2
Algae
CLASSIFICATION
Over the past 30 years, molecular phylogenetic studies have led to extensive modification of traditional classification schemes for algae; nowadays no easily definable classification system acceptable to all exists for this group of organisms, since taxonomy is under constant and rapid revision
at all levels following everyday new genetic and ultrastructural evidence. Keeping in mind that the
polyphyletic nature of the algal group is somewhat inconsistent with traditional taxonomic groupings, though they are still useful to define the general characteristics and levels of organizations,
and aware of the fact that taxonomic opinion may change as information accumulates, we will adopt
a tentative scheme of classification mainly based on the most recently published classifications. In
particular, we will integrate the most recent publications on revised classifications of eukaryotes
and specific groups to obtain a classification scheme highlighting the presence of algae in the four
kingdoms of Bacteria, Plantae, Chromista, and Protozoa. The main purpose of the classification
here reported is to categorize the diversity of the algae in a very practical manner, providing names
useful for teaching students and searching the literature.
Prokaryotic members of this assemblage are grouped into the kingdom Bacteria, phylum Cyanobacteria, with the single class of Cyanophyceae. Members of the proposed division
Prochlorophyta, considered artificial, are currently included in this class.
Eukaryotic members are grouped into the three kingdoms of Plantae, with four phyla, Chromista,
with four phyla, and Protozoa, with two phyla. Table 1.1 shows the different classes comprised in the
11 phyla. Figure 1.1 shows examples of representatives of each class.
OCCURRENCE AND DISTRIBUTION
Algae can be aquatic or subaerial, when they are exposed to the atmosphere rather than being submerged in water. Aquatic algae are found almost everywhere from freshwater spring to salt lakes,
with tolerance for a broad range of pH, temperature, turbidity, O 2 , and CO 2 concentration. They can
be planktonic, as most unicellular species do, living suspended throughout the lighted regions of all
water bodies including under ice in polar areas. They can also be benthonic, attached to the bottom
or living within sediments, limited to shallow areas because of the rapid attenuation of light with
depth. Benthic algae can grow attached on stones (epilithic), on mud or sand (epipelic), on other
algae or plants (epiphytic), or on animals (epizoic). In the case of marine algae, other terms can also
be used to describe their growth habits, such as supralittoral, when they grow above the high-tide
level, within the reach of waves and spray; intertidal, when they grow on shores exposed to tidal
cycles; or sublittoral, when they grow in the benthic environment from the extreme low-water level
to around 200-m deep, in the case of very clear water.
Oceans covering about 71% of the earth’s surface contain more than 5000 species of planktonic
microscopic algae, the phytoplankton, which forms the base of the marine food chain and produces
roughly 50% of the oxygen we inhale. However, phytoplankton is not only a cause of life, but also
sometimes a cause of death. When the population becomes too large in response to pollution with
nutrients such as nitrogen and phosphate, these blooms can reduce the water transparency, causing the
death of other photosynthetic organisms. They are often responsible for massive fish and bird kills,
producing poisons and toxins. The temperate pelagic marine environment is also the realm of giant
algae, the kelp. These algae have thalli up to 60-m long, and the community can be so crowded that
it forms a real submerged forest; they are not limited to temperate waters, as they also form luxuriant
thickets beneath polar ice sheets, and can survive at very low depth (more than 200 m), where the
faint light is bluish-green and its intensity is only 0.0005% that of surface light. At these depths, the
red part of the sunlight spectrum is filtered out from the water and not enough energy is available for
photosynthesis. These algae can survive in the dark blue sea since they possess accessory pigments
that absorb light in spectral regions different from those of the green chlorophylls a and b and channel this absorbed light energy into chlorophyll a, which is the only molecule able to convert sunlight
Algae
CLASSIFICATION
Over the past 30 years, molecular phylogenetic studies have led to extensive modification of traditional classification schemes for algae; nowadays no easily definable classification system acceptable to all exists for this group of organisms, since taxonomy is under constant and rapid revision
at all levels following everyday new genetic and ultrastructural evidence. Keeping in mind that the
polyphyletic nature of the algal group is somewhat inconsistent with traditional taxonomic groupings, though they are still useful to define the general characteristics and levels of organizations,
and aware of the fact that taxonomic opinion may change as information accumulates, we will adopt
a tentative scheme of classification mainly based on the most recently published classifications. In
particular, we will integrate the most recent publications on revised classifications of eukaryotes
and specific groups to obtain a classification scheme highlighting the presence of algae in the four
kingdoms of Bacteria, Plantae, Chromista, and Protozoa. The main purpose of the classification
here reported is to categorize the diversity of the algae in a very practical manner, providing names
useful for teaching students and searching the literature.
Prokaryotic members of this assemblage are grouped into the kingdom Bacteria, phylum Cyanobacteria, with the single class of Cyanophyceae. Members of the proposed division
Prochlorophyta, considered artificial, are currently included in this class.
Eukaryotic members are grouped into the three kingdoms of Plantae, with four phyla, Chromista,
with four phyla, and Protozoa, with two phyla. Table 1.1 shows the different classes comprised in the
11 phyla. Figure 1.1 shows examples of representatives of each class.
OCCURRENCE AND DISTRIBUTION
Algae can be aquatic or subaerial, when they are exposed to the atmosphere rather than being submerged in water. Aquatic algae are found almost everywhere from freshwater spring to salt lakes,
with tolerance for a broad range of pH, temperature, turbidity, O 2 , and CO 2 concentration. They can
be planktonic, as most unicellular species do, living suspended throughout the lighted regions of all
water bodies including under ice in polar areas. They can also be benthonic, attached to the bottom
or living within sediments, limited to shallow areas because of the rapid attenuation of light with
depth. Benthic algae can grow attached on stones (epilithic), on mud or sand (epipelic), on other
algae or plants (epiphytic), or on animals (epizoic). In the case of marine algae, other terms can also
be used to describe their growth habits, such as supralittoral, when they grow above the high-tide
level, within the reach of waves and spray; intertidal, when they grow on shores exposed to tidal
cycles; or sublittoral, when they grow in the benthic environment from the extreme low-water level
to around 200-m deep, in the case of very clear water.
Oceans covering about 71% of the earth’s surface contain more than 5000 species of planktonic
microscopic algae, the phytoplankton, which forms the base of the marine food chain and produces
roughly 50% of the oxygen we inhale. However, phytoplankton is not only a cause of life, but also
sometimes a cause of death. When the population becomes too large in response to pollution with
nutrients such as nitrogen and phosphate, these blooms can reduce the water transparency, causing the
death of other photosynthetic organisms. They are often responsible for massive fish and bird kills,
producing poisons and toxins. The temperate pelagic marine environment is also the realm of giant
algae, the kelp. These algae have thalli up to 60-m long, and the community can be so crowded that
it forms a real submerged forest; they are not limited to temperate waters, as they also form luxuriant
thickets beneath polar ice sheets, and can survive at very low depth (more than 200 m), where the
faint light is bluish-green and its intensity is only 0.0005% that of surface light. At these depths, the
red part of the sunlight spectrum is filtered out from the water and not enough energy is available for
photosynthesis. These algae can survive in the dark blue sea since they possess accessory pigments
that absorb light in spectral regions different from those of the green chlorophylls a and b and channel this absorbed light energy into chlorophyll a, which is the only molecule able to convert sunlight