Alga

Definition

A photosynthetic, primarily aquatic organism that lacks the true tissues and embryonic development characteristic of land plants (embryophytes). Algae range from microscopic unicellular forms to multicellular seaweeds tens of meters long. Though not a formal taxonomic group, the term encompasses organisms across multiple kingdoms—most notably green algae (Chlorophyta), red algae (Rhodophyta), brown algae (Phaeophyceae), and cyanobacteria (blue-green algae, historically grouped with algae). Algae serve as foundational primary producers in aquatic and moist terrestrial ecosystems.

Etymology

Latin alga, meaning 'seaweed' or 'kelp'

Example

The aquatic larvae of many caddisflies (Trichoptera) graze on periphyton—filamentous algae and associated microbes coating submerged stones in streams.

Related Terms

• phytoplankton
• periphyton
• Symbiosis
• primary producer
• Photosynthesis
• cyanobacteria
• lichen

Usage Notes

In strict technical usage, 'alga' (singular) and 'algae' (plural) are preferred; 'algae' is frequently treated as a collective singular in general writing, which specialists avoid. The term excludes land plants but historically included cyanobacteria—now classified in Bacteria—so 'blue-green algae' is a persistent misnomer. In entomological contexts, algae appear in studies of aquatic insect nutrition, biofilm ecology, and mutualisms such as those between algae and certain ant or termite lineages.