Algae Guide – A Case Study About the Life Journey of Algae
What is Algae – Defining Algology, Phycology, Phytoplanktons, Endophytes, Occurrence and Structure of Algae, Vegetative, Asexual and Sexual Reproduction, Alternation of Generations, Types of Cycles (Haplontic, Diplontic, Diplohaplontic, Haplobiontic, Diplobiontic Life Cycle)
Algae constitute an important group of Thallophyta, which comprises the lowest and simplest group of Plant Kingdom. The algae includes a large heterogenous grouping of simple plants that has little in common except for their characteristic autotrophic mode of nutrition. The characteristics on the basis of which algae can be defined are difficult to specify, that is why we came across a number of definitions.
Definitions By Fritsch & Smith
The Algae can be defined as “Chlorophyll bearing organisms which possess unicellular sex organs or multicellular ones in which every cell forms a gamete.
Algae Definition by Fritsch (1935): He defined algae as ‘Unless purely artificial limits are drawn, the designation algae must include all holophytic organisms (as well as their numerous colorless derivatives) that fail to reach the level of differentiation characteristics of archegoniate plants”.
Algae Definition by Smith (1955): He defined algae as “these are the plants in which the sex organs are usually unicellular and when they are multicellular as in most algae, all cells are fertile. The group is autotrophic”.
However, the term algae include “Chlorophyllous thallophytes. Algae are small autotrophic plants that fail to show any differentiation and their sex organs are unicellular and if multicellular all cells are fertile”.
The presence of algae is associated with the presence of water. They are aquatic plants occurring abundantly in the sea as well as in fresh water of the streams and stagnant waters of the ponds. Some species can grow on snow and ice at temperature of about 0°C or less, establishing cryoflora; while others inhabit hot springs at temperature of about 50-54°C. They may be found as familiar green coating on the damp shaded sides. The shade may be of trees, flower pots, walls moist soils or even rocks. They may also be observed on quite waters on pools as common green ‘pond scum’. They are the common sea weeds which constitute the vegetation of ocean.
The algae can be found free-floating or attached to the substratum by the lowermost cell known as holdfast. They may be unicellular or colonial forms both can be free-floating. The free-floating algae are known as phytoplanktons. These plants establish the basic food supply on which animals feed. They may be found growing near the shore or in the shallow rocky parts of the sea. These are mostly the “Kelps” and other sea weeds.
Algae may live as epiphytes (living on plants) e.g. Oedogonium, Coleochaete, or epizoics (living on animals) e.g. Cladophora on snails. The algae growing embedded in the tissues of other plants are known as endophytes, e.g. Anabaena is found growing in the roots of Cycas (a gymnosperm) while Nostoc is found growing in the thallus of Anthoceros (a bryophyte). Some establishes symbiosis (Partnership between two dissimilar organisms, in which both, benefit; also referred to as reciprocal parasitism) with other organisms. Lichens show such associations in which algae supply the food while fungi provide mechanical support, protection and water. Several are saprophytes (those organisms which feed on dead organic matter) Mucor, Rhizopus, and there are a few which are parasites (those organisms that feed on alive organic matter / living organisms, and get their nutrients from them and in return harm them.) e.g. Cephaleuros which is an intercellular parasite in the leaves of tea, pepper, mango etc.
General Structure of Algae (Thallus)
In algae we encounter great variation in the size and cellular organization of the plant body. Among the smallest known algae are Chlamydomonas and Chlorella. They are microscopic and unicellular. On the, other hand the largest like, Macrocystis are composed of millions of cells. They may attain a length of several feet (about 350 feet). The plant body is so diverse that it is difficult to describe. They form a mass of chlorophyll bearing plants with undifferentiated tissues. They never form true roots, stems, or leaves; thus, the plant body is known as thallus.
The algal plant may be composed of a single cell: unicellular or number of cells, multicellular (number of cells showing division of labor). The cell may be motile or non-motile, spherical or a variety of other genetically determined shapes. In many species the cells are grouped into aggregates. Two types of aggregations with respect to growth pattern occur.
The aggregates of cells in which cell division is unlimited, and it results in large masses. This type is known as colonies, and the form is colonial form or colony.
The aggregate of cells in which the total number of divisions of component cell is fixed at the time of origin of the colony and is not increased later is called coenobium, and the form is known as coenobial form. The colonies may be motile (by flagella) e.g. Volvox or non-motile e.g. Hydrodictyon. There are two main types of flagella found in algae:
- Whiplash type, it has a smooth surface
- Tinsel type, it has one or more rows of fine minute hairs.
The colonies may be filamentous or may assume other forms. The organism may consist of a filament or chain of cells. These types are usually multicellular. The cells are placed end to end to form branched or unbranched threads. This is the result of cell division in single direction only. Branched filaments arise by repeated division of certain cells in a new direction. The basil cells of a filament are commonly Known as holdfast and functions to grip the filament to the substratum e.g. Oedogonium.
The pattern of cell division may result in the formation of filament when the division is in single plane, e.g Oedogonium or ribbon shaped when the pattern of division is in two, plane, e.g. (Eva, the cell division may be in three planes, e.g. Fucus or Polysiphonia (Fig1.1).
Some marine algae are multicellular and have relatively more or less dense inter-weaving pattern of filaments which has resulted in the formation of pseudoparenchymatous tissue e.g. Codium. Some are multicellular and consisting of parts which resembles with the roots, leaves, stems of higher plants e,g. Caulerpa, Chara, Sargassum.
Cell Structure of Algae
In algae the shape, size and form are variable in different members. The cell which is the basic unit of an organism is the center of all activities.
There are two different patterns of algal cells,
- Prokaryotic (Blue green algae)
- Eukaryotic (Rest of algae)
The prokaryotic types of algal cells are represented in Cyanobacteria or Blue green algae. In it, protoplast does not contain true chromoplast, true nuclei, or vacuoles. Chromoplast is divided into two regions. An outer the chromoplasm which is colored and contains pigments suspended in it. Inner one is a colorless central region known as the centroplasm or central body. The nuclear material, DNA (2-deoxyribonucleic acid) is not separated from the rest of the protoplasm by a nuclear membrane. Nuclear material is dispersed throughout the cell or is concentrated in the central part.
In the same way the plastids are without membrane. The photosynthetic lamella occurs freely in the cytoplasm. The nucleus, endoplasmic reticulum, mitochondria, Golgi bodies, and large aqueous vacuole are also missing. Ribosomes are present. There is no true mitotic cell division because no cell plate is formed. The cell wall is made up of some chemical mucopeptide (complex proteinaceous material giving the wall its rigidity, also found in bacterial wall) which is not found in other algal members. The cell wall is surrounded by a common gelatinous mass (Fig.,2.3).
In Eukaryotic algae they have DNA localized in a minutely perforated nuclear membrane. The protoplast is differentiated into distinct nuclei and cytoplasmic inclusions. The nuclei are essentially similar to those of higher plants. The photosynthetic lamellae are bounded within membranes as well-defined chloroplasts (Fig., 2.4) Pyrenoids, endoplasmic reticulum, mitochondria and Golgi bodies are present in the cells of these algae.
Reproduction in Algae
Reproduction in algae is quite variable. The unicellular algae reproduce mainly by fission or cell division followed by separation of the individual cells. In the higher form of algae, in addition to vegetative reproduction i.e., by separation of parts of the individual (fragmentation) both asexual and sexual reproduction in algae is common. In multicellular types, the same cell may fulfil both vegetative and reproductive functions or special reproductive cells or organs may be developed.
Types of Reproduction
There are three types of reproduction in algae. Following are the list of them.
- Vegetative reproduction
- Asexual reproduction
- Sexual reproduction
A) Vegetative Reproduction
It takes place by the separation of a cell or group of cells from the parent body. The separated body directly develops into new plants. Separation may be voluntary or an accident can do the job.
Vegetative reproduction can take place by a number of methods:
It is found in the unicellular blue green and green algae. It involves simple mitotic division, which results in the formation of two daughter cells. These daughter cells get separated and lives independently as two independent individuals. Common examples are Chroococcus etc.
This type of vegetative reproduction involves break away of many cell segments from the parent body or the wholes parent body may break up into many cell segments. These segments are known as Fragments. The fragments grow in to new filaments by simple mitotic division and thus give rise to new plant. Common examples are Spirogyra and Zygnema.
c) Tubers Fragmentation
Tubers are special structures, which contains a lot of food reserve in them and are produced as out growths from the parent body. These are produced to overcome the unfavorable conditions. These tubers get detached from the parent plant and develop into new plants. Common examples are Chara (Fig., 2.6) and Cladophora.
d) Adventitious Bud Formation
In some cases, certain adventitious branches are produced which break off from the parent body and develop into new plants. Common examples are Chara and Dictyota.
The bud is a gradual developing outgrowth from the parent body. In the process the parent nucleus divides by mitosis and the daughter nucleus migrates to the increasing bud. The bud after receiving the nucleus is cut off from the parent body, and starts living independently. It has been reported from Protosiphon.
B) Asexual Reproduction in Algae
Their are number of methods by which asexual reproduction in algae takes place.
a) Zoospores and Aplanospores
Zoospores are naked bodies of protoplasm. They are motile and move by means of flagella or cilia. They are produced in the variable numbers from the contents of mother cell, the zoosporangium. Their formation is favored typically when the conditions are favorable. Zoospore usually germinates directly to produce new plant.
Aplanospore in contrast are produced when there is a deficiency of free water. Aplanospore are covered by a thin wall and do not have flagella like the zoospores. They also germinate directly to give rise to a new plant.
b) Akinetes, Resting Spores or Hypnospores
Akinete: Under relatively drier and hot weather some time the entire cell as such becomes thick walled This is known as Akinete. They are usually produced at the approach of dry and hot conditions. On the approach of favorable conditions, they germinate directly to give rise to a new plant or form zoospores.
Resting Spore or Hypnospore: In relatively drier conditions, the content of mother cells shrinks and secretes a thick wall around them. This helps the plant to tide over the unfavorable conditions. These thick-walled structures are called resting spores or hypnospores.
c) Palmella Stage
On the approach of dryness, when the plants are left on the moist bank by receding water of the ponds, the cells of many algae continue to divide but their contents are not liberated. The mother wall becomes gelatinous thus, forming a mass or colony of rounded cells which lie embedded in a jelly-like substance (Jelly-like material is developed to retain water) formed from the cell walls. This aggregate of embedded cells is known as palmella stage. On the return of favorable conditions, the cells come out either as zoospore or as aplanospores and germinate to give rise to normal plants.
In some plants the resting spores develop all the structures of the parent plant cell in which they are formed. They are exactly similar to the latter except the size. These spores are called autospores. This condition is observed in Scenedesmus and some other Chrococcales.
These are thick walled spores formed during unfavorable conditions or even when food supply in abundant. During their formation, as in Vaucheria, the thallus becomes septate. Each chamber thus formed produces a thick-walled cyst. Sometimes the cyst may be formed in rhizoids as in Botrydium when they are called rhizocysts. Sometime, a cyst may divide further to form a number of small microcysts.
C) Sexual Reproduction in Algae
Sexual Reproduction in algae takes place by the fusion of sex cells or gametes which are produced in cells called the gametangia. The fusion may take place in different ways.
The fusing gametes are identical i.e., have similar size and form, both are motile. Such gametes are called isogametes (iso means similar) and the sexual reproduction is known as isogamous reproduction or conjugation.
The fusing gametes are similar in appearance and are motile but are different in size i.e., small or micro-gametes and large or macro- gametes. This type of sexual union is known as anisogamous reproduction.
The fusing gametes are different in size and form i.e. one of the gametes is small and motile while other is large and non-motile. The small is described as a male gamete or sperm and the large is female gamete egg or oosphere. This type of sexual reproduction is termed as oogamous reproduction and the product is called oospore or zygote. This is usually found in the higher type of green and brown algae.
In the red algae the reproductive process is rather elaborate and very special type. The oospore, zygote or zygospore may germinate directly or more usually undergo a period of rest and produce after reduction division, a number of meiospore or aplanospores which on liberation germinate to produce new plant.
Life Cycle of Algae – Alternation of Generations and Types of Cycles
Life cycle of algae is defined as the sequence of different phases, both morphological and cytological, through which an alga completes its cycle. The life cycle of algae is also described in another term known as life history. The term life cycle signifies the process and events which transpires starting with one individual unit a new generation of similar individual is again produced. The important features in these life cycles are
- Gametic union (plasmogamy and karyogamy) and
- Meiosis and the interveinal nature of the tissue or plant between nature of the tissue or plant between these events.
Alternation of Generations
Alternation of generations is described as the type of life cycle takes place in those plants as well as algae that have distinct sexual haploid and asexual diploid stages. Depending upon the sexual reproduction in algae, the algae exhibit three fundamentally different types of alternation of generations. Fig.,2.8. Type A and type B are haplobiontic. In that only a single form of the plant (biont) occurs in nature. It may be predominantly haploid as in A (Zygotic), or it may be predominantly diploid as in B (Gametic). The type C (Sporic) is found in diplobiontic life cycle. In it an alternate or regular sequence of two free living individuals (haploid and diploid) are observed. Several modifications of these three basic types are found. In type C when organisms are morphologically similar the alternation is said to be isomorphic and when the organism differ morphologically, they are said to be heteromorphic alternation.
Drew (1955) and Chapman et al., (1961) reviewed the alternation and generation in the life cycle of algae. They were of the opinion that algal life cycle, should be cytologically and morphologically acceptable. If we considered aspects then the life cycle can be of three types.
a) Monomorphic in which both gametophytic and sporophytic generations are similar to each other.
b) Dimorphic in which both generations are morphologically unlike or different.
c) Trimorphic in which a succession of three morphological types occurs.
Life Cycle of Algae and Its Types
There are at least five types of life cycles now accepted in algae. They are as follows:
- Haplontic Life Cycle e.g., Chlamydomonas, Oedogonium, Spirogyra etc.
- Diplontic Life Cycle e.g., Bacillariophyta, Siphonales, Fucales etc.
- Diplohaplontic Life Cycle
a) Isomorphic or homologous e.g., Cladophora, Ulva.
b) Heteromorphic or heterologous e.g. Egtocarpus, Dictyota, Laminatia.
- Haplobiontic Life Cycle e.g., Batrachospernium, Nemalion, etc.
- Diplobiontic Life Cycle e.g., Polysiphonia etc.
Haplontic Life Cycle
This life cycle is found in the majority of the green algae. In it, the vegetative plant body is composed of a single thalloid gametophyte which is haploid. The gametes are produced by this haploid body. The gametophyte multiply vegetatively. Mitospores (zoospores) are produced but they play no role in alternation of generation. At the end of life cycle the gametes fuse to form the zygote. The zygote on germination divides by meiosis and produce four haploid spores. The diploid generation is represented by only a single cell the zygote, which can least be considered as sporophytic generation. Thus, there is only one generation and therefore no alternation of generation. Such type of life cycle is termed as haplontic life cycle, e.g., Oedogoniun.
Diplontic Life Cycle
In diplontic life cycle the plant body is diploid (sporophyte) and it bears gametangia as sex organs. They produce gametes after meiosis. The gametes are haploid. They are gametophytic cells. The fusion of the gametes re-establishes the diploid phase. Thus, the gametophytic phase is very short lived. There is no true alternation of generation of two phases of life cycle. In such a condition where the life is a diploid and the gametophytic stage is almost nil the life cycle is called diplontic life cycle. e.g., Fucus, Sargassum, etc.
Diplohaplontic Life Cycle
In this type the plant has two somatic phases; the haploid or gametophytic and the diploid or sporophyte. Both regularly alternate during the life cycle. Such type of life cycle which deals with an alternation of two vegetative individuals the gametophyte and sporophyte is called diplohaplontic, e.g., Ectocarpus.
Diplohaplontic life cycle is of two types:
i) Isomorphic Life Cycle
When the two alternating individuals are similar in their morphology, they are called isomorphic or homologous diplohaplontic life cycle. In such plants the zygotes do not divide immediately but developed into a multicellular diploid (sporophyte) plant. This sporophyte produces sporangium which undergoes meiosis. After sporophyte produces sporangium, which undergo meiosis. After this sporogenic meiosis the haploid (gametophyte) plant are produced e.g., Ulva, Cladophora.
ii) Heteromorphic Life Cycle
When the two alternating vegetative individuals are dissimilar in their morphology the life cycle is termed as, heteromorphic or heterologous diplohaplontic. In this case one of the phases of life cycle is dominant on the other phase. In Laminaria the sporophyte is much larger in comparison to a few cells’ smaller gametophyte, But in Cutlaria the gametophyte is dominant over its sporophyte.
Haplobiontic Life Cycle
In some members of the red algae the plant is a gametophyte. It produces sex organs which form gametes, which fuse into a zygote. The diploid zygote undergoes a meiotic division and branches into filamentous outgrowth. These outgrowths are haploid. These are called carposporophytes. The carposporophytes remains attached to the parent gametophyte as a parasite. The terminal cells of the carposporophytes are known as carposporangia. The latter cell forms carpospores by simple divisions.
Carpospores are liberated and no germination from the parent gametophytes. Thus, the haploid parent gametophyte phase alternates with a haploid asexual carposporophytic phase. The diploid phase is represented only by a zygote of very short duration. The life cycle in such cases is biphasic or haplobiontic with two morphologically dissimilar but cytologically similar generations with a diploid zygote between the two generations, e.g., Nemalion.
Although the life cycle of Batrachospermum is haplobiontic but further difficulty has been noted in its life cycle. In this case the corpospores on germination gives rise to the Chantransia stage instead of forming the parent gametophyte. The Chantransia multiply by monospores (asexual) and the parent gametophytes come out as lateral outgrowth from the Chantransia stage. Thus, in Batrachospemum there is alternation of three somatic haploid generations (gametophyte, carposporophyte and chantransia) with a diploid zygotic phase of short duration. Such a life cycle is known as triphasic haplobiontic life cycle.
Diplobiontic Life Cycle
It is found in some red algae. It shows more complicated life cycle as compared to the life cycles already studied. The gametophytes or the individual plants are unisexual. The male and female gametangia respectively are called as spermatangia and carpogonia. The gametes are called spermatia and carpogonia. They occur in different haploid plants. As a result of fertilization, the diploid zygote is obtained. The zygote does not divide immediately by meiosis but divides mitotically to form a diploid carposporophyte. The latter produces diploid carposporangia and remains attached as a parasite on the parent gametophyte.
The carposporangia produce diploid carpospores. These develop after they have been shed into diploid tetrasporophytes, which in their turn produce haploid tetraspores. Meiosis occurs during tetraspore formation. Tetraspore germinate into haploid gametophytes. This type of life cycle is represented by two kinds of haploid as well as diploid. It is called diplobiontic life cycle.
Polysiphonia is the best studied example of diplobiontic life cycle. The life cycle is triphasic. A succession of three generations gametophytic stages occur. The tetrosporophyte plant is produced. Thus, the life cycle in Polysiphonia is triphasic or isomorphic diplobiontic. Heteromorphic diplobiontic life cycle is found in certain Nemalioales.