PinkMonkey Online Study Guide-Biology
Economic importance : Algae are of certain economic importance to man. Chlorophyta serve as initial food producers and the first link in the aquatic food chain, both fresh water and marine. Some like Ulva and Chlorella are used as vitaminized food.
Evolutionary significance : It is believed that chlorophyta has evolved from some flagellate unicellular ancestors like Chlamydomonas. These ancestors, during evolution, gave rise to complex filamentous forms which supposed to have given rise to land plants like bryophytes.
Brown algae (Phaeophyta)
These are multicellular, simple, filamentous or plant-like giant forms called kelps (sea weeds). These are mostly marine, found in cool shallow water.
(i) The body is the multicellular thallus showing
the highest degree of differentiation. The unicellular colonial forms
are absent. Some brown algae called kelps or sea weeds exhibit giant forms
extending over 50 meters in length and showing parenchymatous organization.
(ii) The cell wall is made up of two layers. The
inner firm layer is of cellulose while the outer layer is gelatinous containing
compounds like algin, fucosin etc.
(iii) The cytoplasm show one or many vacuoles and single large, distinct nucleus, with one or more nucleoli. Chloroplasts are either discoidal or band-shaped, without pyrenoids.
(iv) Yellow-brown pigment fucoxanthin a type of xanthophyll
is predominant which gives golden - brown color to algae, while other
pigments such as chlorophyll-a and chlorophyll-b are also present.
(v) The reserved food is in the form of soluble carbohydrates called laminarin and mannitol.
Figure 14.31 Types of Phaeophyta
(vi) Reproduction is vegetative by fragmentation, asexual by formation of haploid or diploid zoospores and sexual by gamete formation, by isogamy, anisogamy or oogamy.
(vii) The life-cycle shows isomorphic alternation of
generations where the gametophyte and sporophyte are morphologically similar
(e.g., Ectocarpus), heteromorphic alternation or generations where
the gametophyte and sporophyte are morphologically differentiated (e.g.
Laminaric) while in there is no alternation of generations as gametophytic
generation represented only by gametes.
Examples: Fucus, Sargassum, Laminaria, Ectocarpus etc.
Economic importance : "Komba", a product
made from Laminaria, is widely used as food in Japan and oceanic
islands. It is boiled with fish, meat or in soups or as a cooked vegetable.
A gel called algin, obtained from Macrocystis, Laminaria, etc.,
is used in the preparation of toothpastes, ice-cream, etc. Sea weeds like
Sargassum are used as a directory source of iodine and bromine.
Evolutionary significance : The universal appearance of motile gametes probably suggests that Phaeophyta took its origin from unicellular brown, flagellated ancestors. These do not seem to be related to green algae because of the multicellular vegetative body and presence of fucoxanthin and laminarin which belong exclusively to this group.