Algae cell structure, reproduction and life cycle

1. Algae
Cell Ultrastructure
and reproduction
Dr. Renu Jangid

2. ALGAE
• The term algae was first introduced by Linnaeus in 1753 but he had used this term for the plants we know as
bryophytes now a days. It was A. L. de Jusssieu (1789) who delimited the term for the algae only known to
us at present.
• Algae are chlorophyll bearing thalloid plants with no differentiation into tissue or tissue system; however
some algae have advanced complex thalli with slight differentiation of true tissues (Ulva, Sargassum, etc.).
• In India, literature provided the evidence of phycology since 18th century where major interests were on
macroscopic forms of algae. F. E. Fritsch (1907) published a marvelous work on subaerial and fresh water
algae from Ceylon. He published the classification of algae in his book ‘Structure and Reproduction of the
algae’.

3. CELL ULTRASTRUCTURE
• Algal cells are:- Prokaryotic, Eukaryotic and Mesokaryotic.
• Ultrastructure of Prokaryotic :- The prokaryotic algal cell can be divided into two parts- Outer
cellular covering and cytoplasm.
• (A) Outer cellular covering: a) Slime layer or mucilaginous sheath- Is is a characteristic feature
of all cyanobacteria (blue-green algae). In the sheath, fibrils of peptic acid and
mucopolysachharides are arranged reticulately so that the sheath appears to be homogenous. Its
main function is to retain absorbed water to protect the cell from desiccation.
• B)Cell wall- The cell wall is rigid and made up of mucopeptide. It consists of four layers which are
named as L1, L2, L3 and L4.
• c) Plasma membrane- It is made up of lipid bilayer.

4. (B) Cytoplasm:- It is differentiated into chromoplasm and centroplasm.
a) Chromoplasm- It is the outer and peripheral pigmented region. It consists of parallel
photosynthetic lamellae or thylakoids. These lamellae contain chlorophyll a, carotenoids and
phycobilins. The membrane bound organelles are not found in chromoplasm. However, 70s
ribosomes, αgranules, β-granules, structural granules, polyhedral bodies, gas vacuoles etc are
found in chromoplasm. Gas vacuoles are made up of vesicles. These vacuoles provide buoyancy
to the cell.
Centroplasm- It is the central colourless region which consists of chromatin material or DNA
material that is not bounded with histone proteins. Hence no organized nucleus is found.

5. Ultrastructure of Eukaryotic algal cell
• a) Cell wall- It made up of cellulose. In some brown algae, alginic acid is present in their cell wall.
Certain algae, particularly the diatoms possess silicified cell wall. Xylan, agar and carrageenin are
present in cell wall of red algae.
• b) Plasma membrane- It is made up of protein lipid bilayer.
• c) Cytoplasm- Inside the plasma membrane dense cytoplasm is present. In cytoplasm, membrane
bound cell organelles are present. Ribosomes are of 80s type. Cells of most algae contain one
chloroplast per cell with the exception of few species whose cells have more than one chloroplast.
single nucleus is present in most of the algae, but multinucleate eukaryotic algal cell are also found
in considerable number. DNA is bounded with the histone proteins.
• e) Flagella- In motile algal cell, thallus bears flagella which originates from the basal granules or
blepharoplast. It shows a typical 9+2 arrangement.

7. 3. Mesokaryotic cell
The mesokaryotic cell is the intermediate cell of the Prokaryotic cell and the Eukaryotic cell. Dodge has first
used the term mesokaryotic in 1966 for the cell that contains an intermediate nucleus (both eukaryotic and
prokaryotic characters are present). Mesokaryotic cell contains a membrane-bound organized nucleus. The cells
are medium-sized and have membrane-bound cell organelles like – mitochondria, plastids, endoplasmic
reticulum, etc. The organisms having mesokaryotic cells are called mesokaryotes.

8. Plastids and Photosynthetic Pigments
• The most prominent feature of an algal cell is the plastid, which makes an important characteristic of an algal
cell for classification. Plastids which consist of chlorophyll a and chlorophyll b are called
CHLOROPLASTS and the one which lacks chlorophyll b are called CHROMATOPHORES.
• (i) Cup shaped: Chlamydomonas , Volvox
• (ii) Discoid: Chara , Vaucheria , Dinophyceae, Bryopsidophyceae and many diatoms
• (iii) Girdle or C shaped: Ulothrix
• (iv) Ribbed: Volvocales
• (v) Reticulate: Oedogonium , Hydrodictyon and Cladophora
• (vi) Spiral or ribbon shaped: Spirogyra
• (vii) Stellate: Zygnema

10. REPRODUCTION
• Reproduction in algae takes place by vegetative, asexual and sexual methods.
• A. Vegetative Reproduction
• a) Fragmentation- In this process, filament breaks into fragments and each fragment give rise to a new
filamentous thallus. The common examples are Ulothrix, Spirogyra, Oedogonium, Zygnema, Oscillatoria,
Nostoc etc.
• b) Fission- This process is common in desmids, diatoms, and other unicellular algae. The cell divides into two
by mitotic division and then separation occurs through septum formation.
• c) Adventitious branches- Protonema develops in certain algae like Chara and give rise to new thalli when
detached from parent thallus. These adventitious branches develop mainly on the rhizoids. Other examples
include Dictyota and Fucus.
• d) Tubers- Tubers are spherical or globular bodies which are found on lower nodes or rhizoids of Chara.
These tubers when detach from parent plant can give rise to new thalli.

11. f) Budding- In some algae like Protosiphon, budding takes place which results in new individuals.
e) Amylum stars- In Chara, star shaped bodies filled with amylum starch are formed that give rise to new
individual after detaching from the parent plant.
g) Hormogonia- In some cyanobacteria like Nostoc, Cylindrospermum hormogonia develop that may give rise
to new thalli. These hormogonia are of varying lenths and may develop at the place of heterocysts in the thallus.
These hormogones are produced by breakage of filament into two or more cells.
h) Hormospores or hormocysts- Hormospores are thick walled hormogones which are produced in drier
conditions.
Budding
Fission
Fragmentation

12. B. Asexual Reproduction
• a. Zoospores- These are flagellated asexual spores which are formed in zoosporangium or directly from the
vegetative cells. The zoospores may be bi, quadric or multiflagellate. e.g., Chlamydomonas (biflagellate),
Ulothrix, Cladophora (quadriflagellate), Vaucheria, Oedogonium (multiflagellate).
• b. Hypnospores- Hypnospores are thick walled, non flagellated spores with plenty of food reserves. They are
produced under unfavourable conditions by some green algae. They germinate into new plants with return of
favourable environmental conditions. e.g., Chlamydomonas, Protosiphon.
• c. Akinetes- In filamentous forms, certain vegetative cells become thick walled elongated structures called as
akinetes. It can survive under unfavourable conditions and can give rise to new individual on occurrence of
favourable conditions. e.g., Anabaena
Multiflagellate zoospore

13. d. Aplanospores- These are non flagellated thin walled asexual spores that are formed in majority of aquatic
algae by the failure of flagella formation due to some unfavourable conditions.
e. Tetraspores- Tetraspores are non motile asexual spores that are formed in some members of Rhodophyceae
and Phaeophyceae.
f. Monospores- Single spore formed in the sporangia is called monospore. Eg. Rhodophyceae.
g. Autospores- These are actually aplanospores which appear identical to the parent cell. Hence referred as
autospores.
h. Heterocysts- According to some phycologists, heterocysts are sometimes able to reproduce asexually. These
structures are found in blue green algae and depending upon the position in thallus they may be terminal or
interstitial.
i. Auxospores- auxospores are produced in the member of Bacillariophyceae.
j. Carpospores- Carpospores are produced in carposporophyte of red algae.
k. Paraspores- In some members of Rhodophyceae, paraspores are formed that give rise to new individual.

14. l. Statospores- Statospores are produced in the members of Bacillariophyceae and Xanthophyceae. Statospores
formed by diatoms are thick walled.
m. Neutral spores- In some alga, the protoplast of vegetative cells directly functions as spores and these are
called neutral spores. e.g., Asterocystis, Ectocarpus.
n. Nannocytes- In the members of chroococcales, the cell content divide repeatedly to produce numerous very
small spores. The name nannocytes to these very small spores was given by Geitler. E.g., Macrocystis,
Gloeocapsa.
o. Gongrosira stage of Vaucheria- In Vaucheria, the protoplast divides into several cysts like structures or
hypnospores. This stage looks like an algal form ‘Gongrosira’. Each hypnospore or cyst may give rise to new
thallus.

15. C. Sexual Reproduction-
• a) Isogamous- When fusing gametes are morphologically similar and physiologically different (+ and -) then
the sexual reproduction is called as isogamous. E.g., Chlamydomonas, Ulothrix, Zygnema, Spirogyra.
• b) Anisogamous- In anisogamous sexual reproduction fusing gametes are morphologically as well as
physiologically different. The gametes are produced in different gametangia. The microgametes are male
gametes while macrogametes are female gametes. e.g., Chlamydomonas.
• c) Oogamous- Oogamy is the most advanced type of sexual reproduction in which microgamete or male
gamete fuses with a large female gamete or eg. Male gametes are produced in antheridium while female
gamete or egg is produced within a structure called as oogonium. During fertilization male gamete reaches the
oogonium to fertilize the egg and a diploid zygote is formed. e.g., Chlamydomonas.

16. LIFE CYCLES IN ALGAE
• 1. Haplontic Life Cycle- In this type of life cycle the main plant body is gametophytic (haploid). Eg. Green
algae.

17. • 2. Diplontic Life Cycle- The dominant plant thallus is diploid. The thallus reproduces sexually by gametes
that are formed by meiosis in sex organs. Eg. Diatoms

18. 3. Diplohaplontic Life Cycle- In this type of life cycle two different generations alternate each other.
a. Isomorphic- In this life cycle, alternating
sporophyte and gametophyte are
morphologically similar. E.g. Ectocarpus, Ulva,
Cladophora, Dictyota etc.
b. Heteromorphic- In this life cycle, alternating
generations are morphologically dissimilar. E.g.,
Laminaria, Desmarestia etc.

19. 4. Haplobiontic Life Cycle- This is either diphasic or triphasic life cycle. In Nemalion a red
alga exhibits two haploid phases (gametophyte, carposporophyte) and a diploid zygote.
Batrachospermum (red alga) do exhibit haplobiontic life cycle but it is triphasic as it consists of
three prominent haploid phases (main gametophyte, carposporophyte and chatransia phase).
5. Diplobiontic Life Cycle- This life cycle consists three phases of which two phases are diploid
and one is haploid. Tetraspores eventually develop into main gametophytic plant thallus. This
type of life cycle is exhibited by some members of red algae such as Polysiphonia.