Download 02/01/05 1 Cellulose-Degrading Symbioses BI 358 I. Intro: Guts of

02/01/05
Cellulose-Degrading Symbioses
BI 358
I.
Intro: Guts of many animals harbor extensive microflora.
A. Many are transient but some persist more or less permanently
B. Some are pathogenic (e.g. certain strains of E. coli)
C. Many are obligate - we’ll consider the mutualistic ones here
1. herbivorous vertebrates, phytophagous insects and terenid molluscs
(shipworms) lack enzymes needed to degrade plant complex
polysaccharides such as cellulose and pectin
D. Metabolic premise:
1. Gut symbionts are extracellular
2. Hydrolyze the carbos into simple sugars.
3. These are degraded anaerobically by fermentation into volatile fatty acids
(short chain fatty acids), such as acetic and butyric acid and are then
absorbed into the host body wall where they are substrates for aerobic
respiration - fuel.
a) Process analogous to production of EtOH by yeast
4. Symbionts can also contribute to nitrogen needs a) host diet often nitrogen-poor - feeding on mostly polysaccs
b) symbionts use host nitrogenous waste or fix nitrogen into amino acids
and proteins
5. The host provides habitat, a stable environment and some nutrients. The
symbionts do the innovative metabolism
a) Another good example of symbiosis as a source of metabolic
innovation
II. Microbial Communities:
A. These guts are anaerobic communities, miniature ecosystems involving a
consortium of symbionts. Each symbiont type has a unique metabolic role in the
metabolism of the gut. Microbial and metabolic interdependence – key to
understanding these guts
B. Microbial players – these are the possible players:
1. Prokaryotes
a) eubacteria: different taxa for
(1) cellulose-degrading
(2) fermenting
(3) nitrogen metabolism
b) archaebacteria: methane production
2. Eukarya
a) ciliates
(1) cellulose degrading
(2) fermentation
(3) also evidence that they eat bacteria – moving carbon and nitrogen
to different trophic level
b) chytrid fungi: group endemic to guts
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(1) cellulose degrading
3. All are interwoven and dependent on one another metabolically
4. Eukaryotes have not been cultured away from guts
C. Consortia can change depending on host diet and environment – makes these
symbioses very difficult to nail down because they can differ dramatically
depending on diet and location. Really is like studying an ecosystem
III. Vertebrates
A. Virtually all herbivorous verts have gut symbioses to aid in digestion of plant
material
1. foregut symbioses - where enlarged chamber (fermentation chamber) is in
foregut show slide
2. hindgut symbioses where enlarged chamber is in hindgut. These are the in
the majority
B. Foregut symbioses: by far the best studied because this is the ruminant gut –
agronomically and economically important group
1. Can think of these gut ecosystems as one of the most important on earth for
humans – 3.5 billion domesticated ruminants worldwide
a) Important as sources of:
(1) Food
(2) Animals products – leather, wool
(3) Beasts of burden – oxen, camels
2. Evolved independently in groups of verts that feed on plant material
a) 6 separate evolutions Show slide
(1) Marsupials (order) in kangaroo family
(2) Primates (order) in colobus monkeys
(3) Edentates (order) in sloths
(4) Artiodactylas (order)
(a) hippos (family)
(b) camels (family)
(c) ruminants (suborder)
3. Anatomy of gut show slide
a) Rumen large muscular chamber in front of stomach
b) Omasum - another enlarged area preceding stomach
c) Stomach d) Remainder is hindgut - much like other vertebrates
4. Foregut environment show slide
a) Food chewed very thoroughly - increases surface area to vol. Ratio of
food
(1) food sometimes chewed more than once - chewing cud, rumination
(2) food remains in gut for hours - fermentation takes time
b) fluid in rumen, liquor contains microflora which are fermenting the
food - give off VFAs
(1) 50% bacteria by biomass
(2) 40% ciliates
(3) 10% chytrids
c) despite these acids, pH kept at neutrality with lots of buffering saliva
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d) gas, head space, above liquor is mostly CO2 and methane - very little
oxygen - so whole chamber remains anaerobic
5. Biochemistry of rumen:
a) Nutrient exchange.
b) Remember rumen is essentially an ecosystem - so each biochemical
component is performed by a subset of microbial flora
c) Carbon chemsitry
(1) Carbohydrates show slide
(a) Ruminant diet is almost all carbos
(i) over ½ is cellulose or hemicellulose
(ii) 25% is soluble sugars
(2) Symbionts break carbos down to monosaccharides
(3) Ferment these sugars by glycolysis
(4) synthesize VFAs from pyruvate
(a) if symbiont oxidized them all the way to CO2 via aerobic
respiration then the host could not derive any nourishment
(5) VFAs then absorbed and used in metabolism. So almost no
soluble sugars are absorbed
d) Methanogenic bacteria - derive energy from producing methane from
CO2 and hydrogen. Contribute to the anaerobic environment
(1) So all of the domestic animals on the planet belching methane –
contributes to green house gas problem
e) Nitrogen chemistry and recycling - very nitrogen poor diet - so
symbiosis is very good at conserving and keeping as much nitrogen as
possible show slide
(1) Symbionts breakdown nitrogen in proteins to ammonia and then
use it in their amino acid synthesis
(a) Food protein – not much and many non-essential aas
(2) Other set of symbionts metabolizes urea - a host waste product and
turn it into ammonia and then use in aa synthesis
(3) Host absorbs almost no nitrogen until stomach - where it digests
about 30% of symbionts and derives a mixture of amino acids from
them - including essential aas it cannot synthesize
6. Transmission: Juvenile ruminant gut lack symbionts
a) initially gut has symbionts to aid in milk digestion
b) cellulolytic bacteria and protists are obtained from mother licking
calves or from airborne saliva, containing the symbionts
C. Hindgut symbioses: Similar function - both for carbo breakdown and nitrogen
conservation - and similar flora to the foregut ones
1. Animals from many groups engage in this symbiosis. Show slide
2. But symbiosis can vary dramatically in importance
a) humans derive <2% of their nutrition from the hindgut symbionts
whereas horses derive 25%
3. Coprophagy: particularly important in some rodents and rabbits
a) Since the symbioses are after the stomach – symbionts cannot be
digested easily
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b) Functions in nutrient recycling. Pellets from the symbiotic caecum are
different in appearance - are reingested and passed back into caecum
for additional digestion
c) Also a method of transmission between parent and offspring
IV. Phytophagous Insects
A. Wide variety of insects also harbor gut symbionts - all kinds found fore-, mid-,
and hindgut have various functions.
1. Best described are termites and cockroaches.
B. Termites
1. 2000 sp of termites in order Isoptera.
2. Social insects with castes - division of labor
3. 6 families
a) 5 families of lower termites all have hindgut symbioses involving both
bacteria and protists
b) 1 family of higher cultivate fungi and have intrinsic cellulases don’t
have hindgut assoc
(1) this family comprises 75% of all species
4. Many species feed on wood
C. Termite gut symbiosis
1. Hindgut paunch
2. Very analogous to rumens - anaerobic environment
a) Carbon dynamics similar – termites assimilate up to 90% of carbon
brought in
3. Microbial Players similar to ruminants but some key differences
a) several orders of protozoans - all of which have only been found in
termite guts - high degree of endemism.
(1) Now known that ciliates are primary players in carbon metabolism
– they possess cellulases and fermentation pathways
(a) results in the production of VFAs and assimilation by the host
b) Bacteria seem to be there in other capacities roles - in nitrogen
economy and for methanogenesis - no good evidence that they
breakdown cellulose
(1) bacteria - some of the same species as in vert foregut and hindgut
associations
c) no fungi
D. Nitrogen dynamics - some similarities and some differences with ruminants
1. Wood is very poor in nitrogen.
2. Symbionts very good at recycling and conserving nitrogen
3. Evidence of nitrogen fixation in termite guts.
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