Introduction to Fungi, Third Edition

72 STRAMINIPILA: MINOR FUNGAL PHYLA
within which individual vegetative cells are
contained. For this reason, they are sometimes
referred to as ‘slime nets’ (Porter, 1990). The
vegetative cells possess a wall which, uniquely,
is produced from Golgi-derived scales of a
polymer of l-galactose (Dick, 2001a). These
scales are located between the plasma membrane
and the inner membrane of the slime net. The
slime net is delimited by an inner and an outer
membrane and is produced by specialized organelles
termed sagenogens or bothrosomes; the
net membranes are continuous with the plasma
membrane at the sagenogen (Perkins, 1972).
Labyrinthulomycota feed by absorption (osmotrophy)
of nutrients. The nets contain degradative
enzymes which can lyse plant material or
microbial cells. Two orders are distinguished.
4.4.1 Labyrinthulales
Members of this order, especially of the genus
Labyrinthula, can be readily isolated from marine
angiosperms such as Zostera and Spartina or from
seaweed by placing a small piece of one of these
substrata directly on low-nutrient sea water agar
augmented with penicillin and streptomycin
(Porter, 1990). Within a few days, a fine network
of strands can be seen extending over the agar
surface (Fig. 4.6). Labyrinthula spp. can be kept in
monoxenic culture with yeasts or bacteria as
food source. These are presumably lysed by the
enzymes contained in the slime net.
A closer examination shows that the network
consists of branched slime tubes within which
spindle-shaped cells move backwards and
forwards (Fig. 4.7a; see Webster, 2006a).
Movement of a speed up to 100 mm min 1 has
been reported and is due to a system of
contractile actin-like proteins in the slime net
(Nakatsuji & Bell, 1980). Cells occasionally aggregate
to form sporangia containing numerous
round cysts. Following meiosis, eight heterokont
zoospores (Figs. 4.6a, 4.7b) are released by each
cyst. These possess a pigmented eyespot not
found in other types of heterokont zoospore
(Porter, 1990). It is, however, unclear whether
zoospores can establish new colonies (Porter,
1990). Asexual reproduction occurs by division
of spindle cells within the slime net, and
fragments of such a colony can establish new
colonies (Porter, 1972). Further details of the life
cycle appear to be unknown at present.
Labyrinthula spp. were implicated as pathogens
in a wasting epidemic of eelgrass (Zostera
marina) at the west coast of North America in the
1930s (Young, 1943; Muehlstein et al., 1991),
causing considerable disturbance to the littoral
ecosystem and collateral damage to the
local fisheries industry. However, although
Labyrinthula spp. are still frequently associated
with pieces of moribund Zostera shoots, no
further epidemics seem to have occurred since.
Instead, a new species, L. terrestris, has recently
been identified as the cause of a rapid blight of
Fig 4.6 Labyrinthula. (a) Zoospore with
long anterior straminipilous flagellum
and a short posterior whiplash flagellum
with a pointed tip (after Amon &
Perkins,1968). (b d) Portions of colonies
at different magnifications. In (c) spindle
cells are seen in swellings in the slime
tracks.