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MICROSCOPY RESEARCH AND TECHNIQUE 33:32-46 (1996)
Lignocellulolysis by Ascomycetes (Fungi) of a Saltmarsh
Grass (Smooth Cordgrass)
S.Y. NEWELL, D. PORTER,AND W.L. LINGLE
Marine Institute, University of Georgia, Sapelo Island, Georgia 31327 (S.Y.N.);
Department of Botany, University of Georgia,
Athens, Georgia 30602 (D.P., W.L.L.)
KEY WORDS
Lignin, Lignocellulose, Polyphenolic degradation, Fungi, Transmission electron
microscopy, Spartina alterniflora, Phaeosphaeria spartinicola
ABSTRACT
Lignocellulose (LC) makes up greater than 70% of the mature shoots of the prodigiously photosynthetically productive saltmarsh grass Spartina alternif Zora. Naturally decaying
shoots of this cordgrass were examined by transmission electron microscopy (after high-pressure
freezing and freeze-substitution) as a means of directly detecting lysis of the LC-rich tissues.
Portions of the cordgrass were selected that contained ascomata (sexual reproductive structures) of
only one of each of four species of fungi (Kingdom Fungi; Subdivision Ascomycotina): Phaeosphaeria spartinicola and Buergenerula spartinae from leaf blades, Phaeosphaeria spartinae from
leaf sheaths, and Passeriniella obiones from naked stems. All four of the ascomycetes were LC-lytic.
Phaeosphaeriu spartinicola caused both thinning of LC-rich secondary walls of fiber cells from cell
lumina outwards (type 2 soft rot, akin to white rot) and digestion extending from hyphae within
longitudinal cavities in the secondary walls (type 1 soft rot). The other three species caused either
one or the other type of soft rot. Bacterial erosion of cordgrass cells was found only in the samples
of naked stems. Ascomycetous decomposers of standing-dead grasses may have potential for biotechnological applications involving alterations of lignocellulose or toxic polyphenolic substances.
0 1996 Wiley-Liss, Inc.
INTRODUCTION
Net productivity by the predominant saltmarsh
grass (smooth cordgrass, Spartina alterniflora Loisel.)
in coastal Georgia (USA) marshes is very high. For
example, one estimate of annual production of shoot
dry mass is 2,840 g m-' (Schubauer and Hopkinson,
1984). About 70-75% of the dry mass of mature
cordgrass shoots consists of lignocellulose (LC) (Hodson
et al., 19841, and the lignin component of LC can be a
key controlling factor for rates of decomposition of
plant material (e.g., Dyer et al., 1990; Robinson, 1990;
Taylor et al., 1991). During decay of cordgrass leaves in
the natural, standing position, ascomycetous fungi are
the principal secondary producers based on decomposition of the leaves. Consider the following example findings: a) living-fungal crop in the dead-cordgrass-leaf
system of 106 mg organic mass per gram system organic mass; b) 75% capture of system nitrogen in living-hngal mass; c) standing crop of 1,000 ascomata,
minute (approximately 100 km diam) but visible, dark
brown sexual reproductive structures immersed in leaf
tissue, per square centimeter decaying-leaf abaxial
surface; d) ratio of funga1:microalgal:bacterial mass in
the decaying-leaf system of 600:4:1 (Leuchtmann and
Newell, 1991; Newell, 1993, 1994; Newell and Wasowski, 1995).
Background Experimental Testing of
Saltmarsh-FungalBreakdown of LC
Given that fungi are the principal secondary producers during decay of standing cordgrass and that asco-
0 1996 WILEY-LISS,INC.
mycetes clearly can be capable of destruction of LC and
mineralization of the phenolic components of the lignin
of LC (Haider and Trojanowski, 1980; Nilsson et al.,
1989), the findings of one of us (Benner et al., 1984)
regarding the capabilities of cordgrass ascomycetes
seem counterintuitive. Three species of ascomycetes
could not mineralize (to 14C02)physicochemically preexposed cordgrass LC faster than 0.09% per day (neither specifically labeled polysaccharide nor lignin portions). For comparison, a mixed assemblage of bacterial
species mineralized polysaccharide and lignin components of the cordgrass LC at rates of 0.9 and 0.5% per
day, respectively.
Two subsequent sets of investigators obtained results contrary to the first tests of cordgrass LC-lysis by
ascomycetes. Torzilli and Andrykovitch (1986) measured 42 day laboratory-microcosm (statically incubated, *NH,NO,) losses of LC and its carbohydrates
from living shoots of smooth cordgrass, after oven-drying and grinding, and found that 16-35% of LC was
lost but that lignin losses were not detectable by the
method used. Bergbauer and Newell (1992) repeated
the fungal portion of the 1984 testing (previous paragraph) of fungal capability for cordgrass LC usage,
changing some of the incubation conditions. Three key
~~
~~~
Received January 10,1994; accepted in revised form April 10, 1994.
Address reprint requests to Steve Newell, Marine Institute, University of
Georgia, Sapelo Island, GA 31327.
FUNGAL LYSIS OF LC IN SALTMARSH GRASS
changes were as follows: 1) malt and yeast extract (800
mg - 1-I total) were present along with the cordgrass
LC (1 g * l-'), simulating the presence of labile carbohydrates and proteins that would be present in senescent leaves (in the 1984 work, only autoclaved marsh
sediment was added to the LC); 2) uniformly labeled
(14C02)and lignin-specifically (['*CIcinnamic acid) labeled LC were used rather than lignin- and polysaccharide-specifically ([14CClglucose)labeled LC; 3) the
incubations were performed statically, not with 90 rpm
rotary agitation. Bergbauer and Newell (1992) found
that for the principal fungal decomposer of cordgrass
leaves (Phaeosphaeria spartinicola Leuchtmann [see
Newell and Wasowski, 199511, mineralization of LC
polysaccharide took place a t 0.83% per day, mineralization of LC lignin occurred at 0.07% per day, and
fungal growth efficiency on LC was about 40%(the last
with urea as a nitrogen source and no malt or yeast
extract present).
Potential causal factors for the disparities between
the 1984 and 1992 results for LC lysis by cordgrass
ascomycetes include the following: 1) agitation depressed fungal LC-lysis in the 1984 experiments; 2) the
glucose radiolabel in the 1984 experiments was incorporated into LC by the cordgrass in positions that favored fungal anabolism of the radiolabel; 3) the presence of non-LC carbohydrates in the 1992 experiments
favored lignin alteration and thereby enhanced LC oxidation (see Eriksson et al., 1990; Reid, 1991; Reid and
Deschamps, 1991). In all three of the above experiments, it is possible that nitrogen sufficiency (from
marsh sediment, green leaves, and other added N) suppressed fungal oxidation of lignin (McCarthy et al.,
1984; Reid, 1991), and in the 1984 experiment the
physicochemical preexposure of LC may have made it
available to bacterial enzymes that would not naturally have been physically capable of contacting the LC
(Backa et al., 1993; Flournoy et al., 1993; Robinson,
1990; Srebotnik and Messner, 1990).
Background Electron Microscopy
of Lignocellulolysis
As an alternative to the empirical route for determination of the LC-lytic capabilities of cordgrass ascomycetes, we chose to follow the lead of wood-decay science (Adaskaveg et al., 1991; Blanchette, 1991;
Eriksson et al., 1990) and examine cordgrass-ascomycete LC-lysis in as direct a fashion as possible: by
electron-microscopically examining the interactions
that occur in nature between fungal hyphae and
cordgrass LC. Gessner et al. (1972) recorded the colonization of internodal surfaces of smooth cordgrass by
Buergenerula spartinae Kohlm. & Gessner via scanning electron microscopy. We excavated a bit deeper
(using transmission electron microscopy [TEMI), and
display below the results of hyphal activity by four
common cordgrass ascomycetes, each an inhabitant of
one of three vegetative components of shoots (leaf
blades, leaf sheaths, or stems). We focus most strongly
on decay caused by P. spartinicola, because we know
more about the productivity of this species in the
cordgrass-decomposition system than we do for the
other three species (Newell, 1993; Newell and Wa-
33
Fig. 1. Diagrammatic representation of fiber cells of a grass,
drawn from our TEMs of smooth cordgrass (Spartinu alterniflom).
The upper cell (about 10 km across) is undecayed;the lower two cells
partially drawn are undergoing fungal lysis. Letter labels in this and
all following figures either lie directly upon the structure indicated or
are shown with a bar that ends upon the structure. ML, the middlelamellar layer between cells (the whole area between the s, layers of
adjacent cells includes the primary walls and is termed the compound
middle lamella); CA, a cavity being digested out of compound middlelamellar and secondary-walllayers; CC, the cell-corner portion of the
middle lamella; FCW, fungal cell wall; GR, granular or amorphous
darkened digesta of fiber-cell material; HS, sheath material formed
surrounding hyphae; HY, cross-section of fungal hypha causing LC
lysis; L, the cell lumen; PC,a pit canal joining two adjacent cells; RS,
reticulate structure resulting from lignocellulolysis (LC lysis); S,-S,
the three principal portions of the secondary wall of a fiber cell.
sowski, 1995), and we compare the results for P. spartinicola to those for the other species of ascomycetes.
We directed our examination of fungal activities to
those that occur in the most lignocellulose-rich portions of the shoots, namely the sclerenchymatous fiber
cells of the bundle sheaths or subepidermal structural
layers (Anderson, 1974; Juniper, 1979). Lignocellulose
(LC) is a compact matrix of lignin, hemicellulose, and
cellulose macromolecules (see Fig. 1.1 of Eriksson et
al., 1990). Readers unfamiliar with lignocellulosic
structures may refer to the diagram in Figure 1 for
orientation; boldface labels used in Figure 1 and in our
electron micrographs are given in the text below also
for reference. The highly lignocellulosic fiber (F) struc-
34
S.Y. NEWELL ET AL.
Fig. 3. Ascoma (A at its base) of P. sportinicola forming a small
through the abaxial epidermis of a leafblade
pore at its ostiolar tip (T)
of smooth cordgrass, showing partial, tangential sections of ascospores (AS) within and released onto the leaf exterior. Note that the
aGacent fiber tissue (F) is not as extensively digested as that of Fig.
2. Bar = 30 wm.
Fig. 2. Transverse section through a portion of the adaxial side of
a standing decaying leaf blade of smooth cordgrass, from a leaf segment exclusively occupied by the sexual structures (aseomata; Fig. 3)
ofPhaeosphaeria sportinicola. Hyphae (HY)are present in cell lumina
and within the thick, lignocellulosiesecondary walls (S)of the adaxial
fiber-cell band (F). Note that the formerly chloroplast-containing
and mesophyll chlorenchyma (MI are
outer-bundle-sheath cells (0)
nearly completely digested. Scale bar = 20 km.
tures of grasses have lignin distributed throughout the
thick, multilayered secondary wall (S1 [outermost,
thin], S2[central, thick], and S3[innermost, thin]) (Fig.
1).However, lignin is most concentrated in the middle
lamella (ML), the thin hemicelluloseAignin layer lying
between fiber cells, including the wider cell corners
(CC), and in some of the xylem cells (He and Terashima, 1991). In wood, the middle lamellakell corners
may have three to four times the concentration of lignin found in the secondary walls (Higuchi, 1980), but
in grasses the enrichment may be lower (two times [He
and Terashima, 19911). Regardless of the enrichment
factor, the greater width of the fiber-cell wall than the
middle lamella results in the fiber secondary walls containing most (6040%)of the lignin of LC (Eriksson et
al., 1990; Zhai and Lee, 1989).
Fungal decomposers of LC have traditionally been
categorized as causing white rot (equal loss rates for
LC components [“simultaneous” white rotl, or disproportionately high loss of lignin + hemicellulose [“delignification” white rotl), brown rot (disproportionately
high loss of hemicellulose + cellulose), or soft rot (variable proportionality of loss among the three components of LC) (Blanchette, 1991; Eriksson et al., 1990).
White- and brown-rot fungi situate their hyphae (HY)
primarily within fiber-cell lumina (L) and cause erosion of cell walls (white rot only) or depolymerization of
macromolecules progressing from the lumina toward
the middle lamellae (Fig. 1, bottom right). Soft-rot
fungi often degrade LC primarily by forming cavities
(CAI within the walls of lignocellulosic cells (type 1
decay) (Fig. 1, bottom left), but they may also cause
outward-progressing erosion of secondary fiber-cell
walls adjacent to hyphae situated in cell lumina (type
2 decay) (Nilsson et al., 1989). When only type 2 decay
is present, differentiation from white rot is not clear
(Nilsson et al., 1989). Ascomycetous fungi are generally considered to be agents of soft rot, but some species
are capable of causing white rot (Eriksson et al., 1990;
Nilsson et al., 1989).
Ultrastructural aspects of lignocellulolysis have
been documented for species of wood-decomposing
fungi (especially Phanerochuete chrysosporium Burds.)
FUNGAL LYSIS OF LC IN SALTMARSH GRASS
35
Fig. 4. Hyphae (HY) of P . spartinicola digesting lignocellulosic cluding the triangular cell-corner (CC) portion of the middle lamella.
secondary fiber-cell walls (S,) primarily from within cell lumina (L) Note the secondary front (digestive?) (F2) located at a distance from
but also directly adjacent to or within the middle lamella (ML),in- the hyphae. Bar = 5 pm.
36
S.Y. NEWELL ET AL.
Fig. 5. Hyphae (HY) of P. spartinicola digesting lignocellulosic secondary fiber-cell walls (S,) primarily from within longitudinal cavities lysed out of the walls. Note resistance t o lysis of innermost layer
of secondary wall (Q. Bar = 5 pm.
FUNGAL LYSIS OF LC IN SALTMARSH GRASS
37
seleau and Ruel, 1992; Kim et al., 1993; Nilsson et al.,
1989; Ruel, 1990).
Fig. 6. Longitudinal section through fiber cells of a leaf' blade of
smooth cordgrass showing hyphae (HY) of P. spartinicolu within the
cell lumen (left upper hypha) and digesting a longitudinal cavity
within the cell wall and middle lamella (right lower hypha). ML,
middle lamella; S,, secondary fiber-cell wall. Bar = 3 km.
whose capacity for oxidation of lignin and LC carbohydrates is well known (Daniel et al., 1990; Eriksson et
al., 1990; Hale and Eaton, 1985; Nilsson et al., 1989;
Ruel, 1990; Ruel et al., 1988). The characteristic TEM
appearance of LC of secondary walls of fiber cells under
attack by lignocellulolytic fungi is a darkening of wall
material and formation of reticulate structure (RS)
and/or a crumbling into amorphous bits or granules
(GR) (Fig. 1) (Ruel, 1990). The darkened material is
believed t o be modified (lysed and spontaneously partially repolymerized) lignin. In monocotyledenous
shoots, the low-lignin mesophyll-chlorenchyma tissue
(M) adjacent to highly lignocellulosic fiber bundles can
be nearly totally destroyed by LC-lytic fungi (e.g., rice:
Yu et al., 1990; date palm: Adaskaveg et al., 1991). A
sheath (HS) commonly appears surrounding the hyphae (HY) that cause the destruction of the LC. The
sheath can contain LC-lytic enzymes and, probably,
bits of the modified lignin (Hale and Eaton, 1985; Jo-
MATERIALS AND METHODS
Standing decaying leaf blades and leaf sheaths of
smooth cordgrass (Spartina alternifloru) were collected
from marshes of Sapelo Island, Georgia (USA) (see
Pomeroy and Wiegert, 1981); naked, fallen stems were
collected as portions of drift wrack in the same
marshes. Living, green leaf blades were collected and
used to prepare the diagram in Figure 1.Portions of the
decaying shoot material were selected on the basis of
apparent occupancy by single species of ascomycetes.
Sole occupancy was determined by direct-microscopically determined exclusive presence of mature ascomata (sexual reproductive structures); in such areas,
there is evidence (immunoassay vs. chemical-index assay) that most, if not all, of the hyphae within
cordgrass tissue is formed by the sexually reproductive
species (Newell, 1994). The four species of ascomycetes
selected for study were Phaeosphaeria spartinicola (leaf
blades), Buergenerula spartinae-(leaf blades), Phaeosphaeria spurtinae (Ellis & Everhart) Shoemaker &
Babcock (leaf sheaths), and Passeriniella obiones
(Crouan & Crouan) Hyde & Mouzouras (naked stems)
(see Kohlmeyer and Volkmann-Kohlmeyer, 1991). We
recognize that evidence of LC lysis in our micrographs
could have resulted from microbial activity that took
place before thorough pervasion by the fungal species
that we were targeting; this caveat would be least
likely to apply to the two species involved in decay of
leaf blades, since they are the first microbial invaders
of the senescent blades (Newell, 1993).
Small pieces (1-2 mm2 surface area) were dissected
from the shoot tissue and prepared for transmission
electron microscopy (TEM) by high-pressure freezing
followed by freeze-substitution (Lingle et al., 1992).
The pieces were vacuum-infiltrated with 15% dextran
prior to placement in planchettes. Void space in the
planchettes was filled with dextran solution. The
planchettes were inserted into a Balzers HPM 010
high-pressure freezer (see Dahl and Staehelin, 1989)
where specimens were sprayed with liquid nitrogen immediately after pressurization to 2,100 bars. After
freezing, specimens were collected in liquid N, and
transferred to substitution fluid a t -80°C where they
were held for 72 h and then gradually (over a 6 h period) warmed to room temperature. The substitution
fluid consisted of 2% osmium tetroxide and 0.01% uranyl acetate in HPLC-grade acetone. After several acetone washes, the specimens were infiltrated slowly
with either Embed 812 or Quetol651 epoxy resins (EM
Sciences, Ft. Washington, PA). Ultrathin sections were
cut and poststained with uranyl acetate and Reynolds'
lead citrate (Lingle, 1989) prior to examination in a
Zeiss EM 902A transmission electron microscope. We
prepared the following number of electron micrographs
for each species, each from a separate field of view: P.
spartinicola, 66;B . spurtinae and P. obwnes, 36 each; P.
spartinae, 14. The micrographs presented as figures below were chosen to be representative of the images
found for each species.
38
S.Y. NEWELL ET AL.
Fig. 7. Transverse section through fiber cells of a leaf blade of smooth cordgrass containing hyphae
( H Y ) of P. spartinicola both within the cell lumen and in lysed cavities in the secondary wall (S.J Note
glycogenrosettes (G)and electron-densehyphal-sheath(HS)
material and granular wall lysate (GR). Bar
= 2Fm.
Fig. 8. Leaf-blade xylem of smooth cordgrass containing hyphae
= 5 pm.
(HY)of P. spartinicola. Bar
Fig. 10. Digestion of xylem (X)and phloem (P)in leaf blades of
smooth cordgrass by hyphae (HY)of Buergenerulu spartinue. Note
that cells of the outer bundle sheath (0)
have been fully digested and
lost. Bar = 20 pm.
RESULTS
Fig. 9. Production of both reticulate dark lysate (RS)and dark
amorphous lysate (GR) (see Background) as a result of digestion of
fiber cells of smooth eordgra~sleaf blade by hyphae (HY)of P. spurtinicolu. Bar = 5 y.m.
Phaeosphaeria spartinicola and Cordgrass LC
Abaxial portions of cordgrass leaves occupied by mature ascomata of P. spartinicola (see Leuchtmann and
Newell, 1991; Newell and Wasowski, 1995) exhibited
areas of the former leaf mesophyll chlorenchyma, outer
bundle sheath, and phloem in which only thin remnants of plant cell walls and empty fungal hyphae remained (Fig. 2). Fiber tissue was also extensively degraded, with nearly all cells containing fungal hyphae
in some sections (Fig. 2). In other sections, fewer fiber
cells were as heavily damaged (Fig. 3). Hyphae of P.
spartinicola brought about LC lysis from within cell
lumina (Figs. 4-9), from positions along compound
middle lamellae (Figs. 4-7, 9), and from cavities
formed within secondary walls of fiber cells (Figs. 5-7).
In some cells, there were two electron-opaque (digestive?) fronts visible within secondary fiber walls, one
dark and close to the hyphae, one fainter and often
distant from the hyphae (Fig. 4). Hyphae within lumina of xylem cells (which can be the most heavily
lignified in grasses [He and Terashima, 19911) caused
dissolution of the cell walls, leaving remnant pit membranes in some cases (Fig. 8). Darkened, amorphous,
cell-wall lysate often was directly juxtaposed to the
fungal hyphal wall, appearing to become a part of
the hyphal sheath (Fig. 7). Alongside some fiber cells,
the.middle ktmelh appeared
be resistant to degradation, and in others it was clearly darkened and al-
40
S.Y.NEWELL ET AL.
Fig. 11. Hyphae (HY)of B . spartinae in a fiber cell of smooth cordgrass. Note that digestion of the
lignocellulosic secondary wall (S,)and production of granular lysate (GR)apparently extends primarily
from within the cell lumen and pit canals (PO. Bar = 5 Fm.
tered if not dissolved or mineralized, with fungal hy- walls, P. spartinicola could also produce reticulate
phae growing directly within it (Figs. 4-7, 9). In darkened lysate (Fig. 9). The thin S3layer of the fiber
addition to the darkened amorphous lysate of fiber walls was often apparently resistant to lysis (Figs. 5,
FUNGAL LYSIS OF LC IN SALTMARSH GRASS
41
Fig. 13. Hyphae (HY) of Phaeosphaeria spartinae causing erosional thinning from within lumina of fiber-cell walls of a leaf sheath
of smooth cordgrass. Bar = 5 pm.
Fig. 12. Hyphae (HY) of B. spartinae in the lumen and wall of an
abaxial epidermal cell of a leaf blade of smooth cordgrass. Note the
darkening of the epidermal-cell wall just under the plant cuticle (CU)
around vacant (V)spaces that appear to have previously contained
hyphae (compare the vacant spaces to the intact hypha (IH)a t top
center). Bar = 10 pm.
7). Some hyphae within fiber cells contained dense concentrations of glycogen rosettes (Fig. 7) (glycogen is a
fungal storage carbohydrate) (see Cooke and Whipps,
1993; Lingle, 1989).
Buergenerula spartinae and Cordgrass LC
B . spartinae caused xylem dissolution and fiber-wall
erosion, with alteration of middle lamellae (Figs. 10,
11). Although we observed hyphae of B . spartinue
within the secondary walls of fiber cells, it was most
commonly seen to cause secondary-wall digestion from
within cell lumina and their pit canals (Fig. 11).Hyphae of B . spartinae grew within the outer portions of
epidermal cells or between the cuticle and the wall of
the epidermis and appeared to cause the surrounding
material to become very electron opaque (Fig. 12; compare epidermal walls at the top of Fig. 2).
PhaeosphaeM spartinae and Cordgrass LC
The hyphae ofp. spartinae caused thinning to total
from cell lumina outwards (l?igs.
loss of fiber cell
1 3 9 14); these hyphae were not observed to grow in
longitudinal cavities in fiber-cell secondary walls. Mid-
Fig. 14. Hyphae (HY) of P. spartinae causing erosional lysis of
subepidermal abaxial fiber and adjacent parenchyma cells. Note short
bore holes (B)between some cells, sometimes passing directly through
cell-corner portions of middle lamellae (lower marked hole). Bar = 20
pm.
S.Y. NEWELL ET AL.
42
Fig. 15. Hypha (HY) of P.spartime at the eroding edge of a fiber cell. Note that the hyphal sheath
(HS)is continuous with dark cell-wall lysate, under which reticulate wall digesta (RS)can be seen. Bar
= 2 pm.
dle lamellae were largely unlysed by P. spartinae; this
species grew fkom cell to cell via pit connections or
through short, transverse bore holes (Fig. 14). The
thinning of fiber walls characteristically left a serrate
edge to the wall, with a very electron-opaque, thin covering that was often a t least partially continuous with
hyphal-sheathing material (Fig. 15). Beneath the dark
covering, areas of reticulate-digested secondary wall
could be seen (Fig. 15). Hyphae of P. spartinae growing
in the outer edges of epidermal-cell walls caused a
marked darkening (Fig. 14, upper left) like that of B.
spartinae (Fig. 12).
the stem, sometimes in close proximity to fungal hyphae (Fig. 181, and in other instances in areas wherein
remaining fungal hyphae appeared empty of cytoplasm
(not shown). Bacteria outside of the plant cuticle in
epiphytic positions were commonly observed for all
four types of fungal samples, but bacterial epidermal
erosion was found only for the P. obiones-occupied portions of shoots.
DISCUSSION
All four of the cordgrass ascomycetous decomposers
were capable of lysis of lignocellulose (LC). Both type 1
(cell-wall decay via wall-cavity formation) and type 2
Passeriniella obiones and Cordgrass LC
(cell-wall erosion from the interior [see Nilsson et al.,
Both cell lumina and secondary-wall cavities were 19891) soft rot were found. B. spartinae and P. spartinae
occupied by hyphae of P. obiones (Figs. 16,17). Hyphae caused only or predominantly type 2 decay. P. obiones
within secondary walls commonly caused cavities to caused type 1 only or predominantly, and P. spartinienlarge until only disjunct innermost and outermost cola caused both types 1 and 2. The middle lamella, one
layers remained (Fig. 17). Middle lamellae were some- of the portions of cordgrass LC likely to be the most
times not apparently changed (Fig. 17), and in other lignin-rich (He and Terashima, 1991), was often clearly
cases parts of the middle lamellae were altered (Fig. altered by P. spartinicola, the principal decomposer of
16, top center). Colonies of bacteria were found in leaf blades (Figs. 4, 6, 9) (Newell, 1993). TEM images
eroded pits in the very thick-walled epidermal cells of for cordgrass ascomycetes acting upon LC-rich cells
FUNGAL LYSIS OF LC IN SALTMARSH GRASS
43
Fig. 16. Hyphae (HY) ofPusserznieZluobiones in the lumen (L) and
in a longitudinal cavity in the wall of a subepidermal fiber cell of a
naked stem of smooth cordgrass. Note continuity of dark hyphal
sheath (HS) with dark wall lysate (GR)and darkening of a portion of
the cell corner (CC) of the middle lamella. Bar = 2 q.
Fig. 17. Hyphae (HY)of P. obiones as in Fig. 16, but here much of
the secondary wall of the cell is digested and collapsed, apparently
from broadening of longitudinal cavities (CA) formed as in Fig. 16,
between what may be multiple SIB, layers (see Adaskaveg et al.,
1991). Bar = 2 pm.
were characteristic of lignin-destroying fungi: formation of electron-dense granules or networks as the cellulose/hemicellulose fabric of the secondary walls of fiber cells was unraveled (Figs. 2, 4, 5, 9, ll, 15, 16).
Compare the figures of Ruel et al. (1988) for Phanerochaete chrysosporium, a very well-known lignin-oxidizing basidiomycetous fungus, and figures of Nilsson et
al. (1989)for Daldinia concentrica [Bolt.:Fr.l Ces. & De
Not, a white-rotting xylariacean ascomycete capable of
causing 40%losses of lignin. For cordgrass ascomycetes
and for LC destruction documented for other fungi, the
dark granules or networks often were continuous with
fungal extracellular hyphal-sheathing material (e.g.,
Figs. 7, 15, 16). This continuity suggests that the hyphal sheaths contained a mixture of partially oxidized
and partially repolymerized lignin with fungal extracellular polysaccharides and LC-lytic enzymes (Daniel
et al., 1990; Faix et al., 1985; Hale and Eaton, 1985;
Joseleau and Ruel, 1992; Nicole et al., 1993; Nilsson et
al., 1989; Ruel et al., 1988). Because all four of the
cordgrass ascomycetes of this report make melanin
(polyphenolic, brown, electron-dense pigments; see the
melanin deposits in ascospore walls of P. spartinicola in
Fig. 3) and because fungal melanin is usually deposited
in walls and hyphal sheaths, it may be that melanin
plus repolymerized lignin is formed as a conglomerate
by the cordgrass ascomycetes (see Bell and Wheeler,
1986; Kohlmeyer and Volkmann-Kohlmeyer, 1991).
The hypothesized lignomelanin conglomerates might
be measured as lignin by some methods (e.g., acid-detergent methods [Torzilli and Andrykovitch, 19861),resulting in underestimates of lignolysis (Newell, 1993).
Nilsson et al. (1989) examined by TEM the patterns
of LC lysis in wood for several ascomycetes. In parallel,
they chemically measured losses of lignin and LC carbohydrates. They found that 1) there was no difference
between type 1 and type 2 soft-rot fungi with respect to
losses of lignin and LC carbohydrates (12-44% loss of
lignin, 22-91% of glucan, 28-92% of xylan, 4 weeks
decay), 2) percentage losses of lignin ranged from x 1 to
X 0.5 the percentage losses of total dry mass (see also
Abe, 1989) and 3) losses of syringyl lignin units were
greater than losses of vanillyl units, leading to decreases in the syringy1:vanillyl (S:V) ratio in the remaining lignin. The data of Bergbauer and Newell
(1992) and Haddad et al. (1992) suggest that similar
chemical changes to those found by Nilsson et al.
(1989) occur for the ascomycete attack on cordgrass
depicted herein. Haddad et al. (1992) found that for
naturally decaying, standing leaf blades of smooth
cordgrass, lignin phenols were lost at a rate greater
than that for loss of total organic mass, implying that
fungal lysis of cordgrass LC does not result in rising
concentrations of fungal-resistant lignin. Bergbauer
44
S.Y. NEWELL ET AL.
Fig. 18. Epidermal cells of naked stem of smooth cordgrass, with
erosion bacteria (B)in pits in the outer secondary walls and hyphae
(HY)of P. obiones in lumina of the same cells. Bar = 5 pm.
and Newell (1992) found that cinnamyl and syringyl
phenols of extracted cordgrass LC were especially susceptible to alteration by P. spartinicola, with S:V falling (from 0.9 to 0.6) over 45 days incubation. However,
Haddad et al. (1992) did not find a decrease in S:V for
standing decaying cordgrass leaves, suggesting that selective syringyl alteration may not be as marked in the
field as it can be in vitro. Based on our TEM results and
the findings of Bergbauer and Newell (1992) and Haddad et al. (1992), we suggest that the higher ligninphenol concentrations for standing-dead than for green
smooth cordgrass shoots reported by Alberts et al.
(1992) were probably more the result of plant maturation and senescence processes than, as Alberts et al.
inferred, the result of inability of cordgrass ascomycetes to cause lignin losses.
Some portions of the LC-rich fiber tissues of
cordgrass were not heavily invaded and lysed, even in
direct juxtaposition with ascomata of the most aggressively LC-lytic species, P. spartinicola (Fig. 3). Patchiness of decay extensiveness is well known for wooddecay fungi (Eriksson et al., 1990). It seems reasonable
that it would be advantageous to the fungus to avoid
causing near-total loss of structural integrity in the
decaying leaf, even though it does decay both chlorenchymal nonstructural and lignified structural tissue,
since a large proportion of the fungal propagule-projecting bodies (the ascomata) (see Newell and Wasowski, 1995) might lose the ability to deposit spores on
new substrate within the shoot canopy if the leaves
were to rapidly crumble to the marsh floor.
Our TEM inspection of decaying portions of smoothcordgrass leaf blades and sheaths confirms Newell’s
(1993) conclusion, based on epifluorescence-microscopic assay of bacterial biovolume, that bacteria contribute negligibly to microbial standing crop of decaying, standing cordgrass leaves. We found no images of
bacteria causing lysis of plant cells for leaf preparations. For naked stems, however, that had fallen to the
marsh sediment prior to collection, we did find bacterial colonies that were apparently cell-wall-lytic (Fig.
18). Newell (1993) suggested that tunneling bacteria
might be important decomposers of fallen portions of
decaying smooth cordgrass, based on unpublished information from the study of Benner et al. (1984) (see
Newell, 1993). The bacteria that we found in pits in
epidermal cells of cordgrass, though, were typical of
erosion bacteria (Singh and Butcher, 1991; see also
Barnabas, 1992). These erosion bacteria in cordgrass
stems appeared to be embedded in an extracellular matrix (Fig. 18), perhaps as an adaptation to periodic dryness (Roberson et al., 1993).
Finally, our TEM findings for cordgrass ascomycetes
have implications with respect to fungal biotechnology.
All four of the ascomycetes presented images characteristic of LC-digesting fungi, and a t least one of them
destroyed LC-rich cells to an extent comparable to
white-rot basidiomycete fungi (see Adaskaveg et al.,
1991; Eriksson et al., 1990; Ruel, 1990). Thus, one
might expect that the cordgrass ascomycetes (and ascomycetes of other standing-dead grass shoots) would
be candidates for the types of biotechnological usage to
which the LC-degrading basidiomycetes have been put,
especially destruction of toxic phenolic substances (including chlorinated molecules), alterations of paper-industry raw materials or wastes, and improvement of
ruminant fodder (e.g., Abernethy and Walker, 1993;
Akin et al., 1993; Blanchette, 1991; Hammel, 1992;
Kirkpatrick et al., 1990; Loske et al., 1990; Moyson and
Verachtert, 1991; Ollikka et al., 1993; Yadav and
Reddy, 1993). Bergbauer et al. (1992) have preliminarily confirmed this expectation: they discovered that the
anamorph (asexual form) of an ascomycete of standing,
decaying shoots of a sedge from freshwater wetlands
could effectively dechlorinate and degrade polyphenolrich effluents from a pulp-mill bleachery.
ACKNOWLEDGMENTS
Financial support for this research was received from
the National Science Foundation (grant OCE-9115642)
and the Sapelo Island Research Foundation. We thank
Darrell Casey for expert assistance with preparation of
our figures. This is UGMI contribution 747.
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