Submitted to: American Scientist
Publication Type: Other
Publication Acceptance Date: December 5, 2005
Publication Date: March 1, 2006
Citation: Hall, M., Weimer, P.J. 2006. Ruminating on films. American Scientist. 94:100. Technical Abstract: We applaud Harrison et al. (Biofilms, November-December) for highlighting the wide distribution and importance of biofilms in nature, but inaccuracies in the article incorrectly make the ruminal process in cattle appear overly similar to sewage sludge digestion. The article states or gives the impression that protein-rich microbial mass from the fermentation is the primary source of nutrients to the animal and that methanogens play a major role in ruminal fermentation of organic acids. Although microbial mass is a source of nutrients, organic acids (predominantly acetate, propionate, butyrate) produced from carbohydrates are quantitatively the major source of nutrients that cattle derive from ruminal fermentation. These acids are absorbed into the blood stream from the rumen and can provide roughly two-thirds of an animal's energy requirement. If organic acids were extensively fermented by ruminal methanogens, energy derived from the ration would be limited and the cow would likely starve. Ruminal methanogens readily convert hydrogen gas and carbon dioxide (also produced by carbohydrate fermentation) to methane , but, ruminal methanogenesis from organic acids is negligible because the short digesta retention times (typically 12 to 48 hours; cf. sewage sludge digesters at 2 to 3 weeks) preclude establishment of the slower growing, acetate-fermenting methanogens and their propionate- and butyrate fermenting symbionts. That biofilms are an important in microbial ecology is not in question. However, the functioning of ruminal microbes portrayed in the article would likely have led to the demise of the ruminant digestion strategy and the central role of ruminants in animal agriculture.