Red Algae

Kingdom Rhodoplantae

The red algae, or rhodophytes, are a diverse group of marine algae with fewer freshwater representatives. They are photosynthetic organisms with chlorophyll that is masked by secondary pigments, giving them a red color. Some have toxic terpenoids to prevent herbivory, and others (coralline red algae) incorporate calcium carbonate into their structure to provide a rigid defense. They are related to land plants, but have one of the most complex life cycles among living organisms, with three different phases. Agar from red algae is used as binding agent and emulsifier, as well as growing medium for culturing bacteria in lab settings. Nori (Porphyra), which is used in Japanese maki rolls, is the single most valuable marine crop grown by aquaculture with a value in excess of US$1 billion.

Above: Close-up of a red alga

Above: Coralline red algae

Form and Ecology

Extremely diverse in marine environments, but rare in freshwater
Non-motile aquatic group, which does not produce flagella in any part of its life cycle including the male gametes (see spermatia)
Mostly multicellular algae, and structurally complex
Rhodophytes lack centrioles in their cells
They are photosynthetic organisms using chlorophyll a & d
- They also use phycoerythrin and phycocyanin as accessory pigments (giving them their red color)
- The result is glucose converted into floridean starch, unlike plants which have "true" starch
Cell walls are made of cellulose and covered by agar and carrageenan

Coralline Red Algae

Some species incorporate calcium carbonate into tissues
Calcium carbonate acts as a structural deterrent to herbivorous organisms

Life cycle

Complex haplo-diplontic lifecycle
- Carposporophyte (2n): diploid stage that forms from fertilization and produces asexual carpospores
- Tetrasporophyte (2n): diploid stage that forms from carpospores, and produces haploid tetraspores through meiosis
- Gametophyte (1n): haploid stage that forms from tetraspores, and produces gametes
  - Spermatia: non-motile sperm
  - Trichogyne: female stalk that catches spermatia
Carposporophyte → mitosis → Carpospores → Tetrasporophyte → meiosis → Tetraspores → mitosis → Gametophyte → mitosis → Gametangia → mitosis → Spermatia /eggs → fertilization → Zygote → mitosis → Carposporophyte

Pit Connections & Plugs

Pit connections and pit plugs are characteristic features of red algae that develop during cytokinesis after mitosis
In red algae, cytokinesis does not fully complete, leaving a small pore in the center of the newly formed partition
This pore creates a pit connection, allowing the daughter cells to remain in contact.
Soon after the pit connection forms, cytoplasmic continuity between the cells is disrupted by the formation of a pit plug.
The pit plug is deposited within the gap in the cell wall that links the two cells.

Classification

Eukaryotes

└Archaeplastida

└Red algae

Diversity

2,500 - 6,000 species, about 670 genera
e.g. Polysiphonia, Chondrus, Porphyra

Geological Range

Mesoproterozoic - present
Rafatazmia chitrakootensis and Ramathallus lobatus are red algae discovered from Mesoproterozoic sediments in Northern India
Rafatazmia was a filamentous multicellular alga about ½ millimeter long with possible evidence of pyrenoids
Ramathallus was a thalloid algae ~½ millimeters to over 3 mm across with a pseudoparenchymatous construction

Above: Polysiphonia carposporophyte, inside the pericarp cup. The branches are the female gametophyte

Above: Male gametophytes of Polysiphonia

Above: Tetrasporophyte of Polysiphonia

Above: life cycle of the red algae

Ethnobotany

Nori

These are sheets of dried algae used for making maki (sushi) rolls
- The genus Porphyra is a red algae that provides the basis of these sheets
Nori used in Japanese maki rolls is the single most valuable marine crop grown by aquaculture, with a value in excess of US$1 billion

Dulse

Palmaria palmata, also called dulse, is an important and nutritious food source in countries along the northern Atlantic and Pacific, where it grows.

Terpenoids

These are toxic alkaloid chemicals used by red algae to prevent herbivory
A fair amount of cancer research using these chemicals has created anti-cancer drugs

Above: red algae, Porphyra, used for nori

Agar

This is a chemical that coats the cell walls of red algae, used as binding agent and emulsifier
Agar is also used as a growing medium for culturing bacteria, and frequently seen in Petri plates
Agar is also a non-animal substitute for gelatin

Questions for Thought

Why do red algae produce toxic terpenoids?
How is the red algae life cycle different from other haplo-diplontic life cycles?
How is the red algae life cycle similar to plants?
What is the function of the tetrasporophyte?
Where does the carposporophyte reside?
Why are Rhodophyta, red in color?

Additional Resources

Scientists Turn Supermarket Seaweed Into Heart Tissue (ScienceBlog 22Oct2025)

└Development and optimization of decellularized seaweed scaffolds for tissue engineering (Chithiravelu et al., 2025)

Seaweed cells could give solar panels a boost (Phys.org 11Sep2025)

└Arnould-Pétré et al. (2025) Structural colour in red seaweeds is more common and diverse than has been presumed

- New research identifies potential source populations of invasive red alga threatening Hawaii's protected coral reefs (Phys.org 24Jun2025)

└Fumo et al. (2025) A predictive framework for identifying source populations of non-native marine macroalgae: Chondria tumulosa in the Pacific Ocean

- Discovery suggests that red algae use colors for inter-species communication (Phys.org 5May 2025)

└Kawai & Motomura (2025) Structural colour in Asparagopsis taxiformis (Bonnemaisoniales, Rhodophyta) and its possible role in communicative functions

- Invasive seaweed may better adapt to changes than native species in Hawaii waters (Phys.org 14Oct2024)

└Gibson et al. (2024) Integrated physiological response by four species of Rhodophyta to submarine groundwater discharge reveals complex patterns among closely-related species

- Revealing the Ramifications of Ocean Acidification for Coralline Algae (University of Tsukuba 25Oct2021)
- Red algae feed to cows makes them less gassy (The Washington Post 27Nov2020)
- Making Pana Cotta with Red Algae (Black Forager 2020)
- Global agar shortage (New Scientist 2015)

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