Algal Blooms and Plant Evolution

Summary

Charophyte green algae are closest relatives of land plants, sharing key cell division and wall traits.

In depth

Charophytes such as Chara and Coleochaete share plasmodesmata, phragmoplast cell division and sporopollenin. Genome analysis supports the Zygnematophyceae as closest relatives.

Shared traits with land plants include cellulose, phragmoplast-based cytokinesis and sporopollenin-coated spores. Gene families for stress and hormone signalling already existed in algal ancestors.

Remember: land plants did not evolve from seaweeds such as kelp, but from freshwater green algae.

Land plants' closest relatives, the Zygnematophyceae (and Charophytes such as Chara), have features that preceded land: plasmodesmata, phragmoplasts, cellulose walls, sporopollenin around zygotes and apical growth. Gene toolkits for stress responses, hormone signalling (ABA, auxin) and symbiosis were already in place in algal ancestors. They lived in shallow freshwater or wet ground, where drying out and UV were pressures.

The land colonisation changed Earth: plants increased weathering of rocks, lowered atmospheric CO2 in the Devonian, built soils and formed forests that created coal in the Carboniferous. Fungal partners helped early plants take up nutrients. The rise of lignin and trees changed carbon cycles, also leaving a time before fungi evolved lignin decomposers, when carbon burial may have been high.

Example: Chara has large cells used in studying cytoplasmic streaming and membrane potentials.