Algae

Algae are basically Plant-like organisms that are usually photosynthetic and aquatic, but do not have true roots, stems, leaves, vascular tissue and have simple reproductive structures. Their body is unicellular, colonial, or multicellular. They are either simple or branched filaments or have large unspecialised, flattened bodies. They constitute a Paraphyletic (a group containing its last common ancestor but not all descendants) and Polyphyletic (a group whose members' last common ancestor is not a member of the group) group, as they do not include all the descendants of the last universal ancestor nor do they all descend from a common Algal ancestor, although their Plastids (small particles in the cytoplasm of the cells) seem to have a single origin.  Diatoms are also examples of Algae. There are about 30,000 Species of Algae. Algae produce an estimated 30 to 50 percent of the net global oxygen available to humans and other terrestrial Animals for respiration. Algae are also known as 'Sea-Weeds', 'Pond-Scum' and 'Grasses of Water'. Algae recycle 97% of all water in the growth and harvest process.

Scientific Classification

Domain - Eukaryota.

Common Classification

1.  Division Chlorophyta (green algae)

Features:

1. These contain Chlorophylls A and B.
2. They have starch stored inside chloroplast and have Mitochondria with flattened Cristae (a structure resembling a ridge or crest).
3. Some have flagella, which lack tubular hairs (mastigonemes). There are about 9,000-12,000 Species.

Class Chlorophyceae - These are primarily Freshwater Algae. These include Chlamydomonas, Chlorella, and Oedogonium.

Class Charophyceae - It includes the macroscopic Pond Weed Chara, filamentous Spirogyra and Desmids.

Class Micromonadophyceae - These are primarily marine and include the smallest Eukaryotic Algae, Micromonas.

Class Pleurastrophyceae - These are both Freshwater and Marine and include Marine Flagellate, Tetraselmis.

Class Ulvophyceae - These are primarily marine and include Sea Lettuce Ulva.

2.  Division Chromophyta

Features:

1. Most of these type of Algae have Chlorophyll A, with exception of one or two containing Chlorophyllide C.
2. These have Carotenoids (yellow to red pigments) present. These have storage product beta-1,3-linked Polysaccharide outside Chloroplast.
3. They are characterised with having Mitochondria with tubular cristae and bi flagellate cells and zoo-spores with tubular hairs on one flagellum.
4. Mucous organelles is common among them.

Class Bacillariophyceae (diatoms) - These have Silica Cell Walls, or Frustules and Centric Diatoms. They are mostly Pennate Diatoms, usually attached or gliding over solid substrates, with valves bilaterally symmetrical; primarily in freshwater, marine, and soil environments. They are commonly Planktonic. There are about 12,000 to 15,000 living Species. There are about thousands more Species described from fossil Diatomite deposits. Examples - Cyclotella and Thalassiosira (centrics) and Navicula and Nitzschia (pennates).

Class Bicosoecophyceae - These have colourless Flagellate Cells in vase-shaped Loricas (wall-like coverings). The cells are attached to Lorica using Flagellum as a stalk, Lorica attaches to Plants, Algae, Animals, or water surface both freshwater and marine. There are fewer than 50 Species described like Bicosoeca. These may be included in the Chrysophyceae or in the Protozoan group Zoomastigophora.

Class Chrysophyceae (golden algae) - These are mostly Unicellular or Colonial Flagellates. They are also capsoid, coccoid, amoeboid, filamentous, parenchymatous, or plasmodial. Many of these produce silica cysts (statospores). They are predominantly freshwater. There are about approximately 1,200 Species. Examples - Chrysamoeba, Chrysocapsa, and Ochromonas.

Class Dictyochophyceae - These are predominantly Marine Flagellates, including Silicoflagellates that form skeletons common in Diatomite deposits. There are fewer than 25 described Species.

• Order Pedinellales - These are characterised by having 6 chloroplasts in a radial arrangement when they are pigmented. They have Flagella bases attached almost directly to nucleus.

• Order Dictyochales (silicoflagellates) - These typically have siliceous skeletons like spiny baskets enclosing the cells. Their Flagella bases attach almost directly to nucleus. Silicoflagellate skeletons are common in Diatomite deposits. Examples - Dictyocha, Pedinella and Pseudopedinella.

Class Eustigmatophyceae - These are mostly small, pale green, and spherically shaped. There are fewer than 15 Species. Examples - Eustigmatos and Nannochloropsis. These are newly described Class and more are still to be discovered.

Class Phaeophyceae (brown algae or brown seaweeds) - These Range from microscopic forms to large kelps more than 20 metres long. These are almost all marine, examples - Ectocarpus, Macrocystis and Sargassum. There are at least 1,500 Species.

Class Prymnesiophyceae (Haptophyceae) - These are predominantly marine and planktonic. Many of these are equipped with Haptonema, a hairlike appendage between two flagella; no tubular hairs; many with organic scales. Some of these deposit calcium carbonate on scales to form Coccoliths; Coccolithophorids are considered to be partly responsible for global warming as they can remove large amounts of carbon from the ocean water. There are approximately 300 Species with more fossil Coccolithophores known, like - Chrysochromulina, Emiliania, and Prymnesium.

Class Raphidophyceae (Chloromonadophyceae) -  They are Flagellates having Mucocysts (mucilage-releasing bodies). These are occasionally found in freshwater or marine environments. There are fewer than 50 Species including Heterosigma, Vacuolaria and Olisthodiscus.

Class Synurophyceae - Their cells are covered with elaborately structured Silica Scales. They were previously placed in Chrysophyceae; silica-scaled; unicellular or colonial flagellates sometimes alternating with Capsoid Benthic Stage. There are approximately 250 Species which include Mallomonas, Synura and Tesselaria.

Class Xanthophyceae (yellow-green algae) - These are Primarily coccoid, capsoid or filamentous and found mostly in freshwater environments. There are about 600 Species which include Bumilleriopsis, Tribonema and Vaucheria.

3.  Division Cryptophyta

Features

1. These are Unicellular flagellates.

Class Cryptophyceae - Most of these have Chlorophyll A, Chlorophyllide C2 and Phycobiliproteins. They have starch stored outside of chloroplast and have  mitochondria with flattened Cristae. They have tubular hairs on one or both Flagella and special Ejectosomes in a furrow or gullet near base of Flagella. Their cells are covered with periplast, often elaborately decorated sheet or scale covering. Their Nucleomorph may represent reduced nucleus of symbiotic organism. There are approximately 200 described Species which include Chilomonas, Cryptomonas, Falcomonas and Rhinomonas.

4.  Division Pyrrophyta (Dinoflagellata)

Features:

1. These are predominantly Unicellular Flagellates.
2. Approximately half of the Species are heterotrophic rather than photosynthetic. Photosynthetic Forms contain Chlorophyll A, some may have 1 or more Chlorophyllide C Types, and Peridinin or Fucoxanthin.
3. They have Mitochondria with tubular cristae and flagella without tubular hairs.
4. In many members ejectile Trichocysts are below surface.
5. Many have cellulosic plates that form a armour like structure around cell.
6. Some of these are bioluminescent, some containing symbionts; resting (interphase) nucleus contains permanently condensed chromosomes.
7. Several produce toxins that either kill Fish or accumulate in Shellfish and cause sickness or death in humans when ingested.
8. There are more than 1,200 Species described, most in the class Dinophyceae; Alexandrium, Dinophysis, Gonyaulax, Peridinium and Polykrikos.

5.  Division Euglenophyta

Features:

1. They are primarily unicellular flagellates.
2. They are both photosynthetic and heterotrophic.

Class Euglenophyceae - These have Chlorophylls A And B. Paramylon is stored outside chloroplasts. They have Mitochondria with paddle-shaped cristae. Their Flagella lack tubular hairs, but some with hairlike scales; pellicle covering of sliding sheets allows cells to change shape. There are about 1,000 described Species some of which are Colacium, Euglena and Eutreptiella.

6.  Division Rhodophyta (red algae or red seaweeds)

Features:

1. These are predominantly filamentous.
2. They are mostly photosynthetic, a few parasitic. Photosynthetic Species contain Chlorophyll A; Chlorophyll D is however present in some Species.
3. They are predominantly Marine.
4. They have phycobiliproteins (phycocyanin and phycoerythrin) in discrete structures (phycobilisomes).
5. Their starch is stored outside chloroplast.
6. Their mitochondria with flattened cristae.
7. Flagella is completely absent amongst them.
8. Coralline Red Algae contribute to Coral Reefs and Coral Sands.
9. There are approximately 4,100 described Species, some of which are Bangia, Palmaria, Polysiphonia, Porphyra, and Rhodymenia.

Types of Algae based on Cells

Microalgae - The unicellular forms are known as Microalgae. Microalgae comprise a vast group of photosynthetic, heterotrophic organisms which have an extraordinary potential for cultivation as energy crops.

Macroalgae - The multicellular forms are known as Macroalgae. They are photosynthetic, like Plants, and simple because they lack the many distinct organs found in land Plants.

Types of Algae based on Habitats

Planktonic Microscopic Algae - These grow suspended in the water.

Neustonic Algae - These grow on the water surface.

Cryophilic Algae - These occur in snow and ice.

Thermophilic Algae - These live in hot springs.

Edaphic Algae - These live on or in soil.

Epizoic Algae - These grow on Animals, such as Turtles and Sloths.

Epiphytic Algae - These grow on fungi, land Plants or other Algae.

Corticolous Algae - These grow on the bark of trees.

Epilithic Algae - These live on rocks.

Endolithic Algae - These live in porous rocks.

Chasmolithic Algae - These grow in rock fissures.

Endosymbionts - Some Algae live inside other organisms, and in a general sense these are called Endosymbionts. Specifically, Endozoic Endosymbionts live in protozoa or other, larger Animals, whereas Endophytic Endosymbionts live in Fungi, Plants or other Algae.

Characteristics and Physical Features of Algae

• Eukaryotic - Almost all the Algae are Eukaryotes i.e. their cells have nuclear material of deoxyribonucleic acid (DNA) organized within a discrete, membrane-bounded organelle, known as the Nucleus. The exception being the Blue-Green Algae, which are Prokaryotic i.e. they do not have an organized nucleus.

• Photoautotrophic - Most of the Algae are Photoautotrophic i.e. they are capable of synthesizing their own food from inorganic substances using light as an energy source. They conduct photosynthesis within membrane bound structure called Chloroplasts. Chloroplasts are cellular organelles that photosynthesise; they absorb sunlight and use it as an active chemical ingredient in a process that uses water and carbon dioxide to create sugars that the organism uses to feed itself. Some however, are Chemoheterotrophic and obtain energy from chemical reactions and nutrients from preformed organic matter.

• Absence of Roots, Stems and Leaves - Algae do not have vascular tissues and they lack the various structures which are found in land Plants, such as Leaves (phyllids), Stems and Roots. They have Root like structures which are known as Holdfasts which hold the Algae down firmly onto the rock surface.

• Cellular Respiration - Cellular Respiration is the process by which food molecules are metabolised to obtain chemical energy for the cell. Most Algae are Aerobic, i.e., they live in the presence of oxygen, although a few Euglenophyceae can live anaerobically in environments without oxygen.

• Mostly Aquatic - Algae are mostly aquatic. They are generally found in fresh-water ponds, streams, lakes, brackish waters and oceans making up the Plankton. Plankton is the food for many aquatic Animals. The drifting Algae constitute Phytoplankton.

• Presence of Flagellum - A Flagellum is a tail-like projection that protrudes from the cell body of Algae and lash back and forth. Many unicellular Algae move by means of Flagella. Kelps and many other multicellular Algae are non-motile. Each flagellum consists of an Axoneme, or cylinder, with 9 outer pairs of microtubules surrounding two central microtubules. The Axoneme is surrounded by a membrane, sometimes beset by hairs or scales.

• Alternation of Generations - During sexual reproduction, Algae form differentiated sex cells that fuse to produce a diploid zygote with two sets of chromosomes. The zygote develops into a sexual spore, which germinates when conditions are favourable to reproduce and reform the haploid organism having a single set of chromosomes. This pattern of reproduction is called Alternation of Generations.

Size - Most are microscopic, but some are quite large, e.g. some marine seaweeds that can exceed 50 m in length. The Giant Kelps grow to 70 metres (230 ft.) in length.

Geographical Range and Habitat

They are distributed worldwide in the sea, in freshwater and in waste water. Their Habitats range from Tropical Forests to Deserts.

Predators - Algae is eaten by Algae Eating Fish, including Black Sailfin Mollies, Otocinclus, Plecostomus, Siamese Algae Eater, Crossocheilus siamensis etc.

Origin and Evolution of Algae

The fossil records of Algae are not complete as they are for land Plants and Animals. Red algal fossils are the oldest known algal fossils. Microscopic spherical Algae (Eosphaera and Huroniospora) that resemble the living genus Porphyridium are known from the Gunflint Iron Formation of North America (formed about 1.9 billion years ago). Fossils that resemble modern Tetraspores are known from the Amelia Dolomites of Australia (formed some 1.5 billion years ago). The best characterized fossils are the coralline red Algae represented in fossil beds since the Precambrian time (from about 3,800 million years ago until 544 million years ago).

Reproduction

Reproduction in Algae occurs in both asexual and sexual forms. However most Algae reproduce asexually. Many small Algae reproduce asexually by ordinary cell division or by fragmentation, whereas larger Algae reproduce by spores. The process of replication and division of the nucleus that results in the production of genetically identical daughter cells is known as Mitosis. Some red Algae produce Monospores (walled, nonflagellate, spherical cells) that are carried by water currents and upon germination produce a new organism. Some Green Algae produce non motile spores called Aplanospores.

There are Two methods of asexual reproduction utilized by Algae:

• Daughter Colony Formation - A limited number of colonial Algae produce miniature replicates of the colonies. These are termed daughter colonies. These may be produced inside the hollow, spherical colonies or inside the actual cells of the parent colony. Eventually, the parent colony will rupture and release the new daughters.

• Sporulation - It is the most common form of asexual reproduction in the Algae. Sporulation is a process in which any cell of an organism produces one or more reproductive cells inside its cell walls. The original cell is termed a Sporangium and the new cells are termed Spores. Spores are often produced in large numbers for the rapid increase in population size.

Sexual reproduction is characterized by the process of Meiosis, in which progeny cells receive half of their genetic information from each parent cell. Sexual reproduction is usually regulated by environmental events. In many Species, when temperature, salinity, inorganic nutrients (e.g., phosphorus, nitrogen, and magnesium), or day length become unfavourable, sexual reproduction is induced.

The Algae have evolved many variations in sexual reproduction such as different types of gametes, different means of gamete transfer, and different locations of fertilization. The process of gamete formation is called Gametogenesis.

There are two methods of sexual reproduction:

• Isogamy - Isogamy is the form of sexual reproduction in which the gametes produced are identical in shape, size and motility. There is no structural distinction between "male" and "female" gametes . Pairs of Isogametes align themselves with their flagellar poles touching and after several seconds, the motile gametes fuse to form a single, non-motile, diploid zygote. Isogametes, less commonly, may be non-motile structures. A specific example exhibiting non-motile Isogametes, is the reproductive process known as Conjugation, which occurs in the filamentous Green Algae, Spirogyra sp.

• Heterogamy - In Heterogamy, two different types of gametes are produced. The male gamete, the sperm cell, is typically very small, highly motile and is produced in very large numbers. The female gamete, the egg cell, is much larger and non-motile. Fewer female gametes are produced but each is usually afforded some protection. Heterogametes are also produced by higher Plants and Animals. Oedogonium sp. is a Green Algae that produces Heterogametes.

A sexually reproducing organism typically has two phases in its life cycle. In the first stage, each cell has a single set of chromosomes and is called Haploid, whereas in the second stage each cell has two sets of chromosomes and is called Diploid.

Importance and Uses of Algae

• Microalgae can be cultivated under difficult agro-climatic conditions and are able to produce a wide range of commercially interesting by-products such as fats, oils, sugars and functional bioactive compounds.

• Algae produce more oxygen than all the Plants in the world, put together. As they grow, algae absorb large amounts of carbon dioxide (CO2): approximately - 154 tons of CO2 are absorbed from the atmosphere per hectare annually - and Algae absorb other greenhouse gases.

• They form an important food source for many Animals such as little Shrimps and huge Whales. They are at the bottom of the food chain with many organisms depending upon them.

• Seaweeds are used as fertilisers and even food by the Japanese, Irish, Welsh and in the Caribbean. Coastal farmers collect seaweeds by cutting them from seaweed beds growing in the ocean or by gathering them from masses washed up on shores after storms. The seaweeds are then spread over the soil. Dried seaweed, although almost 50 percent mineral matter, contains a large amount of nitrogenous organic matter.

• Extracts from the cell walls of (typically brown & red) Algae provide the Polysaccharides Agar and Carageenan. These are used as thickening agents in food, in surgical dressings and in microbial media.

• The skeletons of a group of Algae, the diatoms, are glass-like and this material is put to a variety of uses, such as abrasives (it used to be used in toothpaste!), reflective road signs, swimming pool filters.

• Biodiesel is another research area for which Algae is being used.

• Sewage can be treated with Algae, reducing the need for greater amounts of toxic chemicals than are already used.