Algae is the broad group — organisms that photosynthesise but are not plants, ranging from single microscopic cells to giant kelp, in fresh water and salt. Seaweed is an everyday name for the large marine kinds you can see and pick up. All seaweed is algae; most algae is not seaweed.
The core difference
- algae — a scientific grouping of photosynthesising organisms that are not plants. Includes microscopic single-celled forms, pond scum and kelp.
- seaweed — an informal term for large marine algae, visible to the naked eye.
Why algae are not plants
They photosynthesise, which makes them look plant-like, but they lack the roots, stems, leaves and vascular tissue that define plants. Kelp's root-like anchor is a holdfast: it grips rock but takes up no nutrients, because the whole surface absorbs directly from the water.
The grammar point
Algae is plural; the singular is alga. "An algae" is a common slip. Seaweed is usually uncountable — you gather seaweed, not seaweeds, though biologists do use seaweeds for distinct kinds.
Related word
Phytoplankton covers the microscopic drifting algae responsible for a very large share of the world's photosynthesis — a reminder of how much of the group is invisible.
Frequently asked questions
Is seaweed a plant?
No. Seaweed is large marine algae. It photosynthesises but lacks the roots, stems and vascular tissue that define plants.
What is the singular of algae?
Alga. Algae is plural, so "an algae" is technically incorrect, though it is common in speech.
Is all algae seaweed?
No. Most algae are microscopic or live in fresh water. Seaweed refers only to the large marine kinds.