen flora is different because the cattle are more efficient digesters. If the former, then maybe cattle could be inoculated against undesirable methanogens and the less injurious methanogens encouraged. If the latter, then maybe breeding would have to come into it. Or the methanogen flora could differ due to differences in the cellulose-digesting flora, in which case control measures would have to act at that level. Whatever the reasons, the simple fact that there are different species involved is yet another reminder: even if we're talking about climate, taxonomy matters.

REFERENCES

Garrity, G. M., & J. G. Holt. 2001. Phylum AII. Euryarchaeota phy. nov. In: Boone, D. R., R. W. Castenholz & G. M. Garrity (eds) Bergey’s Manual of Systematic Bacteriology, 2nd ed., vol. 1. The Archaea and the Deeply Branching and Phototrophic Bacteria, pp. 211-355. Springer.

Zhou, M., E. Hernandez-Sanabria & L. L. Guan. 2009. Assessment of the microbial ecology of ruminal methanogens in cattle with different feed efficiencies. Applied and Environmental Microbiology 75 (20): 6524-6533.