Introduction to Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 31

such as Rhodomonas may form red blooms when
the ice melts in early spring. In other (Antarctic)
lakes, where ice persists, cryptomonad species such
as Cryptomonas sp. and Chroomonas lacustris may
contribute up to 70% of the phytoplankton biomass,
dominating the algal flora for the whole of the limited
summer period (Spaulding et al ., 1994).
Cryptomonads are also typical of the surface
waters of temperate lakes during the clear-water
phase of the seasonal cycle, between the diatom
spring bloom and the beginning of the mixed summer
bloom. During this period, when algal populations
are severely predated by zooplankton populations,
the limited competition between algal cells favours
rapidly growing r-selected organisms such as cryp-
tomonads (Sigee, 2004).
In oligotrophic freshwater lakes, cryptomonads
often form large populations about 15-20 m deep
within the lake, where oxygenated surface waters
interface with the anoxic lower part of the water
column. In this location, these algae form deep-water
accumulations of photosynthetic organisms, referred
to as 'deep-chlorophyll maxima'. Studies on cultures
of Cryptomonas phaseolus and Cryptomonas undu-
lata isolated from deep chlorophyll maxima (Gervais,
1997) showed that these organisms had optimal
growth under light-limiting conditions, suggesting
photosynthetic adaptation to low light intensities.
Other factors which may be important in the ability
of these organisms to form large depth populations
include the ability for heterotrophic nutrition,
relative absence of predation by zooplankton,
access to inorganic nutrients regenerated by benthic
decomposition and tolerance of sulphide generated
in the reducing conditions of the hypolimnion.
The ability of cryptomonads to carry out het-
erotrophic nutrition, using ammonium and organic
sources as a supply of carbon and nitrogen, is
ecologically important. Evidence for uptake of par-
ticulate organic material comes from direct obser-
vation of phagocytosis and has been documented
in both pigmented (e.g. Cryptomonas ovata ) and
various colourless species (Gillott, 1990). Ultra-
structural studies also provide evidence that many
pigmented cryptomonads are mixotrophic, capable
of both autotrophic and heterotrophic nutrition. The
blue-green cryptomonad, Chroomonas pochmanni,

Table 1.10 Variation in Plastid Number and Coloration
in Cryptomonads.

Trophic State Plastid Characteristics

Autotrophic Rhodomonas - a single boat-shaped,
red-coloured plastid (Fig. 4.52)
Chroomonas - a single H-shaped
blue-green plastid with a pyrenoid on
the bridge.
Cryptomonas - two plastids, each with
a pyrenoid (Fig. 4.53).
Heterotropohic Chilomonas - a colourless cell that
contains a plastid (leucoplast)
lacking pigmentation. No pyrenoid.

of water bodies - in contrast to euglenoids, which
tend to occupy mud, sand and water interfaces.

1.8.3 Biodiversity

Cryptomonads are a relatively small group of algae,
withatotalofabout12genera(Hoek etal .,1995)con-
taining approximately 100 known freshwater species
and 100 known marine species.
The low diversity in terms of number of species
is reflected in the uniformity of morphology. There
are no clear colonial forms, though some species are
able to form irregular masses embedded in mucilage
(palmelloid stage) under adverse conditions. Dif-
ferences do occur in relation to plastids - including
presenceorabsence,numberandpigmentation(Table
1.10). Variations also occur in other key cytological
features such as the presence or absence of nucleo-
morphs, occurrence of ejectisomes and structure of
the periplast - providing a basis for the classification
of these organisms (Novarino and Lucas, 1995).

1.8.4 Ecology

Cryptomonads are typical of temperate, high latitude
standing waters that are meso- to oligotrophic. They
appear to be particularly prominent in colder waters,
becoming abundant in many lakes in early spring,
when they may commence growth under ice. Algae