18/10/2024
algae (singular, alga) A large and diverse group of
photosynthetic organisms formerly called simple plants
but now members of their own phyla, the Protoctista,
that also includes the slime molds and protozoa. Algae,
some 17,000 species or more, live in aquatic and moist
inland regions. They do not have roots, stems, or leaves
and have no vascular water-conducting systems. They
reproduce by spores, and in some species, the spores are
mobile with the use of flagella. They range from simple
single cells (e.g., Euglena) to “plants” many feet long
(e.g., kelps such as Macrocytis) and make up marine
seaweed and much of the plankton that provides food
for other species.
Unusual growth outbursts result in “algal blooms”
or “red tides” and occur when there is an increase in
nutrient levels in a body of water.
Cyanophyta, or blue-green algae, are now classified
as cyanobacteria. The phylum Cyanophyta also
includes chloroxybacteria. Cyanophytes contain phycocyanin,
a photosynthetic pigment giving them a blue
color. The red pigment phycoerythrin is also almost
always present. They are diverse and can live as single
cells or as colonies or large filaments. Some are nitrogen
fixers in soil and others, like lichens, display symbiosis
with a fungus, usually with a member of
Ascomycota. The fungus provides the host plant for the
algae cells that are distributed throughout and provide
food to the fungus while the fungus protects the algae
cells. While many lichen have a different fungi component,
they often have the same algae species. Some
common species of lichens are British soldiers (Cladonia
cristatella), pixie cups (Cladonia grayi), cedar
lichen (Vulpicidia viridis), wrinkled shield lichen
(Flavoparmelia caperata), and reindeer lichen (Cladina
subtenuis).
The chloroxybacteria, or green-grass bacteria, contain
both chlorophylls a and b but do not contain the
red or blue pigments of the blue-green algae. They are
nonmotile, aerobic organisms.
Other phyla of algae include the Bacillariophyceae,
comprising the diatoms; Charophyceae, fresh- or
brackish- water algae that resemble bryophytes;
Chlorophyceae, or green algae believed to be the progenitor
of plants; Chrysophyceae, or yellow-green
algae; Dinophyceae, unicellular algae with two flagella;
Phaeophyceae, or brown algae; and Rhodophyceae, or
red algae.
The Bacillariophyceae or diatoms are unicellular
algae that are found in single, colonial, or filamentous
states. Under the microscope they often are beautifully
symmetrical, as their cell walls, or frustules, are composed
of silica and are bivalved, one of which overlaps
the other, and the frustule is often punctated and
ornamented. The two orders, Centrales and Pennales,
occupy two different environments. The centric
diatoms (Centrales) are circular in shape with radial
symmetry and live mostly in marine environments.
The pennate diatoms (Pennales) are elliptical in shape,
have bilateral symmetry, and are found in freshwater
environments.
Deposits of fossil diatoms known as diatomaceous
earth have been mined and used for years in paints,
abrasives, and other products such as chalk. The
famous White Cliffs of Dover in England (rising to 300
feet) are composed of massive amounts of diatoms—
coccoliths—that were laid down some 790 million
years ago when Great Britain was submerged by a shallow
sea.
The Charophyceae, also known as stoneworts, and
which resemble bryophytes, live in fresh and brackish
water especially rich in calcium, where they become
stiff and lime encrusted. The stoneworts consist of a
complex branched thallus with an erect stemlike structure
and many whorls of short branches.
The Chlorophyceae (or green algae) are the closest
to plants in pigment composition and structure and are
related based on a common ancestor. More than 7,000
species live in freshwater and marine environments as
unicellular parts of plankton, in damp soil and even
snow as colonies or filaments, symbiotically with other
eukaryotes, or mutually with fungi as lichens.
The Chrysophyceae or golden algae, named
because of their yellow and brown carotene and xanthophyll
pigments are typically cells with two flagella
at one end of the cell, and many live among freshwater
and marine plankton. While most are unicellular, a few
are colonial.
The Dinophyceae or dinoflagellates, are algae that
are unicellular with two flagella of unequal length contained
in channels on the cell surface. They can change
shape with different water temperatures and are very
tolerant of chemical and physical conditions.
The Phaeophyceae or brown algae, are the largest
and most complex of the algae. All members are multicellular,
and the majority live in marine environments,
especially common in cool water along temperate coastlines.
Many of the marine seaweeds are brown algae.
The Rhodophyceae or red algae, are more recent
and have lost their flagellate stages in their life cycle.
Some species are actually black and not red, as those
that live deeper in waters because of different levels of
the pigment phycoerythrin. While most are found in
warm coastal waters of the tropical oceans, some also
live in freshwater and on land in the soil. Most
rhodophytes are multicellular, sharing seaweed status
with brown algae.
Because the various forms of algae are now
assigned to different phyla, the words alga and algae
are used informally and have no taxonomic status.
