Algae are simple autotrophic plants
that generally lack root, stem,
leaves, conducting vessels and
complex sex organs.
It refers to a heteromorphic group
of organisms which can
photosynthesize as they contain
chlorophyll pigment and they lack
a sterile covering around their
They are apparently simple group
of organisms but possesses diverse
nature in both morphological and
It is often a hard task for phycologists to define the term algae.
It is so diverse that putting all the common characters to define
algae in a single sentence is almost an impossible task.
To support such claim, we can recall two world famous
phycologists’ statement: “Sometimes even the professional
botanists and biologists find algae embarrassingly elusive
of definition” (Bold and Wynne).
The primitive existence of algae is
supported by the Precambrian fossil
record which dates back to about 3
billion years ago.
According to Guiry (2012) the number
of living algae are estimated to range
from 30,000 to more than one million
M.D. Guiry (born 1949),
According to Lee, “The algae are
thallophytes (plants lacking roots, stems,
and leaves) that have chlorophyll a as their
primary photosynthetic pigment and lack a
sterile covering of cells around the
reproductive cells” (Lee, 1989).
Nomenclature of Algae
The Greek word for algae is ”phykos” and according to
International Code of Botanical Nomenclature (ICBN), a
group of algae should necessarily be incorporated by the word
”phykos”. Because of such incorporation, the words such as
Chlorophyceae, Phaeophyceae and Rhodophyceae indicate an
idea of their relationship with algae.
So, as the words like Chlorophyta, Phaeophyta and Rhodophyta
end with ”phyta”, they are equivalent to the status of a division;
words like Chlorophyceae, Phaeophyceae and Rhodophyceae end
with ”phyceae” and are therefore equivalent to the status of a
class and so on.
BASIS OF CLASSIFICATION
Rarely one, and generally a combination of a few or more of the following
characters are considered by the algal taxonomists while classifying algae:
In normal microscope-
Types of pigments,
Types of reserve food materials
Number and position of flagella
Under electron microscope-
Flagella and types
Chloroplast Endoplasmic Reticulum (CER)
Phycobilisomes (out of 450-760 mm range)
Theca or scales
Projectiles – To hold something/prey/to move.
Nuclear structure and division
Through biochemical analysis-
Presence and structure of algal pigments
Cell wall constituents
Schools of thought
Regarding the algal classification, there are different schools of thought:
First school of thought
• This school of thought is supported by Papenfuss, 1946; Bold and Wynne.
• The algae should be divided first into several divisions (i.e. phyta such as
Chlorophyta, Phaeophyta etc.) and then in each division, there should be
different classes (i.e. phyceae such as Chlorophyceae, Phaeophyceae etc.).
Why separate divisions?
Supporters of the first view classified algae into divisions as they considered:
Pigments differ in different phytas (=divisions).
Product of photosynthesis is different in different divisions.
Base of classification
• Storage product
• Cellular organisation
• Cell wall chemistry
Second school of thought
• This school of thought is supported by Fritsch, 1935, 1945 and his
• Algae is itself equivalent to a division, and therefore it can only be further
divided into classes (i.e. phyceae) and not into divisions (i.e. phyta). So in
this school of thought there is no word as ”phyta”.
Base of classification
As the following characters are common in all algae, Fritsch and his followers
opined not to form any divisions rather considered Algae itself as a division.
Chlorophyll-a is ubiquitous to all algae, and so there is definitely a common
origin from a common source. Therefore they all belong to the same group.
• Internal structure of flagella is similar in all.
• Product of assimilation may be different but the process of assimilation is
same in all.
• Methods of reproduction are almost common in all.
Third school of thought (supported by Lee, 1999)
The above two classifications were proposed based on some
physical and chemical characteristics of algae. There is a
third classification as proposed by Robert Edward
Lee based on the evolutionary features.
This is the most modern classification scheme of algae.
The basis of classification also includes-
• Thyllakoid band
• CER membrane
• Carotenoid pigment
The foundation of classification of algae was laid down by Linnaeus
and De Jussieu.
Linnaeus De Jussieu
Fritsch (1935) in his masterpiece on the
‘Structure and reproduction of algae’,
gave a classification of algae into eleven
classes based on
His classification was based on…
types of flagella,
thallus structure and
methods of reproduction.
Professor Felix Eugen
Fritsch 1879 - 1954
Fritsch system of classification(1935, 48)
Smith (1955) classified algae into seven divisions and each division into
several classes. The divisions are
Phaeophyta, Cyanophyta and Rhodophyta.
Gilbert Morgan Smith
Smith system of classification (1933,51,55)
Papenfuss (1955) classified algae into eight
divisions such as
He included blue-green algae along with
On the basis of the pigments in the
plastid, morphological characters and
Chapman (1962) divided algae into
four divisions, which were further
subdivided into several classes.
The divisions are
Papenfuss system of classification (1955)
On the basis of Phylogeny recognized 8 phyla and 12 classes in algae
Sl No Phyla Classes
1 Chlorophycophyta Chlorophyceae
2 Charophycophyta Charophyceae
3 Euglenophycophyta Euglenophyceae
6 Phaeophycophyta Phaeophyceae
7 Rhodophycophyta Rhodophyceae
8 Schizophycophyta Schizophyceae
Prescott (1969) emphasized the presence or
absence of true nucleus in the algal cells for their
classification along with other characters like
pigmentation, biochemical nature of cell wall
and reserve food material, and divided into nine
divisions and fourteen classes.
The divisions are
Round (1973) also recognized the
importance of presence or absence of
well organised nucleus in algal cells
along with their phylogenetic
relationships and other characteristics.
He divided algae into two major groups
and further divided into several
Group 1 Prokaryota includes only one
Group 2 Eukaryota includes the
following divisions such as
Rhodophyta, Cryptophyta &
Round system of
Bold and Wynne (1978) classified
algae into nine divisions such as
Michael J. Wynne
Vanden Hoek et al.,
(1995) classified algae
ultrastructure of cell,
rRNA sequence data
Lee (1989) classified algae into four groups and several Phyla.
• Group 1 Prokaryotic algae includes phylum Cyanophyta and
• Group 2 Eukaryotic algae with chloroplasts surrounded only
by the two membranes of the chloroplast envelope.
It includes phylum Glaucophyta, Rhodophyta and Chlorophyta.
• Group 3 Eukaryotic algae with chloroplasts surrounded by one
membrane of chloroplast endoplasmic reticulum.
It includes phylum Euglenophyta and Dinophyta.
• Group 4 Eukaryotic algae with chloroplasts surrounded by two
membranes of chloroplast endoplasmic reticulum.
It includes phylum Cryptophyta, Chrysophyta, Prymnesiophyta,
Bacillariophyta, Xanthophyta, Eustigmatophyta, Raphidophyta
Guiry and Guiry (2011) classified
algae into two Empires-
Prokaryota and Eukaryota.
Prokaryota includes only one
kingdom i.e., Bacteria comprising
the phylum Cyanobacteria.
Empire Eukaryota includes three
algal kingdoms such as
Barsanti and Gualtieri (2014) grouped algae in four kingdoms of
Bacteria, Plantae, Chromista and Protozoa.
The prokaryotic kingdom bacteria includes phylum Cyanobacteria
with a single class Cyanophyceae.
Kingdom plantae includes four phyla i.e.,
Glaucophyta, Rhodophyta, Chlorophyta and
Kingdom Chromista includes phylum
Haplophyta, Cryptophyta, Ochrophyta and
Kingdom Protozoa includes phylum
Myzozoa and Euglenozoa.
There are four distinct groups within the algae:
1. Prokaryotes: The cyanobacteria are the only prokaryotic algae.
2. Eukaryotic algae with chloroplasts surrounded by the two
membranes of the chloroplast envelope.
3. Eukaryotic algae with the chloroplast surrounded by one
membrane of chloroplast endoplasmic reticulum.
4. Eukaryotic algae with the chloroplast surrounded by two
membranes of chloroplast endoplasmic reticulum.
The standard botanical classification system is used in the
systematics of the algae:
Phylum – phyta
Class – phyceae
Order – ales
Family – aceae
Group 1 Prokaryotic algae Cyanophyta (cyanobacteria):
• The cyanobacteria or blue-green algae form a natural group by virtue
of being the only prokaryotic algae.
• Prokaryotic algae have an outer plasma membrane enclosing
protoplasm containing photosynthetic thylakoids, 70S ribosomes, and
DNA ﬁbrils not enclosed within a separate membrane.
• Chlorophyll a is the main photosynthetic pigment, and oxygen is
evolved during photosynthesis.
• Phycobiliproteins are present which contains the pigments
phycocyanin and phycoerythrin.
• Examples include: Nostoc, Anabaena, Spirulina, Oscillatoria.
Group 2 Eukaryotic algae with chloroplasts surrounded only by the
two membranes of the chloroplast envelope.
• The Glaucophyta include those algae that have endosymbiotic
cyanobacteria in the cytoplasm instead of chloroplasts.
• Because of the nature of their symbiotic association, they are thought
to represent intermediates in the evolution of the chloroplast.
• Photosynthesis is carried out by modified endosymbiotic
• The pigments of the Glaucophyta are similar to those of the
Cyanophyceae: both chlorophyll a and the phycobiliproteins are
• Examples include: Cyanophora paradoxa, Glaucocystis
Rhodophyta (red algae):
• The Rhodophyceae lack ﬂagellated cells, have chlorophyll a, phycobiliproteins,
ﬂoridean starch as a storage product, and thylakoids occurring singly in the
• The major features of a red algal cell are a chloroplast with one thylakoid per
band and no chloroplast E.R.
• The two most important polysaccharides derived from the Rhodophyceae are agar
• Agar is obtained commercially from species of Gelidium and Pterocladia as well as
from various other algae, such as Acanthopeltis, Ahnfeltia, and Gracilaria
Chlorophyta (green algae):
• The Chlorophyta, or green algae, have chlorophylls a and b, and form starch
within the chloroplast, usually in association with a pyrenoid.
• The Chlorophyta thus differ from the rest of the eukaryotic algae in forming the
storage product in the chloroplast instead of in the cytoplasm.
• No chloroplast endoplasmic reticulum occurs around the chloroplasts.
• Examples include: Chlamydomonas and Spirogyra.
Group 3 Eukaryotic algae with chloroplasts surrounded by one
membrane of chloroplast endo-plasmic reticulum.
• Euglenoids are characterized by chlorophylls a and b, one membrane
of chloroplast endoplasmic reticulum, a mesokaryotic nucleus, one
ﬂagella with ﬁbrillar hairs in one row, no sexual reproduction, and
paramylon or chrysolaminarin as the storage product in the
• proteinaceous pellicle in strips under the plasma membrane;
• Typical example includes Euglena.
Dinophyta (dinoflagellates): mesokaryotic nucleus; chlorophylls a
and c1; cell commonly divided into an epicone and a hypocone by a
girdle; helical transverse flagellum; thecal plates in vesicles under the
Apicomplexa: heterotrophic flagellates with colorless plastids.
Group 4 Eukaryotic algae with chloroplasts surrounded by two
membranes of chloroplast endoplasmic reticulum.
• Nucleomorph present between inner and outer membrane of
chloroplast endoplasmic reticulum;
• Starch formed as grains between inner membrane of chloroplast
endoplasmic reticulum and chloroplast envelope;
• chlorophyll a and c2; phycobiliproteins occur inside the thylakoids of
the chloroplast; periplast inside plasma membrane.
• The cell body is asymmetric with a clearly deﬁned dorsi-ventral/
• The asymmetric cell shape results in a peculiar swaying motion during
• Most cryptophytes have a single lobed chloroplast with a central
• Typical example includes Cryptomonas.
Heterokontophyta (heterokonts) :
• The algae in the Heterokontophyta usually have cells with an anterior
tinsel and posterior whiplash ﬂagellum.
• The plastids contain chlorophylls a and c along with fucoxanthin.
• The storage product is usually chrysolaminarin in cytoplasmic
Chrysophyceae (golden-brown algae):
• The Chrysophyceae are distinguished chemically by having chlorophylls a, c1, and
c2 and structurally by two ﬂagella inserted into the cell perpendicular to each other,
• One photoreceptor on the short ﬂagellum that is usually shaded by an eyespot in the
anterior portion of the chloroplast,
• Contractile vacuoles in the anterior portion of the cell, chloroplast endoplasmic
reticulum, and radially or biradially symmetrical silica scales (if they are present).
• The storage product is chrysolaminarin.
• Many members of the class produce statospores enclosed in a siliciﬁed wall with a
• Example includes Ochromonas, Dinobryon
• The Synurophyceae have chlorophylls a and c1, the ﬂagella are inserted into the cell
approximately parallel to one another,
• There is a photoreceptor near the base of each ﬂagellum, there is no eyespot, and the
contractile vacuole is in the posterior portion of the cell.
• Chloroplast endoplasmic reticulum is present in a few species, but absent in most.
• The cells usually are covered by bilaterally symmetrical scales.
• Example includes Mallomonas
• Characteristics of the class include a basal swelling of the tinsel ﬂagellum adjacent
to the eyespot, only chlorophyll a, chloroplasts without girdle lamellae and no
peripheral ring of DNA, and chloroplast endoplasmic reticulum not connected to the
nuclear envelope; an eyespot outside the chloroplast.
• Examples include Pleurochloris magna, Polyedriella helvetica,
• An anterior tinsel ﬂagellum with tripartite hairs and a posterior smooth whiplash
• The chloroplasts have two membranes of chloroplast endoplasmic reticulum, the
outer membrane continuous with the nuclear envelope.
• A girdle band of thylakoids occurs under the chloroplast envelope.
• Pyrenoids occur in the chloroplast and the mitochondria have tubular cristae.
• Chlorophyll a and chlorophyll c-related pigments as well as fucoxanthin,
violaxanthin, zeaxanthin, and carotene are present.
• The class is characterized by unusually high concentrations of polyunsaturated
acids, especially 20:5 (n-3) (EPAeicosapentaenoic acid) in the cells.
• Example is Phaeomonas parva
• These golden-brown algae are characterized by tentacles or rhizopodia on basically
amoeboid vegetative cells.
• Chrysoamoeba is an example.
Pelagomonas calceolata is a very small ultraplanktonic marine alga with a single tinsel
ﬂagellum and basal body, and a single chloroplast and mitochondrion.
The cells contain chlorophylls a, c1,c2,c3, Beta carotene, diatoxanthin, and fucoxanthin,
as do the diatoms; Bolidomonas is an example.
The cells are surrounded by a rigid two-part box-like cell wall composed of silica,
called the frustule. The chloroplasts contain chlorophylls a, c1, and c2 with the major
carotenoid being the golden-brown fucoxanthin, which gives the cells their
characteristic color. Navicula, Achnanthes longipes
The Raphidophyceae, or chloromonads, have chlorophylls a and c, and two membranes of
chloroplast endoplasmic reticulum. The anterior ﬂagellum is commonly tinsel, whereas the
posterior ﬂagellum is naked. Chattonella, Fibrocapsa, and Heterosigma
Xanthophyceae (yellow-green algae):
• The class is characterized by motile cells with a forwardly directed tinsel ﬂagellum and a
posteriorly directed whiplash ﬂagellum.
• The chloroplasts contain chlorophylls a and c, lack fucoxanthin, and are colored yellowish-
green. The eyespot in motile cells is always in the chloroplast, and the chloroplasts are
surrounded by two membranes of chloroplast endoplasmic reticulum. The outer membrane of
the chloroplast E.R. is usually continuous with the outer membrane of the nucleus.
• In most non-motile cells the wall is composed of two overlapping halves.
• Botrydium and Vaucheria
The cells have two membranes of chloroplast endoplasmic reticulum with the outer
membrane of chloroplast E.R. continuous with the outer membrane of the nuclear envelope. The
chloroplasts have a ring-shaped genophore and girdle lamellae. The ﬂagella are inserted
laterally into the motile cells. Tetrasporopsis
Phaeophyceae (brown algae):
The Phaeophyceae, derive their characteristic color from the large amounts of the carotenoid
fucoxanthin in their chloroplasts as well as from any phaeophycean tannins that might be present.
The chloroplasts also have chlorophylls a, c1, andc2. There are two membranes of chloroplast
E.R., which are usually continuous with the outer membrane of the nuclear envelope. The storage
product is laminarin.
Prymnesiophyta (haptophytes): two whiplash flagella; haptonema present; chlorophyll a and c;
fucoxanthin; scales common outside cell; storage product chrysolaminarin occurring in vesicles.
The above discussion certifies well that the science of classification is a complicated one and in
case of algae it has reached its highest point. This article will be incomplete without the words of
Prescott (1969) about the complexity of classification schemes:
…..ideas concerning classification systems continually vary as more and more facts are disclosed
which require modifications of the previous concepts. Further, we may come to realize that as
long as there are three men on the earth, there will be at least two different interpretations
of the facts, possibly three, and we may find that none of them is correct…
DIVISION CHLOROPHYTA (green algae)
Chlorophylls a and b; starch stored inside chloroplast; mitochondria with flattened
cristae; flagella, when present, lack tubular hairs (mastigonemes); unmineralized scales
on cells or flagella of flagellates and zoospores; conservatively, between 9,000 and
Primarily freshwater; includes Chlamydomonas, Chlorella, Dunaliella, Oedogonium,
Includes the macroscopic stonewort Chara, filamentous Spirogyra, and desmids.
Freshwater and marine; includes marine flagellate Tetraselmis.
Class Prasinophyceae (Micromonadophyceae)
Paraphyletic, primarily marine; includes Micromonas (sometimes placed in
Mamiellophyceae), Ostreococcus, and Pyramimonas.
Primarily marine; includes Acetabularia, Caulerpa, Monostroma, and sea
Most with chlorophyll a; one or two with chlorophyllide c; carotenoids present; storage product
beta-1,3-linked polysaccharide outside chloroplast; mitochondria with tubular cristae; biflagellate
cells and zoospores usually with tubular hairs on one flagellum; mucous organelles common.
Class Bacillariophyceae (diatoms)
Silica cell walls, or frustules; centric diatoms commonly planktonic and valves radially
symmetrical; pennate diatoms, usually attached or gliding over solid substrates, with valves
bilaterally symmetrical; primarily in freshwater, marine, and soil environments; at least 12,000 to
15,000 living species; tens of thousands more species described from fossil diatomite deposits;
includes Cyclotella and Thalassiosira (centrics)
and Bacillaria, Navicula and Nitzschia (pennates).
May be included in the Chrysophyceae or in the protozoan group Zoomastigophora; colourless
flagellate cells in vase-shaped loricas (wall-like coverings); cell attached to lorica using flagellum
as a stalk; lorica attaches to plants, algae, animals, or water surface; freshwater and marine; fewer
than 50 species described; includes Bicosoeca and Cafeteria.
Class Chrysophyceae(golden algae)
Many unicellular or colonial flagellates; also capsoid, coccoid, amoeboid, filamentous,
parenchymatous, or plasmodial; many produce silica cysts (statospores); predominantly
freshwater; approximately 1,200 species; includes Chrysamoeba, Chrysocapsa, Lagynion,
Predominantly marine flagellates, including silicoflagellates that form skeletons
common in diatomite deposits; fewer than 25 described species.
When pigmented, has 6 chloroplasts in a radial arrangement; flagella bases attached
almost directly to nucleus;
includes Apedinella, Actinomonas, Mesopedinella, Parapedinella, and Pteridomonas.
Order Dictyochales (silicoflagellates)
Typically with siliceous skeletons like spiny baskets enclosing the cells; flagella bases
attach almost directly to nucleus; silicoflagellate skeletons common in diatomite
deposits; includes Dictyocha, Pedinella, and Pseudopedinella.
Mostly small, pale green, and spherical; fewer than 15
species; Eustigmatos and Nannochloropsis.
Class Phaeophyceae(brown algae or brown seaweeds)
Range from microscopic forms to large kelps more than 20 metres long; at least 1,500
species, almost all marine;
includes Ascophyllum, Ectocarpus, Fucus, Laminaria, Macrocystis, Nereocystis, Pelago
phycus, Pelvetia, Postelsia, and Sargassum.
Class Prymnesiophyceae (Haptophyceae)
Many with haptonema, a hairlike appendage between two flagella; no tubular hairs;
many with organic scales; some deposit calcium carbonate on scales to
form coccoliths; coccolithophorids may play a role in global warming because they can
remove large amounts of carbon from the ocean water; predominantly marine and
planktonic; approximately 300 species; more fossil coccolithophores known;
includes Chrysochromulina, Emiliania, Phaeocystis, and Prymnesium.
Class Raphidophyceae (Chloromonadophyceae)
Flagellates with mucocysts (mucilage-releasing bodies) occasionally found in
freshwater or marine environments; fewer than 50 species;
includes Chattonella, Gonyostomum, Heterosigma, Psammamonas, and Vacuolaria.
Previously placed in Chrysophyceae; silica-scaled; unicellular or colonial flagellates
sometimes alternating with capsoid benthic stage; cells covered with elaborately
structured silica scales; approximately 250 species; Mallomonas and Synura.
Class Xanthophyceae(yellow-green algae)
Primarily coccoid, capsoid, or filamentous; mostly in freshwater environments; about
600 species; includes Botrydium, Bumilleriopsis, Tribonema, and Vaucheria
Chlorophyll a, chlorophyllide c2, and phycobiliproteins; starch stored outside of
chloroplast; mitochondria with flattened cristae; tubular hairs on one or both flagella;
special ejectosomes in a furrow or gullet near base of flagella; cell covered with
periplast, often elaborately decorated sheet or scale covering; nucleomorph may
represent reduced nucleus of symbiotic organism; approximately 200 described species;
includes Chilomonas, Cryptomonas, Falcomonas, Plagioselmis, Rhinomonas,
DIVISION RHODOPHYTA(red algae)
Predominantly filamentous; mostly photosynthetic, a few parasitic; photosynthetic
species with chlorophyll a; chlorophyll d present in some species; phycobiliproteins
(phycocyanin and phycoerythrin) in discrete structures (phycobilisomes); starch stored
outside chloroplast; mitochondria with flattened cristae; flagella completely absent;
coralline red algae contribute to coral reefs and coral sands; predominantly marine;
approximately 6,000 described species;
includes Bangia, Chondrus, Corallina, Gelidium, Gracilaria, Kappaphycus, Palmaria,
Polysiphonia, Porphyra, and Rhodymenia.
DIVISION DINOFLAGELLATA (PYRROPHYTA)
Taxonomy is contentious. Predominantly unicellular flagellates; approximately half of
the species are heterotrophic rather than photosynthetic; photosynthetic forms with
chlorophyll a, one or more chlorophyllide c types, and peridinin or fucoxanthin;
mitochondria with tubular cristae and flagella without tubular hairs; ejectile trichocysts
below surface in many members; many with cellulosic plates that form a so-called
armour around cell; some bioluminescent, some containing symbionts; resting
(interphase) nucleus contains permanently condensed chromosomes; several produce
toxins that either kill fish or accumulate in shellfish and cause sickness or death in
humans when ingested; more than 1,500 species described, most in the
includes Alexandrium, Ceratium, Dinophysis, Gonyaulax, Gymnodinium, Noctiluca, Pe
ridinium, and Polykrikos.
Taxonomy is contentious. Primarily unicellular flagellates; both photosynthetic and
Chlorophylls a and b; paramylon stored outside chloroplasts; mitochondria with paddle-
shaped cristae; flagella lack tubular hairs, but some with hairlike scales; pellicle
covering of sliding sheets allows cells to change shape; approximately 1,000 described
species; includes Colacium, Euglena, Eutreptiella, and Phacus.