Phaeophyta (Brown Algae) - Characteristics, Occurrence, Thallus Organization, Structure, Reproduction

Phaeophyta are commonly known as brown algae. They are mostly marine forms. The members are multicellular and they occur mainly in cold and temperate coastal water.

Very few members are freshwater forms. The colour of the plant body is brown or olive green. This is due to excess amount of fucoxanthin pigment.

The chief pigments are chlorophyll a, chlorophyll c, carotenoids and fucoxanthin. The brown colour of fucoxanthin masks the green colour of chlorophyll. So the members appear brown in colour.

Unicellular and colonial forms are absent in Phaeophyta. All members are multicellular. The plant body may be small filamentous tuft like or very large kelp like body.

The plant body is thalloid. True root, stem and leaves are absent. But the thallus is differentiated into holdfast, stipe and lamina or blade.

Holdfast is root like structure. It fixes the plant body with rocks or sea floor. Stipe is stem like part and it supports the blade.

Blade or lamina is leaf like part. It is the main photosynthetic region. In some brown algae, air bladder like structure are present.

These air bladders are called pneumatocysts. They keep the thallus floating in water. Due to this the blade gets proper sunlight.

The cell wall is made up of two layers. Inner layer is cellulosic. Outer layer is gelatinous and contains alginic acid or algin.

Reserve food material is not true starch. The reserve food is mainly laminarin and mannitol. These are stored inside the cells.

Brown algae are important in marine ecosystem. They act as producer in sea. They also give food, oxygen and shelter to many marine animals.

Large brown algae form kelp forest. This forest is important habitat for fishes and other marine organisms. It also protects many small animals.

Some members of Phaeophyta are used as food. They are used as manure in agricultural field. Alginate is obtained from them and it is used in food, cosmetic and pharmaceutical industries.

General Characteristics of Brown Algae (Phaeophyta)

Brown algae are mostly marine forms. They grow mainly in cold and temperate coastal water. Very few members are found in freshwater.
The members of Phaeophyta are strictly multicellular. Unicellular and colonial forms are absent in this group.

The plant body shows great variation in size. Some members are small microscopic and feathery filamentous tuft like. Some members are very large, as giant kelps.
Giant kelps may reach about 60 to 100 meters in length. So brown algae include the largest algae of sea.
The colour of brown algae is greenish brown to olive green. This colour is due to excess golden brown pigment fucoxanthin.

Fucoxanthin masks the colour of other pigments. Other pigments are chlorophyll a, chlorophyll c and beta-carotene.
The plant body is called thallus. True root, stem and leaves are absent. But the thallus is differentiated into root like, stem like and leaf like parts.
The root like part is called holdfast. It fixes the plant body with rocks or sea floor. It is mainly for attachment.

The stem like part is called stipe. It supports the upper part of the thallus. The leaf like part is called blade or lamina.
Blade or lamina is the main photosynthetic part. In many members, gas filled bladder are present. These are called pneumatocysts.
Pneumatocysts help the fronds to float in water. Due to this the plant body remains near sunlight.

The cell wall is two layered. Inner layer is made up of cellulose. Outer layer is gummy and made up of alginic acid, algin and fucoidan.
Brown algae do not store true starch. The reserve food material is laminarin and mannitol. These are stored as food inside the cells.
Reproduction takes place by vegetative, asexual and sexual methods. Vegetative reproduction occurs by fragmentation. Asexual reproduction occurs by motile spores.

Sexual reproduction occurs by fusion of gametes. It may be different type in different members.
Motile reproductive cells are generally biflagellate. The two flagella are unequal and laterally attached.
One flagellum is long, hairy and tinsel type. It is directed forward. Other flagellum is short, smooth and whiplash type and directed backward.

Life-cycle of most brown algae shows alternation of generations. In this life-cycle, haploid gametophyte and diploid sporophyte phase are present.

Classification of Phaeophyta

Phaeophyta are classified on the basis of life-history, morphology and reproductive structures. The classification given by Kylin (1933) is important. In this classification brown algae are divided into plants without alternation of generation and plants with alternation of generation.

Plants without alternation of generations

This group includes brown algae in which alternation of generations are absent. The main plant body is diploid. The haploid stage is represented only by gametes.

Class Cyclosporeae includes the members without alternation of generations. The important order under this class is Fucales.
Order Fucales has complex diploid plant body. The growth is apical type. Sexual reproduction is oogamous type.
In Fucales, meiosis takes place during formation of gametes. So it shows gametic meiosis. Example- Fucus and Sargassum.

Plants with alternation of generations

This group includes brown algae in which alternation of generations are present. The plants show haploid gametophyte and diploid sporophyte phase.
This group is divided into two main classes. These are Isogeneratae and Heterogeneratae.

Class Isogeneratae

Isogeneratae includes the members having alternation of macroscopic generations. The two generations are usually isomorphic. That means both generations are almost similar in appearance.

The important orders of Isogeneratae are Ectocarpales, Sphacelariales, Cutleriales, Tilopteridales and Dictyotales.
Order Ectocarpales has thallus which may be branched filamentous, pseudoparenchymatous or true parenchymatous. Growth is generally intercalary. Sexual reproduction is mostly isogamous, but sometimes anisogamous also.
In Ectocarpales, alternation of generations is isomorphic type. The gametophyte and sporophyte are similar in structure. Example- Ectocarpus.

Order Sphacelariales includes short and densely branched tufted plants. The thallus shows parenchymatous structure and radial organization. Apical growth is distinct.
Order Cutleriales shows macroscopic alternating generations. But the generations are not always isomorphic. It shows transition towards heteromorphic type of alternation.
Order Tilopteridales resembles Ectocarpus in habit. The plants are mostly deep-water forms. Thallus is filamentous and shows marked trichothallic growth.

In Tilopteridales, reproduction takes place by monosporangia. These monosporangia produce quadrinucleate monospores.
Order Dictyotales has parenchymatous thallus. Growth is apical. Asexual reproduction occurs by non-motile tetraspores.
In Dictyotales, sexual reproduction is oogamous type. Alternation of generations is isomorphic. Example- Dictyota.

Class Heterogeneratae

Heterogeneratae includes the members having heteromorphic alternation of generations. The sporophyte and gametophyte are different in size and structure.
In this class, sporophyte is generally large and macroscopic. Gametophyte is generally small and microscopic. This class is divided into two subclasses.
The two subclasses are Haplostichineae and Polystichineae.

Subclass Haplostichineae

Haplostichineae includes the members in which thallus is made up of pseudoparenchymatous tissue. The important orders are Chordariales, Sporochnales and Desmarestiales.

Order Chordariales has pseudoparenchymatous plant body. Sporophyte shows trichothallic growth. It produces unilocular sporangia.
Order Sporochnales includes deep-water brown algae. The plants have special meristematic tip structure. Alternation is between macroscopic sporophyte and filamentous gametophyte.
Order Desmarestiales shows alternation between large erect sporophyte and microscopic gametophyte. The growth starts from a single filament.

Subclass Polystichineae

Polystichineae includes the members in which thallus is made up of true parenchymatous tissue. The important orders are Dictyosiphonales and Laminariales.
Order Dictyosiphonales has parenchymatous thallus. Longitudinal and transverse intercalary cell division occurs in the thallus.
In Dictyosiphonales, alternation is usually between macroscopic sporophyte and microscopic gametophyte. So the generations are heteromorphic.

Order Laminariales includes large parenchymatous brown algae. The plant body is well differentiated into holdfast, stipe and lamina.
In Laminariales, morphological and anatomical differentiation is very clear. Sexual reproduction is oogamous type. Asexual reproduction takes place by zoospores.
Laminariales shows heteromorphic alternation of generations. The sporophyte is large and dominant. The gametophyte is small and microscopic. Example- Laminaria and Macrocystis.

Types of Common Brown Algae

Large Kelps (Order Laminariales)

Macrocystis is commonly known as giant kelp. It is very large brown alga. It may reach about 60 to 100 meters in length.
Macrocystis forms large underwater forest in sea. These kelp forests give shelter and habitat to many marine animals. So it is ecologically very important.
Laminaria is a large deep-water kelp. Some common forms are sugar kelp and oarweed. It is used as food in many places.

Laminaria is also used as kombu. It is harvested for industrial purpose also. Alginate is obtained from this alga.
Nereocystis is commonly called bull kelp. It grows very fast. It has long stalk and one large gas filled bladder.
The gas bladder of Nereocystis keeps the fronds floating in water. Due to this the upper part gets proper light.

Postelsia is commonly called sea palm. The plant body looks like a small palm tree. It grows attached on rocks where strong waves strike.
Postelsia can live in wave beaten rocks. So it is adapted to heavy crashing wave condition.

Rockweeds and Gulfweeds (Order Fucales)

Sargassum is commonly called gulfweed. It is highly branched brown alga. It may be attached or free floating in the sea.

Sargassum has small gas bladders. These bladders help the plant body to float. It may form large dense floating mats in open ocean.
Large mass of Sargassum is found in Sargasso Sea. This floating mass gives shelter to many small marine organisms.
Fucus is a common rockweed. Fucus vesiculosus is known as bladderwrack. It is generally attached to rocks in intertidal coastal zone.

Fucus is strong and rough seaweed. It can tolerate changing temperature and changing water level. So it is common in tidal region.
Ascophyllum is a common coastal brown alga. It is harvested in large amount for commercial use. It is used as fertilizer and agricultural spray.
Ascophyllum is also important for coastal ecosystem. It gives cover and food to many small marine animals.

Reproduction in Phaeophyceae

Vegetative Reproduction

Vegetative reproduction in Phaeophyceae takes place by simple methods. It occurs mainly by fragmentation and by special propagules.
Fragmentation is the common method of vegetative reproduction. In this method, the thallus breaks into small pieces. Each piece attaches and grows into new plant.
Fragmentation is commonly found in Fucus. It is also found in floating mats of Sargassum. Detached parts continue their growth in suitable place.

Some brown algae produce special branches called propagules. These are small starch rich branches. They separate easily from the parent plant.
Propagules grow directly into new plants. The new plants are similar to the parent plant. This type is found in Sphacelariales and Sargassum.

Asexual Reproduction

Asexual reproduction in brown algae mostly takes place by spores. In many members, motile zoospores are produced.

The zoospores are pear shaped and motile. They have two unequal flagella. The flagella are laterally attached.
The spores are formed inside sporangia. There are two main types of sporangia, such as unilocular sporangia and plurilocular sporangia.
Unilocular sporangium is single chambered. It produces haploid spores by meiosis. These spores later give rise to haploid plant body.

Plurilocular sporangium is many chambered. It produces diploid spores by mitosis. These spores produce diploid plant body.
In some groups, non-motile spores are formed. These spores are called aplanospores or tetraspores. This condition is seen in Dictyotales.
Asexual reproduction is absent in some brown algae. It is completely absent in Fucales. In this order, reproduction is mainly sexual.

Sexual Reproduction

Sexual reproduction in Phaeophyceae takes place by fusion of gametes. It may be isogamous, anisogamous or oogamous type.
In isogamy, two similar motile gametes fuse together. They are same in size and shape. This type is found in Ectocarpales.
In anisogamy, two motile gametes are unequal. Female gamete is generally larger than male gamete. This type is found in Cutleriales.

In oogamy, the female gamete is large and non-motile. The male gamete is small and motile. It is the most advanced type of sexual reproduction.
Oogamous reproduction is found in Fucales and Laminariales. In this type, sperm swims and fuses with egg.
The male gametes are formed in antheridia. The female eggs are formed in oogonia. These are the special reproductive organs.

In Fucus, the sex organs are present inside special cavities. These cavities are called conceptacles. They protect the reproductive organs.
During oogamous reproduction, the egg may secrete chemical substances. These chemicals attract the motile sperm. This helps in fertilization.
Some important pheromones are dictyotene, lamoxirene and fucoserratene. These are released by egg or female structure for attracting male gametes.

Life Cycles

Brown algae show alternation of generations in most members. In this life-cycle, haploid gametophyte and diploid sporophyte are present.
The gametophyte produces gametes. The sporophyte produces spores. The two generations may be similar or different in appearance.
In isomorphic type, the gametophyte and sporophyte look similar. They cannot be easily separated by external appearance. This type is found in Ectocarpus and Dictyota.

In heteromorphic type, the two generations are different. The diploid sporophyte is large and macroscopic. The haploid gametophyte is small and microscopic.
Heteromorphic life-cycle is found in Laminaria. In this, large kelp body is sporophyte. The gametophyte is minute thread like body.
In diplontic type, free living haploid gametophyte is absent. The main plant body is diploid. Meiosis takes place only during gamete formation.

Diplontic life-cycle is found in Fucus. Here the diploid plant produces gametes directly. After fertilization, the zygote again forms diploid plant body.

Life Cycles of Brown Algae

Brown algae have alternation of generations. In this life-cycle two phases are present. One is haploid phase and other is diploid phase.
Haploid phase is the gametophyte. It is 1n. It produces gametes.

Diploid phase is the sporophyte. It is 2n. It produces spores.
Life-cycle of brown algae is of three main types. They are isomorphic haplodiploid, heteromorphic haplodiploid and diplontic.
- Isomorphic haplodiploid life-cycle– In this type, gametophyte and sporophyte are same looking. They are similar in external form. Difference is known only from reproductive parts or chromosome number. Example- Ectocarpus and Dictyota.
- Heteromorphic haplodiploid life-cycle– In this type, gametophyte and sporophyte are different looking. Sporophyte is large and visible. Gametophyte is small, microscopic and filamentous. Example- Laminaria and giant kelps.
- Diplontic life-cycle– In this type, free living haploid plant is absent. Main plant body is diploid. Meiosis occurs during gamete formation only. Example- Fucus and Sargassum.
Meiosis occurs in sporophyte. It takes place in unilocular sporangia and haploid spores are formed. In diplontic members, meiosis forms haploid gametes directly.

Fertilization is fusion of male and female gametes. It is also called syngamy. By this diploid zygote is formed. Zygote grows into new sporophyte.

Economic Importance of Brown Algae

Brown algae are used as human food. Laminaria, Saccharina japonica and Undaria pinnatifida are some important edible forms. Saccharina japonica is called Kombu and Undaria pinnatifida is called Wakame. They are used as vegetables, soup, salad and sushi in East Asian countries.
Brown algae are also used as animal food. They are made into meal and fodder. This is used for cattle and poultry and also in fish culture for increasing immunity and survival of fishes like rainbow trout.

The cell wall of brown algae contains alginic acid. It is also called alginate. It is commercially extracted and used as thickening, stabilizing, emulsifying and gelling agent in ice cream, jellies, sauces, cosmetics, toothpastes, soaps, textile and paper printing industry.
Brown algae are used as fertilizers. Macrocystis gives potash to the soil. Liquid seaweed extract is used as agricultural spray. It gives nutrients and biostimulants and helps in root growth, crop yield and resistance against drought and frost.
Brown algae are important in medicine because they contain iodine. Fucus and Laminaria are used in thyroid disorder. They are used to prevent and treat goiter.

Brown algae contain fucoidan and phlorotannins. These are bioactive compounds. They show antioxidant, antibacterial and anti-inflammatory property and are used in immunomodulatory drugs and also studied in cancer treatment.
Alginate is used in medical and dental field. It is used for wound dressing. It is also used for making dental impression mould.
Mannitol is obtained from brown algae. It is used in pharmaceutical field. It is used as diagnostic agent, osmotic diuretic for reducing intracranial pressure and anti-depressive medicine.

Brown algae have high carbohydrate and fast growth. So they are used in research for making biofuel and biodegradable bioplastics.
Brown algae have some industrial uses also. Alginic acid is used as stable anode component in lithium-ion batteries. Extract of Dictyota is used in anti-fungal food packaging and natural insect repellent for forestry.

Examples of Brown Algae and their Characteristics

Macrocystis– Macrocystis is commonly called giant kelp. It is the largest alga. Macrocystis pyrifera may reach 60 to 100 meters in length and forms large kelp forest in sea. It has sieve like elements and shows very fast growth, about half meter per day.

Fucus– Fucus is commonly called rockweed or bladderwrack. It is strong seaweed and found attached to rocks in coastal and intertidal region. Gas filled pneumatocysts are present. It shows diplontic life-cycle and sex organs are present inside conceptacles.
Sargassum– Sargassum is commonly called gulfweed. It is highly branched brown alga and many forms are free floating in open sea, mainly in Sargasso Sea. Gas bladders are present for floating and large mats give shelter to marine animals. It also reproduce by fragmentation and propagules.
Laminaria– Laminaria is a kelp with differentiated thallus. It has holdfast, stipe and blade. It shows heteromorphic life-cycle, where sporophyte is large and gametophyte is small microscopic filament. It is used as food like Kombu and also for alginate extraction.

Ectocarpus– Ectocarpus is a small filamentous brown alga. It grows as feathery tuft in shallow water and also as epiphyte. It shows isomorphic life-cycle, where gametophyte and sporophyte look same externally. It is used as genetic model due to small size and fast life-cycle.
Dictyota– Dictyota is found mainly in tropical and subtropical water. The thallus is flat, sheet like or ribbon like and branched. Growth takes place by single apical cell. It produces terpenoids which protect the thallus from marine herbivores.
Padina– Padina is a special brown alga. It is the only known calcified brown alga. The thallus is fan shaped or leafy rolled blade like. Calcium carbonate is deposited as aragonite needles and forms crust on the blade surface.

Postelsia– Postelsia is commonly called sea palm. It looks like small palm tree and grows up to about 60 cm in height. It grows on rocky place where strong waves strike. So it is adapted to high wave exposed habitat.
Heribaudiella fluviatilis– Heribaudiella fluviatilis is a rare freshwater brown alga. It forms small dark brown crusts on stones in fast flowing cold rivers. It needs clean water, so it is used as bioindicator of good water quality.
Bodanella lauterborni– Bodanella lauterborni is a very rare freshwater brown alga. It was first known from deep glacial Alpine lakes and also found in freshwater springs of Southeast Europe. It forms brownish black cushion like growth made up of microscopic creeping uniseriate filaments.

Ectocarpus; A- Thallus organization, B- Unilocular sporangium and C- Plurilocular sporangium

Sargassum: A- Thallus, B- Transverse section (T.S.) of main axis, C- Vertical section (V.S.) of leaf.

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Phaeophyta (Brown Algae) – Characteristics, Occurrence, Thallus Organization, Structure, Reproduction