General Characteristics of Algae

Topic Outline

• The notes cover cellular and subcellular organization, reproduction and life cycles, physiology and biochemistry, and the ecology, evolution, and phylogeny of algae.

Intended Learning Outcomes

• The goal is to describe the general characteristics of algae.
• The intention is to list unique algal features regarding structure, cellular components, and cellular organization.
• Understanding how these features relate to algal growth, photosynthesis, pathogenicity, and toxin production is key.
• The aim is to illustrate and describe algal reproduction and life cycles.
• An explanation of the physiological mechanisms, biochemistry, ecology, evolution, and Phylogeny of algae is included.

Distinctive Characteristics of Algae

• Algae are chlorophyll-bearing, thalloid plants without tissue differentiation.
• Nearly all algae are aquatic.
• Algal sex organs are typically unicellular, and if multicellular, each cell can reproduce.
• Algal sex organs lack a surrounding sterile jacket layer.
• Embryo formation does not occur after gametic fusion.
• Sporophytic and gametophytic generations are independent when both are present in the life cycle.
• Algae reproduce through vegetative, asexual, and sexual modes.
• Algae contain chlorophyll.

Major Groups of Algae

• Algae, which are diverse photosynthetic organisms, are systematically classified using a variety of criteria. One aspect is nuclear organization, which refers to whether their nuclei are organized in a simple or complex manner. Additionally, the components of their cell walls, which can vary significantly among different algae, play a critical role in classification. Pigments are also a defining feature, as they influence the color of the algae and determine their ability to absorb light for photosynthesis. Furthermore, the presence or absence of flagella along with their arrangement contributes to mobility classification. Other distinguishing factors include the chemical composition of their reserve materials, such as starch or oils, and the various life cycle strategies they employ, which can range from asexual reproduction to more complex stages involving gametes.

Nuclear Organization

• Algae can be prokaryotic or eukaryotic.
• Cyanophyceae, also known as cyanobacteria (blue-green algae), are prokaryotic.
• Cyanophyceae lack a nuclear membrane, with genetic material present as chromatin threads.

Prokaryotic Cell Structure

• Prokaryotic algae have an incipient/primitive nucleus.
• Nuclear membranes and histones are absent.
• Fibrils of DNA are present.
• DNA is either concentrated in the middle or extended throughout the cell.
• Chlorophyll is found in lamellae or thylakoids.
• Thylakoids are in the periphery of the cytoplasm and arranged parallelly.
• The cell wall contains mucopeptide, which provides strength and is exclusive to prokaryotic algae.
• Mitochondria, Golgi bodies, chloroplasts, and endoplasmic reticulum are absent.

Eukaryotic Cell Structure

• Eukaryotic algae have a distinctive, organized nucleus with a nuclear membrane and membrane-bound organelles.
• Algal cells contain a single or double nucleolus, forming a complex nuclear mass.
• Golgi bodies, made of 2-20 stacked vesicles, are present and involved in cell metabolite synthesis.
• Mitochondria, the powerhouse of the cell, are present.

Eyespot Types

• Type A: Part of the chloroplast, with no association with flagella, e.g., Chlorophyceae and Cryptophyceae
• Type B: Seen in the chloroplast and associated with the flagellum
• Type C: Independent clusters of osmophilic granules, e.g., Euglenophyceae

Ocelloid Structure

• Ocelloid is a subcellular structure in the Warnowiaceae family.
• Ocelloids are analogous in structure and function to multicellular organism eyes.
• Ocelloids include subcomponents analogous to eyes: the lens, cornea, iris, and retina.
• Ocelloids consist of two substructures: the translucent hyalosome and the heavily pigmented melanosome (retinal body/pigment cup).

Other Organelles

• Vacuoles are present and covered by a membrane called the tonoplast; they can be simple or complex.
• Pyrenoids, organelles on chloroplasts, are the sites for starch synthesis and accumulation.
• Flagella are a common feature, except in Rhodophyceae and Cyanophyceae members.

Flagella

• Whiplash or acronematic: Smooth surface
• Tinsel or pleuronematic: Hair-like appendages

Reserve Food and Pigments

• Algae store reserve food as cyanophycean starch, floridean starch, mannitol, and laminarin starch.
• Algal pigments, responsible for thallus color, include chlorophyll, carotenoids, and phycobilins; carotenoids are divided into carotenes and xanthophylls.

Chemical Composition of Cell Wall

• The cell wall is composed of cellulose.
• The inner wall is an insoluble cellulosic layer, and the outer wall consists of pectic substances.
• Phaeophyceae cell walls contain alginic and fucinic acid.
• Bacillariophyceae cell walls contain silica.
• Rhodophyceae cell walls contain xylan and galactan.
• Cyanophyceae cell walls contain mucopeptide.

Pigments

• Chlorophyll: Includes five types - a, b, c, d, and e
• Carotenoids: Yellow or orange, responsible for various colors; include carotene, xanthophylls, and carotenoid acids
• Phycobilins/biliproteins: Special pigments, including phycocyanin, phycoerythrin, and allophycocyanin

Nature of Reserve Food Material

• Starch is a common reserve food.
• Chlorophyceae store starch.
• Cyanophyceae store myxophycean starch.
• Rhodophyceae store floridean starch.

Type of Life Cycle and Reproduction

• Classification considers the presence/absence of sexual reproduction, complexity of reproductive organs, and method of reproduction.
• Reproduction occurs vegetatively, asexually or sexually.

Sexual reproduction characteristics

• Isogamy: Fusing gametes are alike morphologically, e.g., Chlamydomonas and Cladophora.
• Anisogamy: Fusing gametes are morphologically similar but show dissimilar behavior, e.g., Pandorina and Eudorina.
• Oogamy: Male gamete is distinct from the female, with microgametes being male and macrogametes being female.
• Autogamy: Two nuclei of one or different cells fuse to form diploid nuclei; only karyogamy takes place, with no external transfer.
• Hologamy: Zygote formation occurs after the fusion of two mature individuals.

Physiology and Biochemistry

• In water, red wavelengths are absorbed in the first few meters.
• Blue wavelengths are readily absorbed when the water has average or abundant organic material.
• Green wavelengths are the most common light in deep water.
• Chlorophylls absorb red and blue wavelengths.
• Carotenoids and phycobiliproteins strongly absorb green wavelengths.

Ecology: Habitat

• Most algae are aquatic, either submerged or free-floating.
• Algae are found in pools, ponds, ditches, and rivers.
• Two main forms: freshwater and marine.
• Freshwater forms:*
  • Chlamydomonas, Volvox, and Hydrodictyon are found in stagnant water.
  • Cladophora, Oedogonium, Ulothrix, and some Vaucheria species inhabit slow-running water.
  • Common genera include Xanthophyceae, Euglenophyceae, Cyanophyceae, Chrysophyceae, Conjugales, and diatoms.
• Marine forms:*
  • Typically members of Phaeophyceaae, Rhodophyceae, and some Chlorophyceae.
  • Macroscopic thalli, also known as "seaweeds".
• Euplanktons:*
  • Free-floating and free-swimming microalgae that are never attached, such as Microcystis, Chlamydomonas, Scenedesmus, and Cosmarium.
• Tychoplanktons:*
  • Initially attached but later become free-floating, including Cladophora, Cylindrospermum, and Rivularia.

Terrestrial and Specialized Algae

• Terrestrial algae are found on the soil surface (saprophytes) or beneath it (cryptophytes).
  • Vaucheria, Botrydium, Fritschiella, and Oedocladium are examples of saprophytes.
  • Nostoc and Anabaena are cryptophytes.
• Thermophytic algae live in hot water springs at temperatures of 70 degrees or higher like Oscillatoria brevis, Synechococcus elongates, Mastigocladus, and Haplosiphon lignosum.
• Cryophytic algae live on snow-clad mountains, like Haematococcus nivalis which imparts a red color to the snow, and Chlamydomonas yellowstonensis which is responsible for the green color, mainly in the Arctic.
• Halophytic algae are found in saline water with a high salt percentage: Dunaliella, Stephanoptera, and Chlamydomonas ehrenbergii.
• Lithophytic algae grow on moist rocks, wet walls, and rocky surfaces: Rivularia and Gloeocapsa.
• Epiphytic algae grow on other aquatic plants: Oedogonium, Aphanochaete, and Bulbochaete.
• Endophytic algae are found inside higher plants: Nostoc in Anthoceros thallus and Anabaena in the coralloid roots of Cycas.
• Epizoic algae grow on mollusk shells, turtle shells, and fish fins: Cladophora on snails and bivalves, Protoderma and Basicladia on turtles.
• Endozoic algae are found inside aquatic animals: Zoochlorella in Hydra.
• Parasitic algae; Cephaleuros virescens is responsible for red rust of tea (Camelia sinensis). In addition, Polysiphonia festigata is a semiparasite on Ascophyllum nodosum.
• Symbiotic algae; Cyanophyceae (Cyanobacteria) associate with other plants and lichen with symbiotic fungi (Ascomycetes and Basidiomycetes)

Thallus Organization

• Unicellular motile form: e.g., Chlamydomonas
• Unicellular non-motile forms: e.g., Chlorella, Gleocapsa, Cosmarium, and Closterium
• Multicellular colonial forms: non-motile coenobial (coccoid) or motile coenobial.
• Filamentous thallus can be branched (Cladophora) or falsely branched (Scytonema),
• Simple unbranched filaments (Ulothrix, and Spirogyra) are found in algal forms, and can be free living
• Heterotrichous form has a highly advanced thallus with differentiation into prostrate and upright systems e.g., Fritschiella and Ectocarpus.
• Pseudoparenchymatous form looks like parenchymatous thallus but is actually a close association of cells/filaments, e.g., Batrachospermum and Polysiphonta.
• Parenchymatous form has thallus organization which is a modification of the filamentous habit with cell division in various directions. This may appear flat, leaf-like, cylindrical, or well branched e.g., Ulva, Sargassum and Laminaria.