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Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
1
Rhodophyta
1-2 General characteristics
phylum (division) of the kingdom Protista consisting of the photosynthetic
organisms commonly known as red algae. Most of the world's seaweeds
belong to this group. Members of the division have a characteristic clear
red or purplish color imparted by accessory pigments called phycobilins,
unique to the red algae and the cyanobacteria. The chloroplasts of red algae
are believed to be derived from cyanobacteria that formed an ancient
symbiotic relationship with the algae.
Red algae have a number of general characteristics that in combination distinguish them from other eukaryotic groups:
1-Absence of flagella and centrioles.
2-Floridean starch as a storage product and the storage of starch in the cytoplasm
3-Phycoerythrin, phycocyanin, and allophycocyanin as accessory pigments
4-Unstacked thylakoids in plastids
5-No chloroplast endoplasmic reticulum
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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1‐3 Structure
Cells of the Rhodophyta possess chloroplasts that, in addition to the
phycobilins, contain chlorophyll a, carotenes, and xanthophylls. At great
ocean depths, where the wavelength of light available for photosynthesis is
very different from that in shallow water, the phycobilins become more
active than the chlorophylls in absorbing light; this fact may explain the
ability of red algae to exist at depths of up to 879 ft (268 m). The
carbohydrate reserves of red algae are in the form of floridean starch, a
specialized glucose polymer of different structure than the starch of plants.
The red algae, unicellular to multicellular (up to 1 m) mostly free-living but
some parasitic or symbiotic, with chloroplasts containing phycobilins. Cell
walls made of cellulose with mucopolysaccharides penetrated in many red
algae by pores partially blocked by proteins (complex referred to as pit
connections). Usually with separated phases of vegetative growth and
sexual reproduction. Common and widespread, ecologically important,
economically important (source of agar). No flagella. Ultrastructural
identity: Mitochondria with flat cristae, sometimes associated with forming
faces of dictyosomes. Thylakoids single, with phycobilisomes, plastids
with peripheral thylakoid. During mitosis, nuclear envelope mostly remains
intact but some microtubules of spindle extend from noncentriolar polar
bodies through polar gaps in the nuclear envelope. Synapomorphy: No
clear-cut feature available; possibly pit connections Composition: About
4,000 species.
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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Figure 1:Left is a General structure of Rhodophyta and right is Different forms of tetrasporangia commonly found in Rhodophyta. ( a ) Tetrahedral ( b ) Cruciate type ( c ) and ( d ) Zonnate
1‐4 Life cycle and reproduction
They display alternation of generations; in addition
to gametophyte generation, many have two sporophyte generations,
the carposporophyte-producing carpospores, which germinate into
a tetrasporophyte – this produces spore tetrads, which dissociate and
germinate into gametophytes. The gametophyte is typically (but not
always) identical to the tetrasporophyte.
Carpospores may also germinate directly into thalloid gametophytes, or the
carposporophytes may produce a tetraspore without going through a (free-
living) tetrasporophyte phase. Tetrasporangia may be arranged in a row
(zonate), in a cross (cruciate), or in a tetrad.
The carposporophyte may be enclosed within the gametophyte, which may
cover it with branches to form a cystocarp. These case studies may be
helpful to understand some of the life histories algae may display:
In a simple case, such as Rhodochorton investiens:
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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In the Carposporophyte: a spermatium merges with a trichogyne (a long
hair on the female sexual organ), which then divides to form
carposporangia – which produce carpospores.
Carpospores germinate into gametophytes, which produce sporophytes.
Both of these are very similar; they produce monospores from
monosporangia "just below a cross wall in a filament" and their spores are
"liberated through apex of sporangial cell."
The spores of a sporophyte produce either tetrasporophytes. Monospores
produced by this phase germinate immediately, with no resting phase, to
form an identical copy of parent. Tetrasporophytes may also produce a
carpospore, which germinates to form another tetrasporophyte.[verification
needed]
The gametophyte may replicate using monospores, but produces sperm in
spermatangia, and "eggs"(?) in carpogonium. A rather different example
is Porphyra gardneri:
Figure2: Life cycle of Rhodophyta
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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1‐5 Classification
1‐6‐1 Class Bangiophyceae
Members of the Bangiophyceae have a simple alternation of heteromorphic
generations in which the sporophyte is a small, prostrate filament called a
conchocelis that releases meispores called conchospores. The sporophyte
is the stage that has pit connections. The gametophyte can be variable in
this group and range from filamentous to foliose.
Figure 2: Batrachospermum
1‐6‐2 Class Floridiophyceae
The Floridiophyceae contains most of the taxa in the phylum. These plants
tend to be complex, either filamentous or pseudoparenchymatous and tend
to be seaweeds of warmer waters. The polysaccharides common in the cell
walls of many in this group are the sources of agar, agarose, and
carrageenin, common food additives. Chondrus crispus is the red most
commonly harvested on the coast of the eastern US as a source of agar.
Corallina is a taxon that impregnates its cell walls with calcium carbonate
forming filaments that appear armored and segmented.
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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Figure 3: Polysiphonia
1‐7 Important terms
1-carposporophyte (2n): diploid stage that forms from fertilization and produces asexual carpospores
2-tetrasporophyte (2n): diploid stage that forms from carpospores, and produces haploid tetraspores through meiosis
3-gametophyte (1n): haploid stage that forms from tetraspores, and produces gametes
4-spermatia: non-motile sperm
5-trichogyne: female stalk that catches spermatia
Figure 4: Polysiphonia life cycle
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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Phaeophyta (Brown Algae)
The Phaeophyta or brown algae are mostly marine algae. Phaeophyta are
characterized by the pigment fucoxanthin that gives them the brown colour.
The cell wall in Phaeophyta is two layered; inner layer consists of cellulose
and outer layer mainly of algin and fucoidan. The brown seaweeds serve as
important source of the industrial hydrocolloid alginate as well as food in
countries like Japan, Korea and China.
1- General characteristics
(a) Occurrence: Mostly marine.
(b) Pigments: Fucoxanthin is dominant, Chlorophyll a, c and carotene.
(c) Pyrenoids: Stalked pyrenoids present outside the chloroplast envelope..
(d) Reserve food material: Laminarin, mannitol and fats.
(e) Cell wall: Cellulose, alginic acid and fucinic acid.
(f) Structure: Microscopic to branched, filamentous macroscopic
parenchymatous
plants.
(g) Flagella: Zoospores flagellated, flagella unequal, one is tinsel type.
(h) Reproduction: Sexual reproduction (isogamous, anisogamous and
oogamous).
2-Structure Most of brown algae are lithophytes , which require stable hard substrata
for attachment, and a number of the fi lamentous, smaller species are
epiphytes. Unicellular, colonolial and unbranched fi laments are absent in
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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pheophyceae. The freshwater phaeophyta species are simply filamentous
and smaller in size unlike their marine counterparts which have complex
gigantic and bulky thalli Their size ranging from small fi lamentous forms
like Ectocarpus and Hinskia , which are few millimetres to massive
intertidal weeds such as Ascophyllum and Fucus , to subtidal large kelps
and the largest seaweed known Macrocystis pyrifera, They have higher
morphological and anatomical differentiation compared to the other algae
The size range vary greatly, from crustose form which may be 1–2
mm, macroscopic fi lmentous tufts 2–10 mm, subtidal kelp forests that
might be as tall as 20–60 m.
Figure 5: General Morphology of Brown Algae
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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The cell walls of brown algae are generally gelatinous and consist of two
layers. Cellulose makes up the skeleton backbone but is present in small
quantiites i.e. 1–8 % of dry weight. The chloroplasts of brown algae are
usually discoid and surrounded by an envelope. The outer membranes of
the chloroplast endoplasmic reticulum are continuous or discontinuous
depending on the species. Microfibrils of DNA occurring in the plastid
may be linear or circular attached to the thylakoid membranes. The
pigments are located in plastids lacking pyrenoid; their presence may also
vary according to algal stage.
Presence of Physodes (fucosan granules) is one of the characteristic
features of brown algae. In the meristmatic, photosynthetic and
reproductive cells, cytoplasm a large number of colourless vesicles with
highly refractive acidic fl uid staining red with vanillin and hydrochloric
acid are present
3-Reproduction Brown alga reproduces by vegetative, asexual and sexual methods of
reproduction:
1- Vegetative Reproduction Several species of brown algae show vegetative reproduction via
fragmentation. In members of sphaecelariales propagules are found
2-Asexual Reproduction All brown algae reproduce asexually with exceptions of Tilopetridales,
Dictyotales and Fucales. In ectocarpales and spherocarpales asexual
reproduction occurs via bifl agellate zoospores that develops in to
reproductive organs called sporangia which could be unilocular (one-
celled) or many cells plurilocular as observed in Hinskia
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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mitchelliae.Gametes can also reproduce parthenogenetically to form
asexual progenies, for example in Ectocarpus . Asexual reproduction is
absent in Laminaria
3-Sexual Reproduction
In pheophyceae sexual reproduction takes place by the formation of fl
agellate gametes that are formed inside gametangia. Multicellular
gametangia are formed only in some of the brown algae , The haploid thalli
form ranges from isogamous (both male and female gametes exactly
similar), anisogamous (female gamete larger than male) to oogamous
(small fl agellated male and large non-fl agellated female gametes), The
sexual reproduction is through fusion of fl agellated male and female
gametes or fusion of flagellated male and large non-flagellated female
gametesThe haploid (gametangial) and diploid (sporangial) thalli may be
similar (isomorphic) as in Ectocarpales or different (heteromorphic) in
appearance for example in Laminariales, or the gametangial generation
may be extremely reduced (Fucales).
3-Life Cycle Brown algal life cycle shows alteration of generations of haploid and diploid organisms: 1-Haploid gametophytes (n) give rise to haploid gametes by mitosis.
2-male and female gametes (n) fusion give rise to zygote (2n) that forms
diploid sporophyte
3-The sporophyte (2n) produces meiospores (n) by meiosis which
germinates and
4-forms haploid gametophyte.
5-Brown algae life cycle may be isomorphic, heteromorphic and Diplontic.
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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3-1 Life Cycle of Ectocarpus The sexual life cycle of Ectocarpus consists of alternating heteromorphic
gametophyte and sporophyte generations ,Both generations of the sexual
life cycle are multicellular. The sexual life cycle in the sporophyte
generation starts with the formation of a diploid zygote that undergoes a
bipolar germination to produce two germs tubes that eventually forms the
sporophyte. The initial cell division in the zygote is symmetric. Two kinds
of fi laments prostrate and upright are produced in the sporophytic
generation. Two specialized reproductive structures; plurilocular and
unilocular sporangia are produced on the upright filaments A mitotic
event in multi-chambered plurilocular sporangia releases mito-spores,
which after their release, forms sporophyte Unlike plurilocular sporangium,
unilocular sporangium consists of single chamber that contain meiotically
produced meiospores that give rise to multicellular gametophytes.
Morphologically, it is hard to distinguish between a male and a female
gametophyte. Like zygote, the meio-spores undergo a bipolar germination
but the initial cell division of meio-spore is asymmetric that produces
different cell types. Only upright fi laments have been observed in the
gametophytic generation. Specialized reproductive structures called
plurilocular gametangia are produced on the upright fi laments of the
gametophyte Plurilocular gametangia produce male and female gametes
that are released in the surrounding marine water. Male and female gametes
can be distinguished based on the behavior and physiology Unlike male
gametes, female gametes settle quickly and release a sex pheromone that
attracts male gametes. Once a male gamete fuses with a female gamete, a
diploid zygote is produced that marks the onset of the first diploid structure
of the sporophyte generation.
Dr.Ayad M.J. Lecture ‐8‐ Algae 2016
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Figure 5: General life cycle of Brown Algae (Ectocarpus)
Figure 6:Life cycle of Laminaria