Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 24

stalks. Individual cells may produce flagella, how-
ever, swim away and form a new colony elsewhere.
With the relative absence of colonial form, diver-
sity in this group is based mainly on variation in
ultrastructural features - including feeding appara-
tus, flagella and pellicle structure (Simpson, 1997).
Afurtheraspectofdiversityisthedistinctionbetween
green and colourless (autotrophic/heterotrophic)
cells, with approximately two-thirds of all species
being heterotrophic. Some euglenoids (e.g. strains
and species of Euglena ) are facultative heterotrophs,
which are able to carry out heterotrophic nutrition
when photosynthesis is limiting or when surround-
ing concentrations of soluble organic materials are
high.Intheheterotrophicstate,theseorganismsretain
their plastids as colourless organelles and absorb sol-
uble nutrients over their whole surface (osmotrophy).
Other euglenoids ( Petalomonas, Astasia, Peranema )
are obligate heterotrophs and have lost their plastids
completely.Manyoftheseorganismsconsumepartic-
ulate organic material (phagocytic) and have evolved
a complex feeding apparatus.

1.5.4 Euglenoids as bioindicators

Euglenoid algae are not particularly useful as
environmental bioindicators, in terms of either con-
temporary populations or fossil records. Although
present-day algae show some adaptations to specific
environments(seeprevioussection),thereisnoestab-
lished environmental library and species may be dif-
ficult to identify. Within lake sediments, the lack of
calcified or silicified structures that are resistant to
decay means that hardly any remains have survived
in the fossil record.

1.6 Yellow-green algae

Yellow-green algae (Xanthophyta) are non-motile,
single-celled or colonial algae, with a distinctive pig-
mentation that gives the cells a yellow or fresh green
appearance (e.g. Fig. 4.18 - Tribonema ). Although
there is a wide range in morphology (Table 1.9), this
phylum contains relatively few species (compared to
major groups such as green algae) and the algae tend
to be ecologically restricted to small water bodies and
damp soils.

1.5.3 Ecology

Euglenoids are generally found in environments
where there is an abundance of decaying organic
material. This is in line with the heterotrophic nature
of many of these organisms and the ability to take up
complex organic material either in the soluble or par-
ticulate state. Typical habitats include shallow lakes,
farm ponds, wetlands, brackish sand and mudflats.
Within these environments, euglenoids are particu-
larly associated with interfaces such as sediment-
water and air-water boundaries (Walne and Kivic,
1990) and should probably not be regarded as open
water truly planktonic algae (Lackey, 1968).
Certaineuglenoidalgaeareabletotolerateextreme
environmental conditions. One of these, Euglena
mutabilis , is able to grow in very low pH waters.
This alga has an optimum pH of 3.0, can tolerate
values below pH 1.0 and is typical of acidic metal-
contaminated ponds and streams draining mines.
Other euglenoids, found in brackish habitats, are able
to tolerate a wide range of salinity (Walne and Kivic,
1990).

1.6.1 Cytology

Yellow-green algae are most likely to be confused
with green algae when observed in the fresh condi-
tion, but differ from them (Table 1.3) in a number of
key features.

Distinctive pigmentation, with chlorophylls ( a , c 1
and c 2 - but not b ), carotenoids (especially β-
carotene) and three xanthophylls (diatoxanthin,
vaucheriaxanthin and herteroxanthin).

Carbohydrate storage as oil droplets or chrysolam-
inarin (usually referred to as leucosin) granules.

Walls composed mainly of pectin or pectic acid
(sometimes with associated cellulose or siliceous
material). The walls often occur as two spliced
andoverlappingsections,breakinginto'H'-shaped
pieces on dissociation of the filaments.