Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 32

hasaspecialisedvacuoleforcapturingandcontaining
bacterial cells. Bacteria are drawn into this through a
small pore at the cell surface, and subsequently pass
into digestive vesicles within the cytoplasm.

species are small and are important members of the
nanoplankton. They are widely distributed through-
out freshwater systems and are of particular interest
in relation to their morphological diversity, ecology
and as indicator organisms. Details of taxonomy and
ultrastructure are given by Kristiansen (2005).

1.8.5 Cryptomonads as bioindicators

Although they are common in oligo- and mesotrophic
conditions, cryptomonads also occur in eutrophic
lakes (Fig. 2.16) and have limited use as bioindi-
cators. Individual species do not provide the contem-
porary diagnostic range for different habitats seen in
other algal groups, and cryptomonads do not survive
as clearly identifiable remains within lake sediments.

1.9.2 Morphological diversity

Chrysophytes exhibit considerable diversity in their
general organisation, ranging from unicellular to
spherical (e.g. Synura - Fig. 4.37) and branching
(e.g. Dinobryon - Fig. 4.2) colonial types. The
great morphological diversity shown by these organ-
isms (Table 1.11) follows a similar diversity seen in
other algal phyla and includes flagellates, amoeboid
forms, cells in jelly (capsulate), filamentous algae and
plate-like (thalloid) organisms. Each of these types,
however, can transform into another phase (e.g. flag-
ellates can become amoeboid) - so that grouping in
relation to morphology is dependent on the stage of
life cycle. The occurrence of a resistant stage (stoma-
tocyst) phase (see previous section) is also a key part
of the life cycle.

1.9 Chrysophytes

Chrysophytes (Chrysophyta), commonly referred to
as the 'Golden Algae', are a group of microscopic
algaethataremostreadilyrecognisedbytheirgolden-
brown colour (Figs. 1.3 and 4.37). This is due to
the presence of the accessory pigment fucoxanthin
within the chloroplasts, masking chlorophylls- a and
- c (Table 1.3). There are around 200 genera and 1000
species of chrysophytes, present mainly in freshwater
although some may be found in brackish and salt
waters.

1.9.3 Ecology

Chrysophytes are ecologically important in a number
of ways (Kristiansen, 2005). 1.9.1 Cytology

In many ecosystems, they play an important role as
primary producers. This is particularly the case for
adverse conditions - low nutrient and acid lakes
(e.g. Nedbalova et al ., 2006).

Distinguishing cytological features (Table 1.3)
include the presence of two dissimilar (long and
short) flagella (heterokont condition) in motile organ-
isms, plastids with four outer membranes plus triplet
thylakoids,chrysolaminarinasthemainstorageprod-
uct (present in special vacuoles) and cell walls com-
posed of pectin - covered in some species with small
silica spines or scales. Lipid droplets also frequently
occur, and an eyespot may be visible in some species,
associated with the chloroplast. In order to sur-
vive adverse conditions, round-walled cysts (stoma-
tocysts)maybeproduced,withsilicascalespresentin
some species. Although most chrysophyte species are
photosynthetic, some may be partially heterotrophic
orevenfullyphagotrophic(Pfandl etal .,2009).Many

They have a versatile nutrition, with many mem-
bers (e.g. Dinobryon ) being mixotrophic. These
organisms are able, in addition to photosynthesis,
to obtain their carbon from organic sources - either
by surface absorption of organic compounds or by
phagocytosis of particulate organic matter.

They can be nuisance algae, giving a fishy smell
to drinking water reservoirs when they reach high
population levels.