Phaeophyta

General characters Of Phaeophyta

The colour of brown algae varies from olive green to a deep shade of brown due to presence of golden brown xanthophyll pigment fucoxanthin. This is in addition to chlorophyll a, chlorophyll c,B-carotene and C-carotene and other xanthophylls. The cells are uni-nucleated.

The large thalli are tough, leathery or rubbery in texture.

The thalli of brown algae secrete abundant mucilage which readily absorbs moisture.

Reserve photosynthetic products are mannitol and laminarin.

Motile reproductive cells are commonly found in the brown algae, generally they are pyriform,spindle- shaped and biflagellated.

The two flagella are unequal and are inserted laterally,one is whiplash and the other is tinsel type.

Sexual reproduction ranges from isogamy to oogamy through anisogamy but oogamy however,is the general rule.

Distribution of phaeophyta

There are only few fresh water species i.e. Pleurocladia
The marine forms are found in the shallow waters along the coasts of all seas but attain greatest development both in number and large size in the cold waters of the ocean.

They are benthoic and grow as lithophytes attached by holdfasts to the rocks and stones.

The algin coating protects them from dessication by retaining sufficient amount of water.

The brown algae are almost exclusively marine.

The brown algae predominate in the littoral( the part of shore which is twice covered and uncovered by the tides) flora of the cold waters.

They are best developed in the shallow water between low and high tide marks.

Thallus organization in phaeophyta

The members of phaeophyta are multicellular and sessile.

They have a definite form and are both the largest and the most rugged of the algae.

They display the highest degree of body differentiation.

Among the higher members like Dictyota there is a complex and multicellular plant body.It is differentiated into a lower more or less cylindrical portion and an upper dichotomously branched flattened portion which is commonly known as frond.

The cylindrical portion is fixed to substratum by means of rhizoids.Internally the thallus lack any tissue differentiation. It is composed of three layers of cells.

Thallus Organization in phaeophyta

Next in the order of complexity are the fucales.

The plant body consists of a forked, flattened,band- shaped upper part known as the blade and a more less cylindrical lower part called stipe.

The most advanced forms are laminariales show much greater morphological and anatomical differentiation.The main plant body is sporophytic.The gametophytes are comparatively minute,filamentous structures.The sporophytes are differentiated into two main regions.These are the cylindrical stipe or stalk and the flattened blade or lamina.

Structure of the cell

The phaeophycean cell consists of a cell wall enclosing a tiny mass of protoplast.

Cell wall consists of two layers, the inner of cellulose and outer of gelatinous and pectic material. The outer layer is usually thin and reticular in nature. The cellulose layer consists of fine parallel microfibrils. It contains glucose and xylose as sub-units. Due to the presence of the pectic compounds in the outer layer, the cell wall becomes gelatinous or gummy.

Within the cell wall is the protoplast. It is usually differentiated into cytoplasm, single nucleus, one or more vacoules, and often more than one plastid. In addition, there is the presence of ribosomes, endoplasmic reticulum and dictyosomes( Golgi apparatus).

The cytoplasm contains non- living inclusions which are of the nature of food reserves.

Structure of the cell

The important food reserve of the brown algae is a complex carbohydrate,laminarin
Mannitol is another photosynthetic product.

Embedded in the cytoplasm are usually more than one plastids.These are known as chromatophores.

The brown colour of the chromatophores is due to the presence of golden-brown xanthophyll pigment known as fucoxanthin or phycoxanthin.

Asexual Reproduction

It takes place both by vegetative methods and spore formation.

Vegetative reproduction is either by Fragmentation or by propagule formation.

Fragmentation– A portion of the thallus known as the fragment breaks away from the parent thallus and grows into a new individual.i.e. some species of the genus Sargassum
Propagula- Sphacelaria produces special, adventitious branches called the propagules or propagula. The propagule may be wedge- like to di- or triradiate.It has long or short stalk. Each propagule starts as a normal outgrowth. later the apical cells divide vertically and the propagule becomes a bi- or tridadiate branch system. The propagula are absciassed from the parent plant. The detached propagule grows into a new plant.

Spore formation– Asexual reproduction in the ectocarpales take place by the formation of spores which are differentiated by mitosis and thus belong to the category of mitospores. Being motile they are called zoospores.

The zoospores are produced within well- defined multicellular asexual reproductive structures called the plurilocular zoosporangia borne upon the diploid or sporophyte plants.

Sexual reproduction

Sexual reproduction in varies from isogamy,anisogamy and oogamy in phaeophyta. But oogamy is the general rule.

Isogamy– Ectocarpales and Sphacelariales are isogamous in nature.The gametophytic plants may be homo- or heterothalic.The isogametes which look and behave alike are of equal size and resemble zoospores in morphology.

Physiological anisogamy– In this the gametes are morphologically similar but show distinction in their behaviour at the time of gametic union and thus exhibit physiological anisogamy.e.g. some members of order Ectocarpales.

Morphological anisogamy-In this the fusing motile gametes are of unequal size. The smaller gamete is called male or microgamete.It has a single chromatophore.The larger gamete is called female or macrogamete.Both are motile and are produced in multicellular morphologically different gametangia.

Oogamy-Majority of the brown algae are oogamous.The male sex organ is called the antheridium and female oogonium.

•Meiospore formation- The zygote formed as a result as a result of fertilization usually germinates immediately without undergoing a period of rest. There is no zygotic meiosis. The resulting individual is a diploid sporophyte.

Alternation of generation

There is distinct alternation of generation in the sexual life cycle of the phaeophyta with meiosis during spore formation.

In majority of the cases it is isomorphic.e.g. Dictyota And in others can be heteromorphic. i.e. Laminaria.

Economic importance

The brown algae provide food and shelter for fish and other marine animals.

Many species of the brown algae are rich in minerals,and contain carbohydrates and vitamins.

Kombu which is a standard food in Japan and a product of Laminaria and other kelps may be served as a vegetable or made into a confection by coating with sugar.

Kelps is an important source of iodine,mineral salts, bromine and potash.

A viscous, colloidal  preparation,algin obtained from the brown algae (Macroxystis and Laminaria) is used in a number of industrial processes.