Agriculture Reference
In-Depth Information
The role of bacteria in the diet of earthworm species and the extent of species-specific feeding
patterns and digestion are largely unknown. Some bacterial species may be digested; others may
be little affected and survive passage through the gut; yet others may grow or become more active
in the earthwormÔs intestine. This phenomenon is discussed further in the following section. In the
case of the compost earthworm
, 19 species of bacteria were digested in the gut, and this
dietary addition increased the earthwormÔs growth rate significantly (Flack and Hartenstein 1984;
Hand et al. 1988). However, Morgan (1988) found that, of 12 bacterial species tested, only two
allowed
E. fetida
to maintain weight; the earthworms either lost weight or died when they fed on
the other species. Thorpe et al. (1993) showed that the bacteria
E. fetida
Pseudomonas fluorescens
could
proliferate in the gut of
L. terrestris
in the absence of competition from other gut-inhabiting
microorganisms. Doube et al. (1994d) showed that rhizobia and
Pseudomonas corrugata
survived
passage through the intestines of
A. trapezoides
and
A. rosea
.
Different earthworm species may affect the same bacteria in different ways. For example,
Schmidt et al. (1997) fed the same concentration of an inoculum of
P. corrugata
to four species
of lumbricid earthworms, but the density of
P. corrugata
found in the fresh casts of
A. longa
was
tenfold higher than that found in fresh casts of
L. rubellus
,
A. caliginosa,
and
L. terrestris
. On the
other hand, bacteria such as
Enterobacter aerogenes
have been shown to infect and kill the tropical
earthworm
Hoplochaetella suctoria
(Rao et al. 1983), and other species, such as
Pseudomonas
spp.,
Streptomyces
spp., and
Flavobacterium
spp., that produce antimicrobial substances have killed
the earthworm
E. fetida
in cultures (Hand et al. 1988).
The role of algae and cyanobacteria (blue-green algae) in earthworm diets is still not clear
because earthworms can be common in soils that contain a low biomass of algae (e.g., in Medi-
terranean climate regions), but laboratory trials suggest that these algae can play an important role
in the nutrition of some species (e.g.,
E. fetida
; Atlavinyt and Pocien 1973; Piearce 1978;
Stamatiadis et al. 1994).
The Fate of Microorganisms in the Intestines of Earthworms
In the earthworm gut, various enzymes of microbial and earthworm origin are secreted, as well as
intestinal mucus (a readily assimilable C source), CaCO
3
(if calciferous glands are present), and
bacteriostatic and microbicidal substances (including antibiotics of microbial origin and bacteriol-
ysins, peroxidases, and phagocytoses of earthworm origin). All influence the ability of a particular
ingested organism to survive passage through the earthworm gut (Brown 1995;
Figure 12.1
)
. Hence,
different species of bacteria, fungi, protozoa, and algae may be affected in different ways, depending
on the species of earthworm and the particular conditions created in their gut and the ability of
these organisms to take advantage of, or resist, the gut conditions. The survivors (largely fungal
and protozoan spores and resistant bacteria) provide inocula for microbial colonization of the
earthworm casts (Dash et al. 1986; Spiers et al. 1986; Brown 1995).
The microbial composition of the earthworm intestine contents has been considered to reflect
that of the soil or ingested plant remains (Morgan 1988; Brown 1995), but there is evidence of the
possible existence of an indigenous, autochthonous gut flora in some earthworm species (Jolly et
al. 1993; Vinceslas-Akpa and Loquet 1995; Toyota and Kimura 2000). Furthermore, the numbers,
biomass, and activity of the microbial communities in the earthworm gut have also been shown to
be different from that in uningested soil (Fischer et al. 1995; Kristufek et al. 1995; Schnholzer et
al. 1999). Hence, Lavelle et al. (1995) demonstrated the presence of a mutualistic digestive system
in several tropical and temperate earthworms species in which soluble organic C, in the form of a
low molecular weight mucus, was added in large quantities (5 to 80% of the dry weight of soil,
depending on the species) into the foregut (Trigo et al. 1999; Brown et al. 2000) to ÑprimeÒ the
soil microflora. The mixture of mucus with ingested OM and the high water content and near-
neutral pH of the foregut promoted the development of a microflora that could digest cellulose and
