Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 25

When flagella are present they are of unequal
length (heterokont) the longer one pointing for-
wards and the shorter (which can be more difficult
to see using a light microscope) sometimes point-
ing forwards or backwards

systems or an attachment stalk. Some yellow-green
algae form simple globular colonies, which may be
mucilaginous and free floating (e.g. Gloeobotrys )
or attached via a stalk ( Ophiocytium ). Filamentous
forms may be unbranched ( Tribonema - Fig. 4.18)
or branched ( Heterococcus ) and reach their most
massive state in the coenocytic siphonaceous genera
Botrydium and Vaucheria (Fig. 4.3).

Chloroplast fine structure - four outer membranes
(incorporating an endoplasmic reticulum) and thy-
lakoids occurring in threes.

Cells contain two or more discoid and green to
yellow-green chloroplasts, with associated pyrenoids
rarely seen. An eyespot may be present.

1.6.3 Ecology

Yellow-green algae are rather limited in their
exploitation of aquatic habitats, tending to occur on
damp mud (at the edge of ponds) and soil, but not
occurring extensively in either lentic or lotic systems.
Where planktonic forms do occur, they tend to be
in ditches or small ponds. Tribonema species are
often found as free-floating filaments in temporary
waters, and Botrydiopsis arrhiza is found at the
edge of ponds and in patches of water in sphagnum
bogs, where it may form yellowish water blooms.
Even Gloeobotrys limneticus , which has widespread
occurrence as mucilaginous colonies in lake plank-
ton, never forms extensive or dominant populations
(as seen, e.g. by blue-green algae). Various yellow-
green algae are epiphytic, including Mischococcus

1.6.2 Morphological diversity

Yellow-green algae show a range of morphol-
ogy, from unicellular to colonial and filamentous
(Table 1.9). Unicellular forms are non-flagellate in
the mature (vegetative) state, with motility being
restricted to biflagellate zoospores (asexual) or
motile gametes (sexual) at reproductive stages. The
zoospores have unequal and morphologically dissim-
ilar flagella (heterokont algae - Fig. 1.7). Unicellular
algae include simple free-floating forms ( Botrydiop-
sis , Tetraedriella ) or may have associated rhizoidal

Table 1.9 Range of Morphologies in Yellow-Green Algae.

Major Morphotype Attached or Planktonic Examples

Unicells
Coccoid

Planktonic or benthic Botrydiopsis

Pear-shaped, with rhizoidal system Present on mud surface Botrydium

Ovoid cells on stalk Epiphytic on attached or planktonic algae Characiopsis

Amoeboid Benthic, often in hollow cells of Sphagnum. Chlamydomyxa

Colonial
Small to large globular colonies

Planktonic Gloeobotrys

Branched colony on stalk Planktonic or attached Ophiocytium

Filaments
Unbranched Typically planktonic, occasionally attached

when young

Tribonema (Fig. 4.18)

Branched Attached, damp soils Heterococcus

Coenocytic, (siphonaceous) Attached Botrydium, Vaucheria

(Fig. 4.3).