Rhodophyta

Rhodophyta (commonly known as red algae, from the Ancient Greek rhodon, "rose," and phyton, "plant") is one of the oldest and largest phyla of eukaryotic algae, comprising approximately 7,500 described extant species (primarily marine macroalgae/seaweeds alongside ~200 freshwater species and unicellular extremophiles). Within the eukaryotic supergroup Archaeplastida—the monophyletic lineage descended from the single primary endosymbiosis of a β-cyanobacterium by a heterotrophic eukaryote >1.5 billion years ago—Rhodophyta is the sister clade to Viridiplantae (green algae, Chlorophyta + land plants, Embryophyta) and Glaucophyta.

Home to the oldest taxonomically resolved multicellular eukaryote in the geological record—the 1.047-billion-year-old filamentous red alga Bangiomorpha pubescens from the Hunting Formation of Somerset Island, Canadian Arctic (Butterfield, 2000; Gibson et al., 2018)—red algae also served as the plastid donor for Secondary Endosymbiosis across the majority of marine eukaryotic phytoplankton (diatoms, brown kelps, coccolithophores, cryptophytes, dinoflagellates, and apicomplexans).

1. The Rhodoplast, Unstacked Thylakoids, and Blue-Green Light Harvesting via Phycobilisomes: Unlike green algae and land plants (which lost phycobilisomes, evolved chlorophyll b, and stack their thylakoids into grana), the red-algal plastid (rhodoplast)—bounded by two membranes—retains the ancestral cyanobacterial light-harvesting antenna architecture:
   • Thylakoid membranes lie singly and equidistantly (unstacked) inside the stroma and contain only chlorophyll a (no chlorophyll b or c);
   • Studding the stromal face of each thylakoid are hemispherical protein complexes called phycobilisomes, packed with water-soluble linear tetrapyrrole (bilin) chromophores covalently attached to apoproteins: crimson-red phycoerythrin (absorbing blue-green light at 495–565 nm), blue phycocyanin, and allophycocyanin.
   • Because seawater rapidly absorbs red and orange wavelengths within the top 10–15 meters while blue-green light (480–550 nm) penetrates deepest into clear oceanic water, phycoerythrin allows crustose coralline red algae to carry out photosynthesis at depths of up to 268 meters in the Bahamas (Littler et al., 1985)—where ambient irradiance is <0.0005% of surface sunlight, making them the deepest-living photosynthetic organisms on Earth!
2. Complete Absence of Flagella, Cilia, Basal Bodies, and Centrioles at Every Life-Cycle Stage: Uniquely among major algal phyla, no red alga possesses flagella, cilia, basal bodies, or microtubule-organizing centrioles in any cell at any stage of its life cycle—not even in its male gametes! Comparative genomics (Matsuzaki et al., 2004; Brawley et al., 2017) confirmed that the ancestral red alga deleted nearly all axonemal dynein heavy-chain and intraflagellar transport (IFT) genes prior to the diversification of crown Rhodophyta, dividing its cells instead via polar ring-like microtubule organizing centers (MTOCs).
3. Cytosolic Floridean Starch and Proteinaceous Pit Plugs: Whereas green algae and land plants synthesize amylose + amylopectin starch inside their chloroplast stroma, red algae synthesize a branched α-1,4/α-1,6-glucan lacking amylose—floridean starch (structurally similar to animal glycogen and amylopectin)—as granules in the cytosol outside the rhodoplast (often clustered around the nucleus). In multicellular Bangiophyceae and Florideophyceae, incomplete cytokinetic furrowing leaves neighboring sibling cells connected by a central cytoplasmic pore, which is immediately sealed by a lens-shaped glycoprotein stopper called a pit plug (pit connection)—preventing cell-to-cell wound bleeding while providing mechanical tissue shear strength.
4. Sulfated Galactan Cell Walls (Agar and Carrageenan) and Calcitic Biomineralization: Overlying an inner microfibrillar cellulose or β-1,3-xylan framework, red algal cell walls secrete an amorphous mucilaginous matrix of sulfated polygalactans built from alternating D- and L-galactose and 3,6-anhydrogalactose units:
   • Agar (Agarose + Agaropectin): Extracted from Gelidiales (Gelidium) and Gracilariaceae (Gracilaria), forming thermoreversible hydrogels that melt at ~85 °C and solidify at ~32–40 °C (introduced to microbiology by Fanny Hesse and Robert Koch in 1882 as the universal solid culture medium for bacteria, and used as agarose for DNA gel electrophoresis!);
   • Carrageenans (κ-, ι-, and λ-carrageenan): Heavily sulfated galactans extracted from Chondrus crispus (Irish moss), Kappaphycus alvarezii, and Eucheuma, used worldwide as food hydrocolloids and stabilizers.
   • High-Magnesium Calcite Cell Walls in Coralline Algae (Corallinales): Encrusting and articulated coralline red algae (Lithothamnion, Porolithon, Corallina) precipitate high-magnesium calcite (Ca_1−xMg_xCO₃) directly inside their cell walls, acting as the structural "living concrete" (algal ridge) that cements coral-reef rubble together against oceanic hurricane wave energy and secretes chemical cues (tetrabromopyrrole) that induce planktonic coral planula larvae to settle and metamorphose!

Because red algae completely lost flagella, their male gametes (spermatia) cannot swim toward the female egg (carpogonium, which projects a long receptive hair called the trichogyne). Consequently, drifting water currents (or small marine isopods and polychaete worms, such as Idotea balthica which pollinates Gracilaria gracilis underwater like an insect pollinating a flower, discovered by Myriam Valero, Emma Lavaut, and colleagues in Science, 2022!) rarely bring a spermatium into contact with a trichogyne.

To maximize the reproductive output of every rare fertilization event, Florideophyceae evolved a unique Three-Generation (Triphasic) Life Cycle:

1. Generation 1 — Haploid (1n) Male and Female Gametophytes: Produce non-motile spermatia and female carpogonia.
2. Generation 2 — The Parasitic Diploid (2n) Carposporophyte (Clonal Zygote Amplifier): Instead of releasing the fertilized zygote into the sea (where one fertilization = one offspring), the fertilized carpogonium stays attached to the female gametophyte and grows into a tiny, parasitic diploid generation—the carposporophyte (nourished by maternal auxiliary cells inside an urn-shaped cystocarp)—which clones the zygote mitotically into thousands of diploid (2n) carpospores!
3. Generation 3 — The Free-Living Diploid (2n) Tetrasporophyte: Each released carpospore germinates into an independent, macroscopic diploid seaweed—the tetrasporophyte (often isomorphic with the gametophyte, as in Polysiphonia and Chondrus)—which produces millions of tetrasporangia, each undergoing meiosis to release four haploid (1n) tetraspores that grow into new male and female gametophytes! Thus, a single fertilization event is amplified into millions of recombinant meiotic spores!

How did red algae give rise to the chloroplasts of kelps, diatoms, dinoflagellates, and malaria parasites (Secondary Endosymbiosis)?

Whereas red algae, green algae, and land plants acquired their chloroplasts directly from a cyanobacterium (primary endosymbiosis, leaving 2 envelope membranes), a second evolutionary event (secondary endosymbiosis) occurred in the Proterozoic when heterotrophic eukaryotes engulfed a unicellular red alga (closely related to Cyanidiophytina/Bangiophyceae) and reduced it to a three- or four-membraned secondary red plastid (the "Chromalveolate" / CASH serial-endosymbiosis lineage). In Cryptophyta (Guillardia theta), the engulfed red alga's degenerate nucleus still survives today between the second and third plastid membranes as a tiny nucleomorph containing just 3 miniature linear chromosomes (551 kilobases)! Through this red-algal plastid inheritance, Rhodophyta provided the photosynthetic machinery for Stramenopiles (diatoms and giant kelps, which account for ~25% of all photosynthesis on Earth), Haptophytes (coccolithophores), Dinoflagellates (coral-reef Symbiodiniaceae), and even the non-photosynthetic apicoplast organelle of Apicomplexa (the malaria parasite Plasmodium falciparum)!

1. Littler, M. M., Littler, D. S., Blair, S. M., & Norris, J. N. (1985). Deepest known plant life discovered on an uncharted seamount. Science, 227(4682), 57–59. doi:10.1126/science.227.4682.57
2. Butterfield, N. J. (2000). Bangiomorpha pubescens n. gen., n. sp.: Implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology, 26(3), 386–404. doi:10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2026%3C0386:BPNGNS%3E2.0.CO;2)
3. Matsuzaki, M., Misumi, O., Shin-i, T., Maruyama, S., Takahara, M., Miyagishima, S.-Y., ... & Kuroiwa, T. (2004). Genome sequence of the ultrasmall unicellular red alga Cyanidioschyzon merolae 10D. Nature, 428(6983), 653–657. doi:10.1038/nature02398
4. Collén, J., Porcel, B., Carré, W., Ball, S. G., Chaparro, C., Tonon, T., ... & Boyen, C. (2013). Genome structure and metabolic features in the red seaweed Chondrus crispus shed light on evolution of the Archaeplastida. Proceedings of the National Academy of Sciences, 110(13), 5247–5252. doi:10.1073/pnas.1221259110
5. Brawley, S. H., Blouin, N. A., Ficko-Blean, E., Wheeler, G. L., Lohr, M., Goodson, H. V., ... & Prochnik, S. E. (2017). Insights into the red algae and eukaryotic evolution from the genome of Porphyra umbilicalis (Bangiophyceae, Rhodophyta). Proceedings of the National Academy of Sciences, 114(31), E6361–E6370. doi:10.1073/pnas.1703088114
6. Lavaut, E., Guillemin, M.-L., Colin, S., Faure, A., Coudret, J., Destombe, C., & Valero, M. (2022). Pollinators of the sea: A discovery of animal-mediated fertilization in seaweed. Science, 377(6605), 528–530. doi:10.1126/science.abo6661