Fungal gardens, the rumen and the rectum

The complex stomach of ruminants is well known. It is designed by natural selection to provide a habitat for decomposer micro-organisms that possess the metabolic machinery to process the high cellulose food eaten by the ruminant. Ruminants cannot do this for themselves. The nourishment obtained by the ruminant comes principally from the digestion of the micro-organisms and their metabolites rather than form the grass itself.

The external rumen (Swift et al.1979), works in exactly the same way but the decomposers are active outside the animal’s body. This concept has been central to much of my work on the larval stages of aquatic insects (see McLachlan and Ladle 2009 for review). The principal source of food for these animals, just like for ruminants, is composed largely of cellulose in the form of dead organic matter (detritus).

Changes in the micro-organisms associated with detritus (peat) in a bog lake. (a) Peat from moor land before entering the lake. (B) Peat in suspension in lake water following erosion by wave action. Arrows indicate probable bacilli. (C) A faecal pellet of Chironomus lugubris composed of peat particles. (D) Close-up of pellet showing fungal hyphae on the surface. (E) Bacteria associated with a pellet disintegrated by the feeding activities of Chydorus sphaericus. (F) close-up of bacilli in (E). Scale lines A-E, 10μm. F, 1μm. From McLachlan, et al. (1979).

The point I wish to make here follows my reading the Conway Morris’s book (2003). This book has led me to realise that the concept of the external rumen has another quite different application. I refer to the fungal gardens of several species of ants and termites.These highly eusocial insects plant and tend the fungal gardens in adaptive convergence on human agriculture. The gardens are supplied with indigestible detritus by these insects but the fungus which grows on it is nutritious and appears to be the sole food of the ants. The parallel to the external rumen of aquatic detritivores is striking. 

Humans too are composed of a multitude of genomes (Blaser 2011). Indeed, whole genome sequencing techniques have lead to the realisation that the human genome includes the genes of micro-organisms dwelling largely in the rectum (Lupp,C. et al. 2012. Relman 2012). Is ther any connection here to the biblical quote...“…what is your name and he answered “Legion” for many demons had entered him (Luke 8:3). 

References

Blaser, M. (2011). "Stop the killing of beneficial bacteria." Nature 476: 393, 394.

Conway Morris, S. (2003). Life’s Solution: Inevitable Humans in a Lonely Universe. Cambridge University Press, Cambridge, UK.

Lupp, C., Skipper, M., Weiss, U. (2012). Gut microbes and health. Nature 489: 219.

McLachlan, A. J., Pearce, L. J. and Smith, J. A. (1989. Feeding interactions and cycling of peat in a bog lake. Journal of Animal Ecology 48, 850-861.  (1979).

McLachlan, A. J. and Ladle, R. J. (2009). The evolutionary ecology of detritus feeding in the larvae of freshwater Diptera. Biological Reviews, 84, 133-141.

Relman, D. A. (2012). Learning about who we are. Nature, 486, 194.

Swift, M. J., M. J. , Heal, O. W. and Anderson, J. M. (1979). Decomposition in Terrestrial Ecosystems. Blackwell, London.