REPRODUCTION AND LIFE CYCLE IN ALGAE
Previously we learn that algae vary in size from small microscopic
unicellular forms like chlamydomonas to large macroscopic
multicellular forms like Polysiphonia. The multicellular forms show
great diversity in their organization and include filamentous,
heterotrichous, thalloid and polysiphnoid forms. In this unit we will
discuss the types of reproduction and life cycle in algae taking suitable
representative examples from various groups. Algae show all the three
types of reproduction vegetative, asexual and sexual. Vegetative
method solely depends on the capacity of bits of algae accidentally
broken to produce a new one by simple cell division. Asexual methods
on the other hand involve production of new type of cells, zoospores.
In sexual reproduction gametes are formed. They fuse in pairs to form
Zygote may divide and produce a new thallus or it may secrete a thick
wall to form a zygospore.
3.1 TYPES OF REPRODUCTION
Reproductive processes found in various groups of algae can be broadly divided into three types: vegetative, asexual and sexual methods.
The most common type of reproduction in algae is by binary fission. In
unicellular prokaryotic algae like Anacystis it is the only method of
reproduction found in nature. In filamentous and multicellular forms,
the algae may get broken accidentally into small pieces, -each
developing into a new one. The above methods of propagation are known
As vegetative reproduction.
• Asexual Reproduction
When vegetative reproduction takes place through specialized cells
(other than sex cells), it is descried as asexual reproduction.
Anabaena and Nostoc
The cells accumulate food materials; develop thick walls to become
spores or akinetes (Fig. 3.1). Akinetes can withstand dryness (lack of
water) and high temperature for a long time, but when conditions are
suitable they germinate to form new filaments.
Anabaena showing akinetes.
Filamentous algae (like Ulothrix) may reproduce by producing motile cells called zoospores (Fig. 3.2). The protoplast of a single cell divides many times by mitosis to produce several zoospores.
Formation of zoospores in Ulothrix
Each zoospore has 2-4 flagella with which it swims for sometime and
then settles by its anterior end. It subsequently divides into a lowerEach zoospore has 2-4 flagella with which it swims for sometime and
then settles by its anterior end. It subsequently divides into a lowercell which becomes the holdfast and the upper cell which by further
divisions becomes the vegetative filament. Zoospores are produced in
other algae also.
Asexual reproduction in other algae is described below.
Although this is a unicellular motile algae but it produces zoospores.
The parent cell divides inside the cell –envelop and each daughter cell
develops two flagella each. These zoospores look exactly like the parent
cell except they are smaller in size. When the zoospores are fully
developed the parent cell wall dissolves, releasing them free into the surrounding water (Fig.3.3)
Fig. 3.3 :Formation of zoospores and palmella stage in Chlamydomonas
Sometimes when there is less water outside, zoospores may lose
flagella and round up. These non-motile spores are called
aplanospores which develop into thick walled hypnospores.
On moist soil when zoospores can not be released due to lack of free
water, they get embedded within a gelatinous material formed from
parent cell wall. Such cells do not have flagella but whenever they
become flooded with water they develop flagella and swim away in the water These gelatinous masses containing thousands of non-motile
cells are known as palmella stage of Chlamydomonas.
Oedogonium Zoospores are produced singly in a cell. Each has one
nucleus and a crown of flagella at the apex.
Many zoospores are produced from a single cell, as in Ulothrix. They have single nucleus and 2-4 flagella. Ectocarpus
Zoospores are produced in sporangia which are of following two types:
• Plurilocular Sporangia: The sporangium is made up of many
cells and several biflagellate zoospores are produced (Fig. 3.4).
• Unilocular Sporangia: The sporangium is made up of one cell
which produces single biflagellate zoospore (Fig. 3.4)
Fig. 3.4: Unilocular and Plurilocular sporangia of Ectocarpus
3.1.3 Sexual Reproduction
Sexual reproduction in algae like in other organisms involves the
fusion of two cells from opposite sex called gametes, resulting in the
formation of a zygote. Some basic features of this method of
reproduction are as follows:
Gametes are always haploid and may or may not be different in
morphology. If both the sex cells look alike, they could be male called
plus (+) or female called minus (-) mating types of strains. Gametes
can fuse only when one is plus and the other is minus.
Both of them + and – may be produced by a single parent. This is
called monoecious or homothallic condition. When they come from
different plus or minus thallus types it is called dioecious or
There are three types of gametic fusion (Fig. 3.5).
a. Isogamy: When both the gametes are of the same size and
b. Anisogamy: The two gametes are distinctly different in size or
shape, the larger of the two is minus (female) type.
c. Oogamy: The female gamete, egg or ovum is big in size and has no
flagella hence it is non-motile. Male gametes are flagellated and
highly motile. They are also known as antherozoids,
spermatozoids or sperms.
The male gametes are attracted by the female cells because of special hormones called gamones (a violatile hydrocarbon) produced by them. Fusion of the gametes leads to the formation of a zygote. If the conditions are unsuitable for growth, the zygote may develop a thick wall and become a resting zygospore. Gametes being haploid, are produced by mitosis in a haploid thallus. If the thallus is diploid as in Fucus the reproductive cells undergo meiosis or reduction division to form gamete.
Fig. 3.5: Three types of gametic fusion-isogamy, anisogamy and oogamy.
In haploid thallus, after the fusion of gametes, the diploids zygote
undergoes meiosis during germination. However, in diploid algae a
zygote may divide mitotically and give rise to a diploid thallus (Fucus).
Both haploid and diploid thallus are found in Ulva. They look very
similar in size and shape.
3.2 REPRODUCTION AND LIFE CYCLE
We have given above the basic modes of reproduction in algae. Now we take up some specific algal types to illustrate their life cycle in nature. It is to be noted that the life cycle of an alga is very much controlled by environmental factors like temperature, light, seasons, and availability of nutrient, and also salinity, wave action and periodicity of tides in the case of marine forms. Observations made by people during different times from various geographical locations and sometimes experimentally studied under controlled conditions, give us fairly comprehensive if not a complete picture of the life cycle of an alga.
Sexual reproduction in this alga shows all the three different types
depending on the species (Fig. 3.6). Isogamy is found in C. reinhardii, C. gynogama and C. media.
Fig. 3.6 :Sexual reproduction in Chlamydomonas: Isogamy, anisogamy and oogamy.
Isogamy is of two types:
In clonal population:(cells obtained by the repeated divisions of a
single parent cell) fusion may take place between gametes which are
homothallic or in self compatible strains. For example, fusion occurs
between any two cells of C. gynogama and C. media.
In C. moewusii and C. reinhardii fusion of gametes can take place only
when they come from two different unrelated (heterothallic, self
In many isogamous species the parent cell may divide to produce 16 to
64 biflagellate gametes while in some the adult cells themselves may
directly behave as gametes and fuse.
Anisogamous form of gametic fusion is found in C. braunii. A female
cell divides and produces four large cells. Each of these cells have two
flagella but are less active. The male cells are about 8 in number but
smaller in size.
Oogamy is the advanced type of sexual reproduction found in C.
coccifera. A parent cell discards its flagella and directly becomes a non-
motile egg or ovum. While male parent cell by repeated divisions
produces sixteen male gametes. These are biflagellate and highly
The process of gametic attraction, fusion and related phenomena have
been studied in some detail in the laboratory. Under proper light
condition and carbon dioxide concentration, production of gametes can
be initiated by nitrogen starvation. The formation of male or female
gametes (even in the case of isogamy) is attributed to the varying
concentration of gamones produced by them. The attraction between
gametes was found due to the presence of glycosidic mannose at the
tips of the flagella of one strain which in a complementary way binds
with the substance present in the flagella of the gamete of the opposite
stain. Once this sticking of the flagella of plus and minus gametes
takes place, flagella twist about each other bringing the anterior ends
of the gametes close. This is followed by cellular and nuclear fusion.
The zygote secrets a thick wall and accumulates large amount of food
materials like starch. Lipids and orange – red pigments. It is now
known as zygospore which remains dormant till the environmental
conditions are favourable for its germination
It has been shown that during germination of zygospore meiosis takes
place followed by mitosis resulting in haploid Chlamydomonas cells.
Chlamydomonas is unicellular, haploid and reproduces asexually
many times by forming zoospores. Under unfavourable environmental
conditions it produces gametes which fuse to form diploid zygospore.
During germination reduction division takes place and haploid cells
are formed (Fig. 3.7).
Chlamydomonas is of great interest to biologists. Its study has brought
to light several interesting features of biological importance, some of
which are listed below.
i Presence of DNA in the chloroplasts of the alga
ii Presence of cytoplasmic genes
iii Production of genetic mutations – affecting nutrition,
photosynthesis and production of mutants without flagella or cell
iv Dicovery of gamones and their role in sexual reproduction.
v Presence of isogamy, anisogamy and oogamy in a single genus.
vi Control of reproduction by environmental conditions.
Fig. 3.7: Life cycle of Chlamydomonas
• Alternation of Generations
The type of life cycle of an organism in which reproduction
alternates with each generation between sexual reproduction and asexual reproduction is called alternation of generations. The two
generations are termed as gametophytic and sporophytic
generations. The gametophytic generation is haploid(n) and the
sporophytic generation is diploid(2n)
The fusion of two gametes(n) results in zygote(2n) which on
germination forms the plant / thallus called sporophyte. The
sporophyte in turn produces haploid spores by meiosis. When a
spore germinates it develops into gametophyte which bears male
or female gametes or both on the same plant / thallus.
In some bryophytes the gametophytic generation is more
conspicuous. While in ferns the sporophytic generation is more
prominent. In angiosperms main plant body is sporophyte and
the gametophytic generation is reduced to a few cells. You will see
that all type of situations prevail in algae. In some algae
gametophyte is prominent while in others sporophyte is
Sexual reproduction takes place by means of isogamous,
Fusion takes place only between plus and minus mating types.
The gametes are from different filaments (heterothallic). The
zygote develops a thick wall and remains dormant till the
conditions are favourable for germination. When conditions
become favourable meiosis takes place and 4 – 16 haploid
zoospores are produced which settle down and give rise to
vegetative filaments (Fig. 3.8) It has been found that Ulothrix produce gametes when grown
under long day conditions while short day conditions initiate the
formation of zoospores.
Look at Fig. 3.8 : showing the life cycle of Ulothrix.
Which is the diploid stage of the algae?
The thallus of Ulothrix is haploid and the diploid stage is
represented by the zygote only.
We would like to draw your attention to the fact that in some
species (U. speciosa, u. flcca and and in U. implexa) the zygote
develops into an independent, unicellular, thallus which is
diploid in nature. It produces zoospores asexually by meiosis. The
zoospores develop into haploid filaments.
Thus in Ulothrix two types of life cycles can be distinguished:
The thallus is haploid and only the zygote is diploid e.g. U.
In diplobiontic cycle, the alga consists of a haploid thallus that
produces gametes and a diploid unicellular stalked thallus which
produces zoospores after meiotic division. The two generations –
haploid and diploid, alternate with each other. (alternation of
generations). Because the two thalli are very different in size and
morphology it is known as heteromorphic, diplobiontic life
Fig. 3.8: Life cycle of Ulothrix
uocus has advanced type of reproductive structure, termed as receptacles, which are swollen at the tips of branches (Fig. 3.9 A) Distributed over the surface of each receptacle are small pores, known as ostioles which lead into the cavities, called conceptacles (Fig. 3.9B). Each conceptacle may produce only eggs, only sperms or as in some cases both. A thallus may be unisexual – either having male receptacle or only female ones.
Fig. 3.9 :Fucus : A) Structure of thallus, B) Enlarged receptacle
At the base, inside the conceptacle is a fertile layer of cells which
develops into oogonia (Fig. 3.10). Each oogonium has a basal
stalk cell and an upper cell which undergoes reduction division
and produces eight haploid eggs (Fig. 3.10C and D). These are
liberated in the conceptacle (Fig. 3.10E). Some of the cells inside
the conceptacle produce unbranched multicellular hairs called
paraphyses which emerge out of the ostiole as tufts.
Fig. 3.10: Fucus A) T. S. through female conceptacle showing oogonia, B) T.
S. through male conceptacles showing antheridia, C) structure of an
oogonium, D and E) formation and liberation of eggs.
Fig. 3.11: Formation and developmental stages of a zygote
Antheridia are produced on branched paraphyses inside the
concptacle ( Fig. 3.10B). Each antheridium is like a unicellular
sporangium which divides meiotically and then by further
divisions produced 64 haploid sperms. The biflagellate sperm has
a longer flagellum pointing backwards and a shorter one
projecting towards the front. It has a single chloroplast and a
prominent orange eye spot
The release of the gametes is connected with the sea tides. At low
tide, Fucus fronds shrink due to loss of water, and when such
fronds are exposed to an on coming tide, the eggs and sperms are
released into the surrounding sea water.
The egg of Fucus are known to attract sperms (Fig. 3.11 A and B)
by secreting a gamone. Immediately after fertilization a all is
secreted around the zygote. It has been shown that unfertilized
eggs can develop into germlings parthenogenetically if treated
with dilute acetic acid.
The diploid zygote germinates by producing a rhizoidal outgrowth
on one side. It is later cut by wall formation to form a lower
rhizoidal cell and apical cell (Fig. 3.11 C) which by further
divisions (Fig. 3.11 D and E) gives rise to the Fucus fronds.
Reproduction in algae could be by vegetative method (binary
fission), asexual through specialized cells or sexual by fusion of
two cells from opposite sex called gametes.
The life cycles of Chlamydomonas, Ulothrix and Ficus were
discussed. There are other genera in this group. It should be
noted that algae also exhibit alternation of generations in their life
• Reproduction in algae is by asexual and sexual methods.
• Asexual method involves fission of cells are regeneration of
• Sexual method involves fusion of male gamete and female
gamete resulting in the formation of a zygote.
• The life cycle in algae demonstrates clearly a marked
alternation of generations especially in the higher forms like
Ulva, Laminaria and Ficus.
• Dutta A.C. (1981). Botany for degree students.
Oxford University Press. 909p.
IGNOU. (1991). Indra Gandhi Nationl Open University. Plant
Diversity – Algae.
• Dutta A.C. (1981). Botany for degree students.
Oxford University Press. 909p.
• IGNOU. (1991). Indra Gandhi Nationl Open University. Plant
Diversity – Algae.
Dutta: A. C. (1981). Botany for degree students. Oxford University Press
• IGNOU. (1991). Indra Gandhi National Open University. Plant Diversity.