21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 16: Fungi as pathogens of animals, including man
2. Chapter 17: Whole organism biotechnology
   1. Fungal fermentations in submerged liquid cultures
   2. Culturing fungi
   3. Oxygen demand and supply
   4. Fermenter engineering
   5. Fungal growth in liquid cultures
   6. Fermenter growth kinetics
   7. Growth yield
   8. Stationary phase
   9. Growth as pellets
   10. Beyond the batch culture
   11. Chemostats and turbidostats
   12. Uses of submerged fermentations
   13. Alcoholic fermentations
   14. Citric acid biotechnology
   15. Penicillin and other pharmaceuticals
   16. Enzymes for fabric conditioning and processing, and food processing
   17. Steroids and use of fungi to make chemical transformations
   18. The Quorn fermentation and evolution in fermenters
   19. Production of spores and other inocula
   20. Natural digestive fermentations in herbivores
   21. Solid state fermentations
   22. Digestion of lignocellulosic residues
   23. Bread; the other side of the alcoholic fermentation equation
   24. Cheese and salami manufacture
   25. Soy sauce, tempeh and other food products
   26. Chapter 17 References and further reading
   27. Buy a PDF of Chapter 17
3. Chapter 18: Molecular biotechnology

17.20 Natural digestive fermentations in herbivores

We have already discussed the natural digestive fermentations in herbivores:

• the crucial contribution made by fungi to the herbivore’s ability to digest its food (see The Chytrids in Chapter 3; CLICK HERE) and the metabolism of anaerobic chytrids (CLICK HERE to view the Resources Box on chytrid metabolism).
• Ruminant biology and the microbial community of the ruminant gastrointestinal tract, which was considered in the section Anaerobic fungi and the rise of the ruminants in Chapter 15; CLICK HERE to view the page.

What we want to do here, briefly, is add a quantitative view of rumen fermentation; essentially the ‘engineering’ aspects of ruminant animals which will give you some numbers to excite and interest your friends.

The foregut of ruminants consists of four chambers: rumen, reticulum, omasum and abomasum. The rumen is the largest and, combined with the reticulum, forms a fermentation vessel that has a volume of 100 to 150-litre in cattle and about 10-litre in sheep. This fermenter vessel is loaded with:

• solids in the form of the plant biomass on which the animal feeds and
• fermenter fluid in the form of copious quantities of saliva produced by ruminants: about 150 l day^-1 in cattle and 11 l day^-1 in sheep (that is, about one fermenter volume of saliva day^-1).

Retention times for liquids and small particles, including microorganisms, in the rumen are in the range 10-24 h, but for large particles, like plant fragments it ranges from 2 to 3 days. The rumen is a continuous culture system, but it is not well mixed and because of the heterogeneous nature of the substrate plant material it may become stratified. Indeed, the digestive physiology of all animals is quite complicated; the animals depend for their own digestion on the microbial communities in their gastrointestinal tracts and those microbial communities are strongly influenced by diet (Karasov & Douglas, 2013; Dearing & Kohl, 2017).

The ruminant animal relies on microorganisms to convert the plant biomass into nutrients accessible to the animal. The concentrations of these populations in rumen fluid are in the range 10⁹ to 10¹⁰ ml^-1 bacteria; 10⁵ to 10⁶ ml^-1 protozoa and about 10⁹ ml^-1 thallus-forming units of anaerobic fungi (Trinci et al., 1994).

So an average cow contains about 1.5 × 10¹⁴ anaerobic fungi and an average sheep contains about 10¹¹ anaerobic fungi.

The total number of dairy cows in the world is 2.8 × 10⁸, but in addition there are about 1.5 × 10⁹ beef cattle worldwide (sources: https://dairy.ahdb.org.uk/ and https://www.drovers.com/). In total, therefore, there are approximately 1.8 × 10⁹ cattle worldwide, which amounts to a grand total of 2.7 × 10¹¹ litres of rumen fluid walking around containing 2.7 × 10²³ anaerobic fungi in the world’s cattle herd. Just the cattle herd.

There are about 10⁹ sheep in the world, each one a walking 10-litre fermentation culture. And then there are all those antelopes, bison, buffalo, camels, deer, goats, oxen, wildebeest ... and we’ve not even mentioned the hind-gut fermenting horses, zebra, rhinoceros, elephants, tapirs, sloths, pigs, peccaries, guinea pigs, chinchillas…and rabbits!

There are a lot of chytrids doing a lot of fermentation in the world; that’s why we call it the biggest and most widely distributed submerged liquid fermentation process. You do the maths; and you might like to consider the climatic impact of the so-called ‘greenhouse gas’ methane released as a by-product of fermentation in herbivores:

‘...livestock methane emissions, while not the dominant overall source of global methane emissions, may be a major contributor to the observed annual emissions increases over the 2000s to 2010s…’ (Wolf et al., 2017).

Updated July, 2019