Muddiest Points (5): Algae and the Origin of Land Plants

Algae

A functional group that includes photosynthetic and aquatic organisms, comprising both eukaryotes (like green, red, and brown algae) and prokaryotes (like cyanobacteria).

Eukaryotes

Organisms whose cells contain a nucleus, including green algae (Archaeplastida), red algae (Archaeplastida), and brown algae (SAR).

Prokaryotes

Single-celled organisms without a nucleus, such as cyanobacteria, often referred to as 'blue-green algae'.

Key Traits of Algae

Photosynthetic and aquatic characteristics.

Ulva

A type of green algae that shares photosynthesis with kelp but diverged approximately 1.5 billion years ago.

Red Algae

A group of algae characterized by pigments such as chlorophyll a and phycoerythrin, which masks the green color, found in deep water (phycoerythrin absorbs blue light), have no flagella, and never colonized land

Green Algae

A group of algae that contain chlorophyll a and b, found in shallow water/ are land ancestors, flagella is present in many, and they gave rise to land plants (via charophytes)

Charophytes

A subgroup of green algae that evolved sporopollenin and plasmodesmata, key adaptations for land colonization.

Endosymbiosis

The process by which a eukaryote engulfs a prokaryote, leading to the evolution of organelles like plastids.

Primary Endosymbiosis

Involves a cyanobacterium being engulfed by a eukaryote, leading to the development of Archaeplastida.

Secondary Endosymbiosis

Occurs when a eukaryote engulfs a red or green alga, resulting in organisms like brown algae and chlorarachniophytes.

Visual clue of counting plastid membranes

Count of membranes can indicate the type of endosymbiosis: 2 membranes for primary and 3-4 membranes for secondary.

Plants vs. Algae

Plants are monophyletic, descending from charophyte green algae, while algae are paraphyletic, including ancestors and unrelated groups.

Testable Trait of Plants

Plants have embryos (protected zygotes), whereas algae do not.

Reproduction in Red Algae

Red algae rely on water currents to move gametes due to the absence of flagella.

Reproduction in Green Algae

Some green algae, like Chlamydomonas, use flagella, while others, like Ulva, alternate between haploid and diploid generations.

Why does red algae change color with depth

Phycoerythrin absorbs blue/green light (penetrates deep water), masking chlorophyll's green. In shallow water, chlorophyll becomes visible → reddish-green

Is Caulerpa multicellular

No—it's a single giant cell with multiple nuclei (syncytium). Ulva is truly multicellular

Do all algae photosynthesize

Some not exclusively, e.g. euglenids can switch to predation we would refer to them as "mixotrophic"

Algae

ecological term for photosynthetic protists + cyanobacteria

Red vs. green algae

Pigments, flagella, and evolutionary trajectories differ dramatically

Endosymbiosis

explains plastid diversity (membrane counts are clues)

Land plants evolved from charophytes

a small subset of green algae with key adaptations