Algae  

are  

chlorophyll-bearing,  

simple,  

thalloid,  

autotrophic  

and  

largely

aquatic  

(both  

fresh  

water  

and  

marine)  

organisms.  

They  

occur  

in  

a

variety  

of  

other  

habitats:  

moist  

stones,  

soils  

and  

wood.  

Some  

of  

them

also  

occur  

in  

association  

with  

fungi  

(lichen)  

and  

animals  

(e.g.,  

on  

sloth

bear).The   

size   

ranges   

from   

the   

microscopic   

unicellular   

forms   

like

Chlamydomonas,  

to  

colonial  

forms  

like  

Volvox  

and  

to  

the  

filamentous

forms  

like  

Ulothrix  

and  

Spirogyra.  

A  

few  

of  

the  

marine  

forms  

such  

as

kelps, form massive plant bodies.

The   

algae   

reproduce   

by   

vegetative,   

asexual   

and   

sexual   

methods.

Vegetative  

reproduction  

is  

by  

fragmentation.  

Each  

fragment  

develops

into  

a  

thallus.  

Asexual  

reproduction  

is  

by  

the  

production  

of  

different

types  

of  

spores,  

the  

most  

common  

being  

the  

zoospores.  

They  

are

flagellated  

(motile)  

and  

on  

germination  

gives  

rise  

to  

new  

plants.  

Sexual

reproduction   

takes   

place   

through   

fusion   

of   

two   

gametes.   

These

gametes  

can  

be  

flagellated  

and  

similar  

in  

size  

(as  

in  

Chlamydomonas)

or  

non-flagellated  

(non-motile)  

but  

similar  

in  

size  

(as  

in  

Spirogyra).

Such   

reproduction   

is   

called   

isogamous.   

Fusion   

of   

two   

gametes

dissimilar  

in  

size,  

as  

in  

some  

species  

of  

Chlamydomonas  

is  

termed  

as

anisogamous.  

Fusion  

between  

one  

large,  

non-motile  

(static)  

female

gamete  

and  

a  

smaller,  

motile  

male  

gamete  

is  

termed  

oogamous,  

e.g.,

Volvox, Fucus.

Algae  

are  

useful  

to  

man  

in  

a  

variety  

of  

ways.

At  

least  

a  

half  

of  

the  

total

carbon dioxide fixation on earth is carried out by algae through

photosynthesis.   

Being   

photosynthetic   

they   

increase   

the   

level   

of

dissolved   

oxygen   

in   

their   

immediate   

environment.   

They   

are   

of

paramount     

importance     

as     

primary     

producers     

of     

energy-rich

compounds  

which  

form  

the  

basis  

of  

the  

food  

cycles  

of  

all  

aquatic

animals.  

Many  

species  

of  

Porphyra,  

Laminaria  

and  

Sargassum  

are

among  

the  

70  

species  

of  

marine  

algae  

used  

as  

food.  

Certain  

marine

brown  

and  

red  

algae  

produce  

large  

amounts  

of  

hydrocolloids  

(water

holding   

substances),   

e.g.,   

algin   

(brown   

algae)   

and   

carrageen   

(red

algae)  

are  

used  

commercially.  

Agar,  

one  

of  

the  

commercial  

products

obtained  

from  

Gelidium  

and  

Gracilaria  

are  

used  

to  

grow  

microbes  

and

in  

preparations  

of  

ice-creams  

and  

jellies.  

Chlorella  

and  

Spirullina  

are

unicellular  

algae,  

rich  

in  

proteins  

and  

are  

used  

as  

food  

supplements

even   

by   

space   

travellers.   

The   

algae   

are   

divided   

into   

three   

main

classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.