Archaeplastida

Archaeplastida, also called archaeplastids, are a large group of living things. They include green algae, land plants, red algae, and a smaller group called glaucophytes. Most of these organisms can make their own food using sunlight. This process is called photosynthesis.

They have special parts in their cells called chloroplasts that help them do this. These chloroplasts have two layers and contain a green pigment called chlorophyll. Scientists think these chloroplasts came from tiny blue-green bacteria called cyanobacteria. These bacteria lived inside the cells long ago and helped both survive.

Archaeplastids share some common features. They have walls made of a substance called cellulose around their cells. They also store food as a material called starch. However, each group in Archaeplastida has its own special traits. For example, red algae have different pigments than green algae and plants. They store their starch outside their chloroplasts, while green algae and plants store it inside. These differences help scientists learn about how these organisms evolved and are related to each other.

Taxonomy

The group of organisms called Archaeplastida includes glaucophytes, red algae, green algae, and land plants. In 2005, scientists named this group and thought it was one family of related organisms. Some later studies were unsure, but recent research shows it is likely one family.

Archaeplastida has had many names. Some scientists call it plants or Plantae. But this name can be confusing because it has also been used for smaller groups. To avoid confusion, some scientists use names like Primoplantae or Plastida. The name Archaeplastida was suggested in 2005 by many scientists working together.

Archaeplastida includes several smaller groups:

• Glaucophyta – small, fresh-water algae with unique cell structures.
• Rhodophyceae – red algae, mostly found in the ocean and known for their red color.
• Chloroplastida – includes green algae and land plants, all of which have special cell structures for capturing sunlight.

Morphology

All archaeplastidans have special parts in their cells called plastids that help them make food using sunlight. Scientists think these plastids came from tiny blue-green organisms called cyanobacteria that lived inside these cells long ago. In one group called glaucophytes, the plastids are named cyanelles and look very similar to cyanobacteria.

Most archaeplastidans have walls around their cells, often made of a substance called cellulose. These organisms can be very different in how their cells are organized — some are single cells, while others form chains, groups, or even many-celled plants. The first archaeplastidans were single cells, and many still are today. Being many-celled evolved separately in different groups, like red algae, some green algae, and the algae that led to stoneworts and land plants.

Endosymbiosis

Main article: Endosymbiotic theory

Scientists believe the first archaeplastidan got its chloroplasts by taking in a type of bacteria called cyanobacteria. This event is called primary endosymbiosis. In 2013, a green algae named Cymbomonas tetramitiformis was found to get energy in two ways: by eating other tiny creatures and by using sunlight. We do not yet know if this ability was part of the earliest archaeplastidan or if it was gained later.

One clue that supports primary endosymbiosis is that chloroplasts have two layers of membrane—one from the bacteria and one from the cell that took it in. Over time, many genes from the chloroplasts moved into the host cell’s nucleus.

Other tiny creatures got their chloroplasts by taking in an archaeplastidan that already had them. This is called secondary endosymbiosis. These chloroplasts usually have more than two layers of membrane because they were taken in more than once. Some groups, like euglenids, chlorarachniophytes, and some dinoflagellates, seem to have gotten their chloroplasts from green algae. Others, like heterokont algae, cryptophytes, haptophytes, and some dinoflagellates, seem to have gotten theirs from red algae.

Fossil record

The oldest possible remains of Archaeplastida might be red algae called Rafatazmia found in rocks from India that are about 1600 million years old. Other early fossils, like Tuanshanzia from China, are from around the same time. More recent fossils from Australia look like modern green algae and are from the Mesoproterozoic Era, about 1500 to 1300 million years ago.

Later, in the Neoproterozoic Era, more algae fossils appeared. Finally, in the Paleozoic Era, plants began to grow on land and have been thriving ever since.