Algae: simple plants or simply plant-like?

WHAT ARE ALGAE? THE TERM IS used for everything from microscopic bacteria and plankton to pond slime and seaweeds, but unlike animals and many other living things, algal groups are an assortment without a single common ancestor. That said, one feature is shared by most algae in every group: photosynthesis – the ability to make food from carbon dioxide and sunlight, releasing oxygen, just like plants.

- JV CHAMARY

WITH EVOLUTIONARY BIOLOGIST JV CHAMARY

Are algae actually plants? 

It’s the other way around! Chloroplasts – the structures where photosynthesis begins inside plant cells – evolved from symbiotic cyanobacteria that were incorporated into what became green algae (chlorophytes). As descendants of this group, you could say that plants are actually green algae.

Other algal groups originated via similar ‘endosymbiosis’ events (when one organism lives within another) independently, which is how the tree of life ended up with separate branches collectively known as algae. Those groups are distinguished from plants based on distinct features.

So how do they differ?

One feature is anatomy. An alga’s body is generally much simpler. Green algae have up to five types of cell, red and brown have up to 14. Plants have up to 44 cell types, organised into complex tissues and organs, such as leaves or roots, and they often transport liquid nutrients via a network of phloem and xylem vessels.