Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 42

characteristic photosynthetic pigments (chlorophyll-
a only, presence of phycobilins) and distinctive
plastid structure (unstacked thylakoids, no external
endoplasmic reticulum).
Many of the freshwater species (which occur
mainly in streams and rivers) are quite large, being
visible to the naked eye when occurring in rea-
sonable numbers (Sheath and Hambrook, 1990).
The range of morphologies includes gelatinous fila-
ments (e.g. Batrachospermum ), pseudoparenchyma-
tous forms (e.g. Lemanea ) and a flat thallus of tiered
cells (e.g. Hildenbandia ). Many of these shapes can
be an advantage in resisting the forces exerted in
swiftly flowing waters.
Althoughfreshwaterredalgae(suchasthelargeil-
amentous alga, Batrachospermum ) are largely found
in streams and rivers, Rhodophyta may also occur as
marine invaders of lakes and brackish environments.
Certain freshwater red algae in the littoral zones of
the Great Lakes Basin (USA), for example, appear to
be originally marine and to have lost the capacity for
sexual reproduction. These include the filamentous
red alga Bangia atropurpurea (Lin and Blum, 1977),
which reproduces only by asexual monospores -
in contrast to marine species which undergo
alternation of generations and carry out sexual repro-
duction. Attached red algae (e.g. Chroodactylon

ramosum ) also contribute to the epiphytic flora of
lake periphyton.

1.12 Brown algae

As with red algae, brown algae (Phaeophyta) are
almost entirely marine - with less than 1% of species
present in freshwater habitats (Wehr and Sheath,
2003). These species are entirely benthic, either in
lakes or rivers, and have a very scattered distribution.
Cytological features of this group (Table 1.3)
include heterokont flagella (reproductive cells only),
presence of laminarin as the major food reserve,
characteristic photosynthetic pigments (chlorophyll-
a and - c , β- and ε-carotenes) and distinctive plastid
structure (triple thylakoids, enclosing endoplasmic
reticulum). Freshwater brown algae include genera
such Pleurocladia and Heribaudiella and are the least
diverse of all freshwater algae. Their morphologies
are based on a relatively simple filamentous struc-
ture (tufts or crusts), and they lack the complex
macro-morphology typical of the brown seaweeds.
The freshwater brown-algae have not been fully stud-
ied and their ecological characteristics are not well
known. All are benthic and may be recognized by
their possession of large sporangia (Wehr, 2002).