Introduction to Algae

• Algae are chlorophyll-bearing, simple, thalloid, autotrophic, eukaryotic and largely aquatic organisms found on moist stones, soils and wood.
• Some algae appear in association with fungi (lichen) and animals like sloth bears.
• Algae vary in form and size, ranging from microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox, and filamentous forms like Ulothrix and Spirogyra.
• Kelps are marine algae that form massive plant bodies.
• Thallus refers to the algal body, which is undifferentiated into root, stem, or leaves. Vascular tissues and stomata are absent.
• Phytoplankton are photosynthetic algae floating on the water surface in the sunlit layer.
• Benthic algae are attached to a substratum at the bottom of the water, such as brown algae (Phaeophyceae).
• Lichens are composite organisms from a symbiotic relationship between algae (or cyanobacteria) and fungi.
• Epiphytes are algae found on other living plants and bigger species of algae
• Lithophytes are algae living on or within rocks.
• Algae reproduce via vegetative, asexual, and sexual methods.
• Vegetative reproduction occurs through fragmentation where each fragment develops into a thallus.
• Asexual reproduction is through spores, most commonly flagellated motile zoospores, which germinate into new plants.
• Sexual reproduction involves the fusion of two gametes
• Flagellated gametes of similar size, as seen in Chlamydomonas, undergo isogamous reproduction.
• Anisogamous reproduction involves the fusion of dissimilar sized gametes like in some species of Chlamydomonas.
• Oogamous reproduction involves the fusion of a large, non-motile female gamete and a smaller, motile male gamete, examples include Volvox and Fucus.
• Algae fix about half of the total carbon dioxide on Earth via photosynthesis, increasing dissolved oxygen levels and serving as primary producers of energy-rich compounds that form the basis of aquatic food cycles.
• Porphyra, Laminaria, and Sargassum are among the 70 marine algae species used as food.
• Marine brown and red algae produce hydrocolloids like algin (from brown algae) and carrageen (from red algae), which are used commercially.
• Agar, derived from Gelidium and Gracilaria, is used in growing microbes and in ice cream/jelly preparations.
• Chlorella, a unicellular alga rich in proteins, is used as a food supplement, even by space travelers.
• Iodine is obtained from kelps, specifically Laminaria.

Chloroplast Shapes in Algae

• Chloroplasts in algae can have various shapes, including cup-like, ribbon-like, and star-like.

Thallus Organization in Algae

• The thallus, or body, of algae can be organized as unicellular, multicellular, or colonial forms.
• Unicellular forms can be motile or non-motile.
• Multicellular forms can be filamentous, simple, or branched.
• These algae can either be attached to a substrate by a holdfast or be free-floating.

Flagella Structure

• Eukaryotic flagella consist of a "9+2" arrangement, with a ring of 9 peripheral microtubule doublets surrounding 2 single central microtubules.

Algae Classification Basis

• Algae classification into major phyla is based on:
• Major photosynthetic pigments present
• Form of stored food
• Cell wall composition
• Number of flagella and their insertion position
• Habitat.

Main Algae Phyla

• Algae are divided into three main phyla: Chlorophyta, Chromophyta, and Rhodophyta.

Chlorophyta (Green Algae)

• The members of chlorophyta are commonly green algae and their plant body can be unicellular, colonial, or filamentous
• Like land plants, they have chloroplasts with chlorophylls a and b, giving them a grass-green color
• Pigments are localized in definite chloroplasts that can be discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped
• Most members have one or more storage bodies called pyrenoids in the chloroplasts, containing protein besides starch and store food as oil droplets
• Green algae have a rigid cell wall of cellulose inner layer and an outer layer of pectose
• Vegetative reproduction takes place by fragmentation or forming different types of spores
• Asexual reproduction is by flagellated zoospores produced in zoosporangia
• The sexual reproduction varies and it may be isogamous, anisogamous or oogamous
• Common green algae include Chlamydomonas, Volvox, Ulothrix, Spirogyra, Ulva and Chara
• Green algae are divided into charophytes and chlorophytes:
• Charophytes include algae most related to plants.
• Chlorophytes include over 7,000 species, mostly in fresh water, but also marine and terrestrial species.

Chromophyta

• This phylum includes yellow-green algae, golden-brown algae, diatoms, and brown algae
• Phaeophyceae (brown algae) live primarily in marine habitats and vary greatly in size and form
• They range from simple branched, filamentous forms (Ectocarpus) to profusely branched kelps, reaching up to 100 meters
• They possess chlorophyll a, c, carotenoids and xanthophylls
• Brown algae color ranges from light yellow-brown to almost black due to brown pigment fucoxanthin, in addition to chlorophylls a and c, xanthophyll and other pigments in the chloroplasts
• Food is stored as complex carbohydrates, such as laminarin or mannitol
• Vegetative cells have cellulosic walls covered by a gelatinous coating of algin
• The protoplast contains plastids a centrally located vacuole and nucleus
• The body attaches to the substratum by a holdfast, with a stalk (stipe) and leaf-like photosynthetic organ (frond)
• Vegetative reproduction is by fragmentation, asexual reproduction in most brown algae is by biflagellate zoospores that are pear-shaped and bear two unequal laterally attached flagella
• Gametes are pyriform (pear-shaped) & bear laterally attached flagella, and common forms include Laminaria, Fucus, Sargassum, and Ectocarpus
• Diatoms are diploid, photosynthetic, single-celled or colonial protists secreting a protective silica shell
• The shell consists of two overlapping parts that fit together
• Diatoms have a cylindrical shape, but others are triangular, square, or needle-like

Reproduction in Diatoms

• The two rigid halves of each cell separate, and the new cell develops a new half shell inside the old one, resulting in smaller cells with each generation.
• At one point, the diploid nucleus of a reduced-sized cell undergoes meiosis, forming 4 gametes.
• The zygote, produced when two gametes unite, enlarges into an auxospore, which develops into a diatom of the same size as the original diatom.

Rhodophyta (Red Algae)

• Red algae colors are due to the presence of red phycoerythrin and blue phycocyanin accessory pigments called phycobilins, similar to cyanobacteria
• Additional pigments includes chlorophyll a and sometimes chlorophyll d in the chloroplasts
• A carbohydrate called floridean starch is their principal reserve food
• They produce agar and gelatinous substances
• The majority is marine in warmer areas, also occur in deep oceans
• Red algae thalli are multicellular with complex body organisation
• Food is stored as floridean starch
• They reproduce vegetatively by fragmentation; asexually by non-motile spores; and sexually by oogamous reproduction with post fertilization developments
• Common members are Polysiphonia, Porphyra, Gracilaria and Gelidium.

Life Cycle of Red Alga (Porphyra)

• Porphyra, or nori, has a life cycle where:
• The haploid gamete-forming body is sheetlike, like a blade
• Gametes form at its edges
• Fertilization produces a diploid zygote
• The zygote develops into a spore-forming body; sporophytic stage.
• Haploid spores form by meiosis and are released.
• Spores germinate and develop into gamete-forming body

Algal Classes

• Chlorophyceae (Green algae): contains chlorophyll a and b and starch and cellulose, with 2 to 8 equal apical flagella and lives in fresh, brackish and salt water.
• Phaeophyceae (Brown algae): contains chlorophyll a and c. and mannitol and laminarin with cellulose and algin, with two unequal lateral flagella, and mostly live in salt water, and rarely occurs in fresh water.
• Rhodophyceae (Red algae): contains chlorophyll a and d. and phycoerythrin with floridean starch and cellulose with flagella and lives in fresh, brackish, and salt water.