Role of Rumen Fungi in Fiber Degradation
D. E. AKIN and W. S. BORNEMAN
R. 6. Russell Agricultural Research Center
Agricultural Research Servlce, USDA
Po BOX 5677
Athens, GA 30613
ABSTRACT
INTRODUCTION
Anaerobic fungi inhabit the rumen and
actively degrade plant cell walls. Rumen
fungi produce high levels of cellulases
and hemicellulases and are particularly
proficient in producing xylanases. These
enzymes are regulated by substrate (especially soluble sugars) available to the organisms. Fungi degrade unlignified (Le.,
no histochemical reaction for phenolics)
plant walls totally, indicating that enzymes are able to hydrolyze or solubilize
the entire plant wall. These organisms are
better able to colonize and degrade the
lignin-containing tissues than are bacteria; phenolics are solubilized but not metabolized from the plant wall by fungi.
Anaerobic fungi are unique among rumen
microorganisms in that they penetrate the
cuticle. Residues after incubation with
fungi are physically weaker than those
incubated with whole rumen fluid or with
rumen bacteria, suggesting that fungi
could alter the fibrous residue for easier
mastication by the animal. Data indicate
that cocultures of anaerobic fungi with
methanogenic bacteria stimulate cellulose
degradation; other data suggest that fungi
are inhibited by certain rumen microorganisms. The interaction of rumen fungi
with other organisms in relation to fiber
degradation in the rumen requires additional study. Rumen fungi have the potential to degrade the more recalcitrant
plant walls in forages, but this potential is
not always reached in the rumen.
(Key words: rumen fungi, fiber, degradation)
In the mid 1970s, Orpin (37) reported that
organisms that previously had been identified
as rumen zooflagellates actually were fungal
zoospores. In a series of experiments, Orpin
(37, 38, 39) repoxted on several aspects of the
fungi, including their obligately anaerobic nature, the presence of chitin in the cell walls, and
their colonization of plant fiber. Rumen fungi
have been grown on rumen-simulating media,
indicating that pH (about 6.5 to 6.7), temperature (39’0, and nutrient requirements are met
in media designed for rumen bacteria. The
semi-synthetic basal medium of Caldwell and
Bryant (16), in which yeast extract, tryptone,
hemin, and a mixture of VFA replaced rumen
fluid, has supported a variety of anaerobic fungi. However, absolute requirements for growth
have not been determined for most of the fungi.
Orpi (42), in reviewing nutritional requirements for Neocallimustix patriciarum, ~poxted
requirements of heme, D-biotin, and thiamin or
its precursors; sulfur was required in reduced
form, and the nitrogen requirement was met
with ammonium ions or amino acids.
Bauchop (11, 12), using microscopic techniques, reported that rumen fungi extensively
colonized the lignincontaining tissues of
forages and appeared to be active in fiber
degradation. Bauchop (11) further showed that
fungi were more prevalent in ruminants fed
high fiber diets than in those fed less fibrous
ones. Work in Australia (5) with sheep eating a
fibrous, warm-season grass grown with or without sulfur showed that sulfur stimulated rumen
fungi, which were the most active fiberdegrading organisms in this system. This major role
for fungi has not been observed in other feeding systems and probably was an unusual one
due to the feed and feeding conditions, but
results did indicate the potential for extensive
fiber degradation in vivo by anaerobic fungi.
Because of the unusual nature of the zoospores of some of the first isolates and the
Received September 15, 1989.
Accepted December 28, 1989.
1990 J Dairy Sci 73:3023-3032
3023
3024
AKINAND B O R "
obligately anaerobic nature of all isolates, the
classification of these organisms has required
some modifications by fungal taxonomists. A
new family was created (Neocalljmasticaceae)
by Heath et al. (24) to accommodate these
organisms. To date, the following list the
genera and species of monocentric fungi: Neocallimastix frontalis, Neocallimastix patriciarum, Piromyces (formerly Piromnas) communis, Caecomyces (formely Spheromonas)
communis, and Caecomyces equi. The characterization of these rumen fungi has been reported in several recent reviews (17, 19, 23,
31). Other types of fungi with a polycentric
growth pattern recently have been reported (8,
47). Barr et al. (IO) established the genus Orpinomyces for organisms with polycentric
growth and multiflagellated zoospores. Breton
et al. (15) gave the name Neocallimastix joyonii
to a polycentric isolate with characteristics similar to those described by Barr et al. (10). The
systematics of the anaerobic fungi will require
further refinement as information is collected
on these unusual organisms.
Anaerobic fungi have been found in the gut
of ruminants other than sheep and cattle, including deer, reindeer, and impalas (12). Further, similar fungi are present in the horse
cecum (41) and in the stomachs of marsupials,
including kangaroos and wallabies (12). These
types of anaerobic fungi also have been cultured from the feces of elephants (12). Early
work in New Zealand (11, 12) and in Australia
( 5 ) suggested that the fungi in these locatims
were particularly active as fiber-degraders.
Later work (6) comparing mixed fungal popla-
tions in one site in Australia and the US have
indicated that those in Australia were more
active in degrading fiber in incubations with or
without bacteria.
A common characteristic of rumen fungi
related to ruminant nutrition is their ability to
colonize extensively the lignin-wntaining plant
cell walls of forages. Despite their ability to
colonize lignocellulosic fiber and their apparent
cosmopolitan nature, the role of rumen fungi in
ruminant nutrition is not well understood.
Aspects of fiber degradation by these rumen
fungi are discussed herein.
FIBER-DEGRADING ENZYMES
PRODUCED BY RUMEN FUNGI
production of fiberdegrading enzymes and
utilization of carbohydrates by rumen fungi
have been investigated recently by several researchers using cultures isolated from various
parts of the world, including Australia (20).
Great Britain (29,51,52,55), France (25), New
Zealand (33, 34, 46), and the US (14). These
various cultures are similar in fermentation
products formed (most produce formate, acetate, lactate, ethanol, C02, and H2)from soluble and fibrous substrates, and they produce
high amounts of cellulases and xylanases.
Wood et al. (55) reported that one unit of N.
fronralis endoglucanase (evaluation of cell-free
preparations of fungus and methanogen cocultures) was several times more active in
solubilizing cotton fiber than one unit of endoglucanase from Trichoderma reesei C-30,
which is one of the most active cellulases yet
reported. Rumen fungi are even more active in
TABLE 1. hbximum spaific activities' (U/mg)of culture supematants of nunen fansal isolates grown on bermudagrass?
prmsal isolate4
Enzyme3
MC1
MC2
pc1
pc2
PC3
Exoglucanase
EndOglUCaoaSe
j3-Glucosidase
Xylanase
Xylosidase
.16
2.03
1.32
4.5 1
1.32
.20
2.83
1.68
14.45
1.48
.12
258
1.93
12.97
.43
.12
3.01
2.08
20.16
.8 1
.ll
3.04
1.82
15.48
.70
'Btkimwn activity within 9 d of growth.
2From Bomeman et al. (14).
3~ubstrates
used to ansay e w e s are as follows: avicel, carboxymethylcellu~ose,
pni-lmy~ PD-glucopyranoside,
xylan (larchwood), pnitrophenyl PD-xylopynmoside.
4The following names would apply to the isolates according to Gold et al. (19) ruad Barr et al. (10): MC-1 = Piromyces,
MC-2 = Neocallimas&, PC-2, PC-3 = Orpinomyes spp..; PC-1 = unnamed polycentric fuugus.
J o d of Dairy Science Vol. 73, No. 10, 1990
SYMPOSRTM. RUMEN MICROBIAL ECOLOGY AND NUTRITION
producing xylanase, and Mountfo~tand Asher
(34) reported that N.frontalis xylanase was the
most active of all the endo-acting polysaccharides yet studied from the anaerobic fungi. Enzymes produced by several isolates are shown
in Table 1.
The ability to produce high concentrations
of cellulases and xylanases and to produce enzymes active against “crystalline” cellulose
(highly ordered because of hydrogen bonding)
substantiates the finding that fungi are p e n tially active fiber degraders. Although most of
the studies have been canid out with the
monocentric organisms, Borneman et al. (14)
confirmed the high production of fiber-degrading enzymes from the unnamed polycentric
fungi (Table 1). Studies to date, however, indicate that pectin is one cell wall polysaccharide
that does not appear to be degraded or utilized
by the fungi (20, 29, 46, 51). Although
Caecomyces was not able to utilize purified
cellulose, all three genera of rumen fungi were
found to utilize a range of carbohydrates prevalent in forage fiber (20), indicating their ability
to ferment and produce end products for the
animal or for other microorganisms in the mmen ecosystem.
Most studies on the location of fiber-degrading enzymes produced by rumen fungi indicate
that they are extracellular, being free in the
culture fluid (34, 46); results to the contrary
reported by Lowe et al. (29) perhaps were
influenced by the shorter time of incubation
(Le., 72 h) at which time fungal wall autolysis
was not extensive. Enzyme activity has been
reported to be present in both the zoosporic and
vegetative stages as well as the cell-free culture
fluid (51, 52).
Some of the fiberdegrading enzymes are
constitutive, and there is evidence that some of
these enzymes are regulated by soluble sugars
present (32,33,34). Glucose has been shown to
repress cellulase production (33). Xylanase production was influenced by both soluble sugars
and xylan concentration and was higher with
crude wheat straw hemicellulose compared
with xylan fiom different sources (34). The
forces that regulate enzyme production and fiber degradation are complex in the rumen ecosystem and microniches in which the fungi
participate. Therefore, the potential for enzyme
production or activity may not be reached during in vitro or in vivo degradation of plant
walls by the fungi.
3025
pigpre 1. Scanning CICCmicrograph Of bcrmndalcaf blade ~ u c
withu
romtn
~
fhlid
~
P~
~
U ~trcptomyS uI
cin and pmicillin to illhiit bact& and select for fungi.
several Merent sporaogial types m present with fungi
coloniang the kast digestible tissues such as the vascular
bundle (B); the more digestible tissues like mesophyll (
I
@
are degraded. Bar, 50 pm.
ws
COLONIZATION AND DEGRADATION
OF PLANT CELL WALLS
Ultrastructural studies have identified the
type of plant cell walls that are colonized and
the manner of degradation by the mmen fungi
(5, 8, 11, 12). Zoospores produced from
sporangia swim to acceptable sites of colonization (e.g., rigid, lignocellulosic plant walls),
attach to the substram, encyst, and develop a
rhizoidal or rhizomycelial system that penetrates the plant wall and releases polysaccharidases against structural carbohydrates (12, 36).
Fungi degrade the easily digestible, nonlignified (determined by histochemical methods) tissues such as mesophyll completely if colonization has occurred nearby (Figure 1). These
results indicate that enzymes capable of degrading the total cell wall are produced by fungi.
The physical relationship of rhizoid or hypha to
the m e of degradation suggests that cell walldegrading enzymes are released fiom the fungal
structures and are, therefore, extracellular
(Figure 2). These observations are in agreement
with the previously discussed enzymatic
studies, which indicated that cellulases and
xylanases are released from the fungal walls
(34, 46, 51).
Journal of Dairy Scienct Vol. 73, No. 10, 1990
3026
AKIN AND B O R "
tant in colonization of specific plant walls.
After colonizatian of the refractory lignmllulose, fungal rhizoids or hyphae partially or
totally degrade plant walls. Lignin-containing
tissues that are partially degraded by m e n
bacteria, such as leaf blade sclerenchyma (2).
are extensively and often completely utilized or
solubilized by rumen fungi (5, 8). The breakdown of sclerenchyma cells by fungi is shown
in Figure 2 and compared with degradation by
the whole rumen population (Table 2). Other
types of lignincontaining tissues that are totally resistant to backrial degradation, such as
xylem and sclerenchyma ring tissue in stems
(2), are degraded partially by fungi (Figure 4).
Rumen fungi degraded the lignified sclerenFigure 2. Tnulprmss
' ion electrw micrograph o
f bemm- chyma ring of bermudagrass stems by penetratdagrass leaf blade incubated BS inpisure 1 for 96 h. Wal ing entirely through the cell wall, often transfitaments (p) are nearby sclerenchyma cell walls 0, versing across two adjacent cells and causing
which show almost complete dcgmdation, acept for the
erosion zones across primary and secondary
middle Lamella (arrows), apparently by atracellalar cell
walls and the middle lamella region. In this
"Il-degrading cnymoi. Bar. 2 p.
plant., the pit fields of the cell walls appeared to
be a preferred site of attack With some rumen
fungi, appressorial-like structures that produced
The colonization of lignified tissues by m- a penetration peg, a system similar to that in
men fungi is of interest in ruminant nutrition, plant pathogenic phycomycete fungi, appeared
because these tissues are the ones most limiting to be involved in degrading plant walls (26).
to digestibility (1). Eleccr~nmicroscopic inves- With other plants, such as alfalfa stems, the
tigations have shown that colony development highly lignified and most refractory plant walls
is initiated on the ligninumtaining tissues preferentially [Figure 3; (5)]. Although chemotaxis
has been shown with soluble sugars in these
fungi (43) and chemotaxis is recognized for
other chytrids (17), it is not known if
chemotaxis to cell-wall components is impor-
h * W 2 of
)
tLpsae
remabbg ill?
Sccond-orda
Imcnlam3
Midvein
vein
Whole men fluid
Mixcd rumen fungi
1542.7'
355.8b
120.4'
U.8b
'From Akin and Rigsby (8).
2Area includes the total ofabaxial and adaxial scl-
chyma from three to four leaf blades. Values within
differ (PS.05) US@
the I test.
bocula include whole nrmen fluid without antibiotics
in which bacterial degradation predominated. Streptomycin
and palicitlin WQC added to inhibit bacttria and to select
for fungi.
whmm followed by different le-
Journal of Dairy Scieuce VoL 73, No. 10, 1990
Figure 3. Scadng Clactma miaograph of alfalfa stm
incubatad as in pigare 1 for 72 h. Fangi colonize the
lignitied riug cells (R), w e thc easily digc3tible parellchyma cells (P), which arc not ncnr tbe coloniprtiOn site,
arc not degraded. Bar, 100 p.
SYMPOSIUM: RUMEN MICROBIAL ECOLOGY AND NUTRITION
3027
micrograPh of bcnrmpi4. T d s S i ~ n
dagrasa stem b b a t e d as in Figare 1 for 72 h rite
e~h'cmelyrecalcitrant xylem cell wall (W) is partially
de@ed near fungal fitamnts (€9. Bar, 2 pa
to degradation were not penetrated by fungal
filaments but instead were degraded apparently
by solubilization of the outer regions of the
walls Figure 5; (7)].
In addition to the greater degradation of
lignified tissues by rumen fungi compared to
bacteria, another unique attribute of fungi is
their ability to penetrate the cuticle of grass leaf
blades (8). Invasion of leaf stomata by hyphae
has been observed in some studies (5, 12, 26,
38), and attack at these sites allows the fungi to
have a greater penetration of the leaf substrate
and not be limited to damaged sites. Colonization of the stomata was a prevalent means of
penetration of the leaf in one study incorporating sulfur-fertilized warm seasan grass hay (5)
and has been reported to be particularly evident
in wheat straw leaf (12). Penetration of stem
stomata does not appear to be particularly important (12). Other studies with mixed fungal
populations (8) and pure culture isolates (4)
have shown that rumen fungi appear to be able
to penetrate the cuticle barrier at sites other
than the stomata. The mechanism for this
penetration has not been elucidated, but it appears that physical rupaue of this plant barrier
due to the penetrative nature of the rhizoids or
hyphae is important.
Figare 5. Trensnuis
' ion electron micrographs of alfalfa
h Bar, 2 pm. a Incubated with
men fluid with cycloheximide to inhiit eucarptes and
select for bactaia ptw fik-degrading bacteria(arrow) are
anached to an otherwise intact plant cell well 0.b.
Incubated with streptomycin d pmicillin to inhibit w
ria and select for fungi. Faagal filamnts @) am within the
cell lamn, and the plant wall (arrows) is eroded.
stems incubated for 72
preferentially. The ability to degrade and utilize
lignin anaerob~dlywould be an important attribute for m e n microorganisms. merefore,
studies have been undertalcem to assess the ability of the fungi to attack the lignin component
DEGRADATION OF PHENOLICS
of
plant walls. windham and Akin (54) used
AND LIGNIN
antibiotics to manipulate procaryotic (i.e., bacRumen fungi degrade the lignincontaining terial) and eucaryotic (i.e., fungal and proplant walls and usuaUy colonize such tissues tozoal) groups in an in vitro system and then
loamal of Dairy Scieecc Vol. 73, No. 10. 1990
3028
AKIN
AND BORNEMAN
assessed the digestibility of each group; rumen
protozoa were not active as primary degraders
of plant walls in this study. Results indicated
that lignin as determined by the 72% H2S04
method was not lost in the incubations. Similar
techniques using antibiotics to select for microbial groups were used to evaluate the loss of
lignin in [“‘Cllignin or [‘4C]carbohydrate in
lignocellulose of highly lignified and poorly
digested (in vitro digestibility = 36%) cordgrass
(3). Results indicated that, in an in vitro incubation that stimulated a polycentric fungus, lignin
was not converted to C02 but was lost due to
solubilization. Similarly, radiolabeled young
oat plants were evaluated using 18 pure cultures of classified, monocentric fungi by Gordon and Phillips (20). Their results indicated
that, even though 30 to 38% of the initial
radioactivity was solubilized, negligible C02
was produced. To date all data indicate that
rumen fungal attack on lignocelluloseresults in
solubilization but not metabolism of the phenolic component of lignocellulose.
Rumen fungi have recently been shown to
produce esterases that release pcoumaric and
ferulic acids from their methyl esters and from
intact plant cell walls (Borneman et aL, 1989,
Appl. Microbiol. Biotechnol., in press). It is
possible that these enzymes might play a significant role in degrading plant walls high in
phenolics. Phenolic monomers (but not dimers)
are toxic to rumen fungi (8, 22); therefore, the
influence of esterases and release of high concentrations of pcoumaric and ferulic acids on
digestibility require additional study.
ity of fungi to attack and weaken anatomical
barriers to fiber breakdown (5). Wilson and
Engels (53), in reviewing the results of experiments related to breakdown of particles by
rumen fungi, indicated that mastication and rumination were responsible for most of the reduction in particle size, but they also concluded
that microorganisms could weaken plant structures for easier rumination. In a study (7) to
quantitatively evaluate the potential of mixed
bacterial and mixed fungal populations to
weaken structural barriers in vitro, stems incubated with fungi were significantly (Pe.05)
weaker than those incubated with bacteria (Table 3). In this study, particle size was not
altered by any microbial group, but fungal filaments degraded tissues most refractory to
degradation in both grass and legume stems.
Joblin (27) reported that Caecomyces isolates
were able to ‘‘fibrillate” plant fragments due to
the expansion of a bulbous rhizoid, whereas
Neocallimtix and Piromyces isolates did not
alter the plant’s external integrity in this study.
Therefore, rumen fungi appear to have the ability to alter the characteristics of fibrous residue
thus potentially aiding in mastication and fiber
flow in the digestive tract and subsequently
increasing feed consumption. However, a direct
relationship of fungal degradation of fiber to
feed intake has not been established and requires additional research.
INTERACTION OF RUMEN FUNGI
WITH OTHER MICROORGANISMS
Rumen fungi obviously must interact with a
variety of protozoal and bacterial species in
PHYSICAL DEGRADATION OF
their ecosystem. However, information describFIBER BY FUNGI
ing such interactions is relatively scarce. Early
Abilities to degrade extensively certain lig- work (13) showed a synergistic influence of
nified tissues, to partially degrade and weaken methanogenic rumen bacteria on the cellulolythe more resistant tissues, and to penetrate the tic activity of fungi, increasing the extent of
cuticle barrier in forages are characteristics that digestion by 55% and altering the fermentation
indicate rumen fungi are better able than rumen products towards higher amounts of acetate,
bacteria to degrade the structural barriers in methane, and CO, at the expense of formate
plants. Therefore, fungi might be considered to and H2. Similar results occurred with a fungus
have an ability to modify particle size or other grown in triculture with two methanogens (35).
fiber characteristics(18,50) that relate to intake Recent studies by Stewart and Richardson (49)
or passage of fiber through the intestinal tract. indicated that cocultures of rumen fungi and
In a feeding study with sulfur-fertilized warm- methanogenic bacteria enhanced the resistance
season grass, which stimulated the fungal popu- of fungi to growth-promoting antibiotics (i.e.,
lation, the major result was an increase in feed monensin and lasalocid) for animals. However,
intake considered to be influenced by the abil- other studies have indicated that rumen bacteria
Joumal of Dairy Science Vol. 73, No. 10, I990
3029
SYMPOSIUM. RUMEN MICROBIAL ECOLOGY AND XWTRlTION
TABLE 3. Charactmistics of bermudam and alfalfa stems incubated with ramen microbial groups.1s
Diestion charactaistic~~
Strength of residue (Newton)4
% Dry weight loss
Inoculum
Alfalfa
Bm-s
Alfalfa
mudasress
uninoculated
Whole rumen fluid
Bacteria
19.8.
38.4bc
35 .F
40.Sb
21.1'
47.2b
46.4b
53.8'
114.1'
114.9
118.1"
74.Zb
68.7'
56.7b
49.Zb
29.F
Fungi
lonsCm stem sections incubated 72 h with: total mixtd populated (whole rumen fluid), fluid plus cycloheximide to
c to
~ inhibit fiba-digcsthg
inhibit eucaryotes and select for bacteria (bacteria), or fluid plus streptomycin and @
bacteria and select for eucaryotics (fungi). Note: protozoa wen not active as Primary fik-degrnders as observed by
electron microscopy.
2prom ~ k ethal. (7).
3~a1ueswithin columns with different superscripts differ at PSB.
4Newt0n = Force that gives to a mass of 1 kg an acceleration of 1 d S 2 .
may reduce fiber degradation by rumen fungi.
Colonization of forages by fungi (as determined
by sporangial counts on leaf blades) was substantially greater when antibacterial antibiotics
were included in rumen fluid media (Table 4).
Other unpublished work (48) indicated that solubilization of straw by N. frontalis was markedly reduced when incubated in coculture with
ruminococci. Further, mixed fungal populations
were unable to colonize or degrade grass leaves
in the presence of a growing culture of an
unidentified coccobacillus (9).Although information on fungal-bacterial interactions is
scarce, even less information seems to be available concerning the interaction of rumen protozoa and fungi. Some evidence has been presented by Orpin (42) that removal of ciliates by
defaunation increases the fungal population and
that fungal zoospores may be ingested by rumen protozoa. Certainly the interaction of fungi
with bacteria and protozoa needs to be clarified
if understanding of the role of rumen microorganisms in ruminant nutrition is to be expanded.
INFLUENCE OF DIET ON
FUNGAL POPULATIONS
Early work on the fungi by Bauchop (11)
indicated that diet had a substantial effect on
fungal populations; fibrous diets supported
more fungi than the leafy diets. Orpin (40)
reported that plant compounds stimulated zoospore production resulting in a higher population, while more detailed work (44)indicated
that sporogenesis was induced by heme-type
compounds. In Australia, sulfur-fertilized grass
(5) and methionine-supplemented diets (Gordon
et al., 1983, unpublished data) resulted in increased fungal populations. Perhaps related to
these findings is work by Orpin and Greenwood (45) that N. patridarum required a
reduced source of sulfur, However, an increased fungal population with sulfur has not
been found in other studies in the US (Akin,
1983, unpublished data) or Scotland (30). Increased populations occurred with alfalfa versus
bermudagrass and mature versus immature
wheat forage diets (9). Other research showed
that diets with high amounts of starch or solu-
TABLE 4. Rmnen sporaOgia colonizing the leaf surface of
bumudagrass incubated with rumen fluid with or without
antibiotics.~
~-
Sporangia on surface
@er site, avg SD)*
*
Incubation
+ S d P
-SmdP
@)
a
a
6
24
48
72
96
SD
0
1.2
.3
10.0 1.6
8.2 4.4
4.8 3.4
SD
0
.9
1.6
23
0
1.5
0
'From Akh and Rigsby (8).
2 F i e sites for each of 12 leaf blades for iwo cows
examined. + S and P = inclusion of streptomycin and
penicillin to mmtn fluid.
slot determined.
Jonmal of Dairy Science Vol. 73, No. 10, 1990
3030
AKIN
AND B O R "
ble carbohydrates resulted in decreased fungal
populations; in contrast, fungal populations
were high with fibrous diets such as grass
silage, mature ryegrass, and straw (E. Grenet,
1989, personal communication). Research on
the influence of microbial groups on associative
feed effects showed that fungal populations
were stimulated with corn and soybean hulls,
but this increase did not result in higher microbial digestibilities (Windham, W. R., and D. E.
Akin, 1989, unpublished data). The influence of
diet on rumen fungi might occur because of
direct effects such as supplying a growth factor
(40)or because of indirect effects by influencing competing organisms (9, 42).
IMPORTANCE OF ANAEROBIC
FUNGI IN THE RUMEN
Research from several laboratories has
shown the ability of rumen fungi to attack,
weaken, and partially or fully degrade recalcitrant tissues in in vitro samples and from
digesta removed from the rumen. Amino acid
compositions of a few isolates of rumen fungi
have been evaluated, and initial indications are
that absorption by the animal could be extremely high (21, 28). Despite these positive
attributes of the fungi in regard to fiber digestion and their apparent cosmopolitan nature, a
deffitive role for them in the rumen ecosystem
has not been established. For example, increases in fungal populations or the in vitro
digestion of plant material by the fungi was not
reflected in substantial or consistent increases
in fiber digestion by rumen inwula (54). The
interaction with rumen bacteria and protozoa
requires further study, for the apparent potential
of the fungi (as detemined in vitro) is not
always realized in the rumen ecosystem (6).
The unique attributes of the rumen fungi in
degrading fiber,the high amounts and activities
of cellulases and xylanases, and the potential
for improving the efficiency of forage utilization warrant M e r evaluation of these new
and unusual organisms.
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m and L. L.
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