Dr.Ayad M.J. Lecture ‐8‐ Algae 2016 - University of Babylon · 2016-12-28 · Brown alga...

Dr.Ayad M.J. Lecture ‐8‐ Algae 2016

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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


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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.


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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:


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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


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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.


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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


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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


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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


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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


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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.


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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.


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Figure 5: General life cycle of Brown Algae (Ectocarpus)

Figure 6:Life cycle of Laminaria

Dr.Ayad M.J. Lecture ‐8‐ Algae 2016 - University of Babylon · 2016-12-28 · Brown alga...

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