Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 22

situation. This organism occurs in the highly saline
waters of commercial salt pans (salterns) and natural
salt lakes such as the Great Salt Lake of Utah (USA).
Dunaliella is able to tolerate external salt concentra-
tions of

in soil and freshwater environments that are con-
taminated by sewage. The evolutionary relation-
ship of this obligate heterotroph to green algae
is indicated by the presence of starch-containing
plastids.

5 M , by balancing the high external osmotic
pressure (OP) with a high internal OP generated by
photosynthetically produced glycerol.

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1.4.4 Green algae as bioindicators

Specialised environments In addition to the
free-living occurrence of particular algae in lakes and
rivers that have a range of characteristics (see above),
othergreenalgaeareadaptedtomorespecialisedcon-
ditions. Some of these involve a change in the mode
of nutrition from autotrophic to heterotrophic and
include various species of the non-flagellate unicel-
lular algae, Chlorella - a widespread endosymbiont
of freshwater invertebrates. Other green algae retain
their free-living existence, but are able to supple-
ment photosynthesis (mixotrophy) by absorption of
exogenous dissolved carbon (Tuchman, 1996) such
as amino acids and sugars through their cell surface
(osmotrophy). The ability to use external organic
carbon may be important in two situations - when
photosynthesis is limiting and when external soluble
organic is in excess.

Contemporary algae

Habitat preferences of contemporary green algae
(Table 1.8) are frequently useful in providing infor-
mation on physicochemical characteristics of the
aquatic environment. Filamentous green algae, for
example, often dominate environments stressed by
cultural eutrophication, acidification and metal con-
tamination (Cattaneo et al ., 1995).

Fossil algae

Green algae do not typically produce resistant walls
that persist in aquatic sediments and are much less
useful than diatoms and chrysophytes as bioindica-
tors in terms of the fossil record. There are some
exceptions to this, where the cell wall contains fatty
acid polymers known as algaenans that can with-
stand millions of years of burial (Gelin et al ., 1997).
Scenedesmus and Pediastrum are among the most
common members of green algae that have fossil
records, containing algaenans and also some silicon.
The desmid Staurastrum is also frequently seen as
fossil remains, due to the impregnation of cell wall
material with polyphenolic compounds which confer
resistance to bacterial decay (Gunnison and Alexan-
der, 1975).

Limiting photosynthesis. This may occur, for
example, under conditions of chronic low light
intensity (e.g. arctic lakes). It is also typical of
many acid lakes, where low pH reduces the level of
dissolved inorganic carbon to levels that are unable
tosaturatephotosynthesis.Laboratoryexperiments
on Coleochaete , an acidophilic alga, have demon-
strated an ability to use dissolved organic carbon
in the form of hexose sugars and sucrose (Graham
et al ., 1994).

High levels of external dissolved organic car-
bon. Some osmotrophic green algae are photo-
heterotrophic, only able to use organic carbon
when light is present and when their photosyn-
thesis is inhibited by the high availability of dis-
solved organic carbon (Lewitus and Kana, 1994).
The colourless unicellular alga Prototheca has
completely lost the ability to photosynthesise and
obtains all its carbon from external sources present

1.5 Euglenoids

Euglenoid algae (Euglenophyta) are almost entirely
unicellularorganisms,withatotalof40generaworld-
wide (about 900 species) - most of which are present
in freshwater. Cells are typically motile, either via
flagella or (in non-flagellate cells) by the ability of
the body to change shape (referred to as 'metaboly').