Algae: Habitat and Characteristics

• Algae exist in abundance in oceans, seas, ponds, fresh water, streams, and Salt Lake.
• These are found in soil, on rocks, stones, tree bark, and on the surfaces of other animals and plants.
• Small aquatic forms make up plankton, which is free-floating microscopic life in water.
• Phytoplankton consists of algal forms, while zooplankton is composed of animal organisms.
• Some species thrive on snow and ice in polar regions and mountain peaks, as well as in hot springs with temperatures as high as 55°C.
• Endophytic algae are not free-living, instead live within other organisms like protozoa, mollusks, sponges, and corals.

Characteristics

• Algae display a wide range of sizes and shapes.
• Unicellular algae can be spherical, rod-shaped, club-shaped, or spindle-shaped, while many others are multicellular with complex forms.
• Algal cells are eukaryotic.
• Most species have a rigid, thin cell wall.
• Diatoms have silica in their cell walls, making them thick and rigid.
• Motile algae have flexible cell membranes called Periplasts.
• Spirulina is an example of motile algae
• Algae contain a discrete nucleus.
• Chlorophyll and other pigments reside in membrane-bound organelles known as Chloroplasts.
• Thylakoids are flattened membrane vesicles found within the plastid matrix or stroma.

Algal Pigments

• Algae contain three kinds of photosynthetic pigments: chlorophyll, carotenoid, and biloproteins or phycobilins.
• Chlorophyll A is present in all algae.
• Chlorophyll B is found in Euglenophycophyta and Cholorophycophyta.
• Chlorophyll C is present in Xanthophycophyta, Bacillariophycophyta, Chrysophycophyta, and Cryptophycophyta.
• Chlorophyll D occurs in Rhodophycophyta Chlorophyll E in Xanthophycophyta.

Carotenoids

• Two kinds of carotenoids exist: carotenes, which are linear, unsaturated hydrocarbons, and xanthophylls, which are oxygenated derivatives.

Biloproteins or Phycobilins

• These are water-soluble and present in Rhodophycophyta
• Phycocyanin and Phycoerythrin represents the two types of phycobilins.

Reproduction

• Algae reproduce asexually or sexually.

Asexual Reproduction

• Unicellular algae undergo asexual reproduction by mitosis.
• Multicellular algae reproduce asexually through vegetative means or mitotically produced asexual spores called mitospores.
• Zoospores are flagellate motile spores.
• Aplanospores are non-motile spores.
• Autospores are non-motile and produced by chlorella.

Sexual Reproduction

• Asexual reproduction involves gamete formation, fertilization, and meiosis.
• In algae, three types of sexual life cycles are reported: halpontic, diplontic, and haplodiplontic.
• Gametangia are sex organs of algae.
• Antheridium represents the male sex organs.
• Oogonium represents the female sex organ.
• Isogametes are identical gametes, and the infusion process is isogamous.
• Spirogyra is an example of isogametes
• Anisogametes differ in size, and the fusion process is anisogamous.
• Chlamydomonas is an example of anisogamy
• A smaller gamete is male, whilst the larger one is female.
• Oogametes consist of a large, non-motile female gamete (egg) and a small motile male gamete
• Oogamy is exemplified by Volvox and fucus.

Classification

• Algae are classified into three main groups: green, red, and brown algae.
• Primary photosynthetic pigments, storage products, cell wall composition, and the type/location of flagella dictates algal classification.

Green Algae – Chlorophyceae

• Exist in freshwater, marine, and terrestrial environments.
• Photosynthetic pigments include chlorophyll A and B, along with carotenoids.
• Cell wall components include cellulose.
• Starch is stored inside the plastids for food.
• Most cells contain two flagella.
• Asexual reproduction occurs vegetatively by fermentation or by the formation of mitospores.
• Sexual reproduction can be isogamous, anisogamous, or oogamous.
• Chlamydomonas, Volvox, ulothrix, chara, and ulva (sea lettuce) are examples.

Red Algae – Rhodophyceae

• Inhabit marine and freshwater environments.
• Chlorophyll A and D, carotenoids, and phycobilins serve as photosynthetic pigments.
• The presence of phycoerythrin causes the red tint.
• Cell wall components include cellulose
• Floridian starch (alpha -1,4 glucon) is stored in the cytosol as food.
• Flagella is absent.
• Asexual reproduction occurs through fragmentation or by non-motile mitospores.
• Sexual reproduction is oogamous, and gametes are non-motile.
• Polysiphoria, Gracilaria, and Gelidium serve as examples of red algae

Brown Algae – Phaeophyceae

• Thrive in marine and freshwater habitats

• Photosynthetic pigments are chlorophyll A and C, carotenoids, and phycobilins.

• Xanthophyll pigment, fucoxanthin dictates the shade of brown.

• The cell wall comprises cellulose and alginic acid

• A gelatinous coating of Algin covers the cellulose.

• Laminarin and mannitol serve as storage food

• 2 flagella (unequal and lateral)

• Asexual reproduction by fragmentation or by motile zoospores

• Sexual reproduction is isogamous, anisogamous, or oogamous

• Ectocarpus, Laminaria, fucus serve as examples

• Diverse pigments, reserve food, and cilia characterize Algae.

• Morphological and physiological differences dictate the classification.

• Fritsch classified all algae into eleven classes based on pigment type, nature of reserve food material, and mode of reproduction in 1935

• Classes are: Chlorophyceae, Xanthophyceae, Chrysophyceae, Bacillariophyceae, Cryptophyceae, Dinophyceae, Chloromonodineae, Euglinineae, Phaeophyceae, Rhodophyceae and Myxophyceae (Cyanophyceae).

• Fritsch published the classification in his book titled "The Structure and Reproduction of Algae".

Class: Chlorophyceae (Green Algae)

• Occurrence: Most are fresh water, while a few are marine.
• Pigments: Chlorophylls a and b and carotenoids are the main pigments (yellow pigments).
• Reserve food: Starch
• Structure: Unicellular motile to heterotrichous filaments
• Cell walls consist of cellulose. Pyrenoids are surrounded by starch sheaths. Equal flagella characterize motile cells (24).
• Reproduction: Sexual reproduction ranges from isogamous to advanced oogamous
• Example: Chlamydomonas, Volvox, Chlorella, Scenedesmus, Pediastrum

Class: Xanthophyceae (Yellow-Green Algae)

• Occurrence: mostly the freshwater, few marine forms
• Pigments: have abundant yellow xanthophyll
• Oil is the reserve food
• Structure: From unicellular motile forms to simple filamentous structures
• Have cell walls that are rich in pectic compounds. Cell walls composed of two roughly equal pieces and overlap at the edges
• Motile cells will have two very unequal flagella. There are also no pyrenoid
• Reproduction: rare to find sexual reproductions, if present then it will always be isogamous
• Example: Vaucheria

Class: Chrysophyceae

• Occurrence: Mostly occur in cold, freshwater; few occur in marine water
• Pigments: orange or brown chromatophores. Additionally, accessory pigments serve as phycochrysin
• Fat and leucosin are reserve food
• Structure: Motile, unicellular to filamentous branched
• At front end, have two unequal flagella that attached
• Reproduction: rare, if present then of isogamous type
• Example: Chrysodendron, Phaeothamnion

Class: Bacillariophyceae (Diatoms)

• Occurrence: Fresh water, Marine water
• Pigments: Contain chromatophores that are definite, yellow or golden brown
• Volutin and Fat serve serve as reserve food
• Structure: The structure of all bacillariophyceae is cellular or colonial. The presence of 2 halves is present in the cell wall with richly, ornamental texture. The parts within consist of pectin and silica
• Reproduction: diploid with a special condition with protoplast fusion of the cell that ordinary
• Example: Pinnularia

Class: Cryptophyceae

• Occurrence: freshwater and marine
• Pigments: possess diverse pigments by chromatophores (typically brown)
• Starch or Solid Carbohydrates serve as reserve food to some cells
• Structure: The cells can be coccoid, most advanced motile cells will have slightly unequal flagella
• Reproduction: is mainly related to reported forms in cases
• Example: Chroomona

Class: Dinophyceae

• Occurrence: Seawater plankton, sometime found in freshwater as well
• Pigments: Chromaophores (dark yellow, brown), special pigments
• Oil and Starch serve as reserved food material for the cell
• Structure: From filamentous branched to unicellular motile
• Reproduction: in a rare number of cases from isogamous form
• Example: Cermatium, dinoflagellate

Class: Chloromonadineae

• These algae are only found in freshwater habitat
• Have green or bright chromatophores
• The oil is the reserved food inside
• The plants are motile, flagellate with two almost equal flagella.
• Reproduction: can take place by cells dividing via longitudinal division, sexual reproduction is absent
• Example: Tretontonia

Class: Euglenineae

• Occurrence: The plants are found in fresh water habitat or
• Pigments: the chromatophores can range from pure green
• The storage food can be from polysaccharide (paramylon)
• Structure: have 1-2 flagella that arise from the cellular base (front end) Sexual reproduction is not known. Isogamous type
• Example: Eugle

Class: Phaeophyceae (Brown Algae)

• Mostly marine or ocean-based with a diverse pigment consisting of carotenes, chlorophylls (a+c), and xanthophylls
• The reserved food can be divided in the form of mannitol and fats
• Structure: From filamentous unbranched/ simple to branched parenchymatous with internal or external alternation Sexual production ranges from gametes with flagella inserted laterally with alternating generations
• Example: Ectocarpus, Sargassum

Class: Rhodophyceae (red algae)

• In aquatic ecosystems, the freshwater and marine system are included
• Pigments: Chl-α+δ red algae are blue with pigments containing phycoerythrin and anthocyanin
• Reserved Food: can be either carotenes or Floridian starch
• Structure: The structure can have 2 phases in terms of cellularity. There could be cells that are Simple filaments. in contrast they attain considerable complexity that lacks known motile traits
• Reproduction: in most cases sexual type with advanced traits
• Example: Batrachospermum Polysiphonia

11. Class: Myxophyceae (Cyanophyceae or Blue-Green Algae)

• This Class is mostly found in sea and fresh water
• Have a pigment consisting of Chlorophyll, phycocyanin and carotenes
• Sugar and glycogen are stored in the respective regions,
• Structure: have simple cell types to filamentous structures or forms with or without branching, typically a cell lacks the correct form of nucleus or motile stages
• Reproduction in most cases there is no known sexual reproduction present
• Example: The type of algae from this class is known as Nostoc and Oscillatoria