Archaeplastida

Archaeplastida, also called archaeplastids, are a large group of living things that include green algae, land plants, red algae, and a smaller group called glaucophytes. Most of these organisms can make their own food using sunlight through a process called photosynthesis. They have special parts in their cells called chloroplasts that help them do this.

These chloroplasts in archaeplastids have two layers of membrane and contain a green pigment called chlorophyll. Scientists believe these chloroplasts came from tiny blue-green bacteria called cyanobacteria that lived inside these cells long ago, helping both to survive. This is different from other algae that got their chloroplasts from eating other organisms.

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

Taxonomy

The group of organisms called Archaeplastida includes glaucophytes, red algae, and green algae along with land plants. In 2005, scientists named this group and believed it was a single family of related organisms. Some later studies suggested it might not be a single family, but recent research supports the idea that it is.

Archaeplastida has been called different names by scientists. Some simply call it plants or Plantae. However, 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 proposed 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 specific 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 think that the first archaeplastidan got its chloroplasts by swallowing a type of bacteria called cyanobacteria. This event is called primary endosymbiosis. In 2013, a green algae called Cymbomonas tetramitiformis was found to get energy in two ways: by eating other tiny creatures and by using sunlight. We don't 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 swallowed it. Over time, many genes from the chloroplasts moved into the host cell’s nucleus. It’s thought that 6–20% of the archaeplastidan genes came from this process.

Other tiny creatures got their chloroplasts by swallowing an archaeplastidan that already had them. This is called secondary endosymbiosis. These chloroplasts usually have more than two layers of membrane because they were swallowed 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.