Algae Classification: Types, Pigments, and Aquatic Role

The Vital Role of Algae in Aquatic Life

Algae are a very large group of photosynthetic protoctists dependent on the aquatic environment. They all possess chlorophyll pigments, which give them their coloration, and their cells have a cell wall composed of cellulose. Algae colonize all aquatic habitats.

Ecological Importance and Reproduction

Through photosynthesis, algae form the first link in the food chain in ecosystems, discharging large amounts of O₂ into the atmosphere.

Reproduction is typically:

• Asexual: By binary fission or fragmentation.
• Sexual: Involving the interference of gametes and meiospores.

Most algae are microscopic, unicellular, or colonial, forming phytoplankton that floats in the upper, illuminated layers of the oceans and is of vital importance for the ecosystem. Multicellular forms have a homogeneous organization, lacking differentiated tissues and organs.

Classification of Algae Groups

Algae are classified according to their pigments, reserve substances, and degree of organization:

1. Euglenophytes

   Flagellated, unicellular microorganisms, free-living, that abound in stagnant waters. They can live as saprophytes. They have green plastids and accumulate oils as a reserve. They lack a cell wall.

   • Example: Euglena (known to stain water green).

2. Pyrrophytes (Dinoflagellates)

   Unicellular microorganisms provided with two long flagella, forming an important part of marine phytoplankton. Most are photosynthetic and have red plastids. They accumulate oils and starch. They have a cell wall composed of numerous cellulose plates, often with very long extensions that facilitate their suspension in water (H₂O). Reproduction occurs by isogametes and longitudinal division. They are known to cause red tides.

   • Examples: Noctiluca, Peridinium.

3. Chrysophytes (Golden Algae)

   Single-celled organisms that are grouped in colonies or filaments. Their skeletons form rocks (diatomaceous earth). They accumulate oils as a reserve. The cell wall is impregnated with silica. They reproduce by binary fission and gametes.

4. Chlorophytes (Green Algae)

   These are components of plankton, living in the sea or humid places. They reserve starch, and their cell wall is composed of cellulose. They multiply vegetatively and sexually.

   • Example: Chara.

5. Phaeophytes (Brown Seaweed)

   Brown seaweed constitutes the vegetation of the sea. They contain fucoxanthin, which gives them their brown color. They reserve starches and have developed very diverse body organizations. Reproduction involves alternating haploid and diploid phases.

   • Examples: Fucus, Padina.

6. Rhodophytes (Red Algae)

   They owe their color to phycoerythrin. They reserve a polysaccharide intermediate between starch and glycogen. The cell wall is pectic, covered with gelatinous material. Reproduction involves alternation of generations.

   • Examples: Gelidium, Ceramium.