Algae are a diverse group of organisms, not classified as true plants, but united by their ability to photosynthesize and their typically aquatic habitats. Ranging from microscopic single cells to large seaweeds, algae form the base of most aquatic food webs as primary producers. Understanding their composition, from cellular architecture to specialized chemical compounds, reveals their ecological success and growing importance in technology and nutrition.
The Fundamental Cellular Structure
The physical architecture of algae is highly varied, encompassing both prokaryotic and eukaryotic cell types. Prokaryotic algae, such as Cyanobacteria (blue-green algae), lack a membrane-bound nucleus and complex organelles. Their genetic material is concentrated centrally, and their photosynthetic machinery is contained within specialized thylakoid membranes dispersed throughout the cytoplasm.
The majority of algae are eukaryotic, possessing a true nucleus enclosed by a membrane. Eukaryotic algae contain organelles like mitochondria for cellular respiration and chloroplasts, which are the sites of photosynthesis. These forms range from unicellular microalgae to complex multicellular macroalgae, or seaweeds, which can form large body structures called a thallus.
Essential Chemical Building Blocks
The bulk chemical composition of algae, on a dry weight basis, is constructed from the major biological macromolecules: proteins, lipids, and carbohydrates. Proteins serve as structural components, enzymes that catalyze metabolic reactions, and as a source of nitrogen.
The protein content can be particularly high; some species, like Spirulina platensis, contain up to 60-71% protein by dry weight, offering a complete amino acid profile. Lipids, including fats and oils, are used primarily for energy storage and for constructing cell and organelle membranes. Carbohydrates function as energy sources and form the structural backbone for the cell wall. Their ratio and composition vary significantly based on the species and environmental conditions.
Photosynthetic Pigments and Machinery
The ability of algae to harvest light energy depends on photosynthetic pigments. Chlorophyll a is the primary pigment present in all oxygen-producing photosynthetic organisms. It is the molecule that directly converts light energy into chemical energy, absorbing light most effectively in the violet-blue and red regions of the visible spectrum.
Algae utilize accessory pigments to broaden the range of light wavelengths they capture. These include chlorophyll b (in green algae) and chlorophyll c (common in brown algae and diatoms), as well as carotenoids. Carotenoids, which are yellow, orange, or red, help protect chlorophyll and transfer absorbed light energy to chlorophyll a. Red algae, which grow in deeper waters, rely on phycobiliproteins, such as phycoerythrin, which absorb the blue-green light that penetrates those depths.
Specialized Structural Materials and Energy Stores
Beyond common macromolecules, algae produce unique compounds for specialized structural roles or distinct energy stores. Many macroalgae produce specialized polysaccharides that form their cell walls, differing from the cellulose found in terrestrial plants.
Structural Polysaccharides
Brown algae are a source of alginates, linear polymers responsible for the flexible, gelling properties of their tissues. Red algae produce agar and carrageenan, which are sulfated polysaccharides composed of repeating galactose units. These compounds are commercially utilized for their strong gelling and thickening properties in food and pharmaceutical applications.
Energy Storage
While some algae store energy as starch, others, particularly microalgae, store high concentrations of specific oils and triglycerides. These specialized lipids often contain valuable polyunsaturated fatty acids, representing a key difference from general storage compounds in other organisms.