BOT4404 Main Page

FIU Home

FIU Marine Biology Home

Dept. Biology Home

Frank Jochem Home


Rhodophyta (Red algae)
  • Occurrence: ca. 6000 species, but only ~200 freshwater; from polar to tropical waters
  • Economical importance: red seaweed (Porphyra) cultured for nutrition in Japan; sulfated polygalactans used for agar, agarose for food processing, micro- and molecular biology
  • Appearance from coccoid unicells to large, leaflike seaweed and encrusting, calcified „coralline“ algae (support reefs)
  • Atmospheric impact by volatile halogenated compounds (ozone layer) and DMS (cloud condensation nuclei)
  • Extreme environments: deepest photosynthetic organism is a coralline alga at 210 m depth; unicellular red algae grow in acidic hot springs
  • Epiphytes on other seaweeds or animals
  • Parasitic, colorless species live in other, closely related red algae
  • Antimicrobial compounds and anti-grazing compounds; of interest to pharmaceutical research
  • Color is mostly pink to dark red because of phycoerythrin; parasitic forms white, creme or yellow; calcified species white; freshwater species often blue-green due to phycocyanin
  • Phycoerythrin: at least 5 types with slightly different spectra, organized in phycobilisomes on plastid thyllakoids as in cyanobacteria
  • Pigments: Chl. a, c, b-carotene, lutein, zeaxanthin, violaxanthin
  • Primary chloroplasts originated from cyanobacteria; parasitic forms contain „empty“ plastids; plastids are often star-shaped or lobed
  • Multiple plastids is the rule; early forms with single plastid
  • Small and large Rubisco sub-units encoded in plastid
  • Storage product: red algae never contain starch, but granules of differently branched glucan: floridean starch
  • Flagella and centrioles are never present in any life stage
Cell Wall and Pit Plugs
  • Pit plugs formed during cytokinsesis, protein plugs in the center of the cell wall separating adjacent cells; 
  • Cell fusion of vegetative cells occurs and is facilitated by rather soft cell walls as compared to other algae
  • Extracellular matrix composed of cellulose microfibrillar network and amorphous matrix of cellulose, galactanes, mucilages
  • Mucilages are polymers of D-xylose, glucose, glucoronic acid and galactose, produced in large golgi vesicles
  • Calcification occurs in coralline algae and a few other non-coralline algae; in the latter, Ca is substituted by aragonite crystals (strontium)
  • Calcified and uncalcified cells can occur in one specimen, controlled by the alga itself


Cell Division and Pit Plugs
  • Nuclear envelope remains intact during mitosis
  • Nuclear associated organelle (NAO) or polar ring substitutes for lacking centrioles
  • NAO appear as hollow cylinders, which organize the division spindle; chemical structure and function unknown, but microtubuli are associated

  • Daughter nuclei are kept separate by large chloroplast or central vacuole (multi-plastid species)
  • Completion of cell division occurs only in tetraspore formation, vegetative cells exhibit incomplete cell division with pit plug formation
  • Primary pit plugs are found between related cells
  • Secondary pit plugs can be formed between non-sister cells; produced by unequal cell division with primary pit plug formation; then, the smaller cell fuses with adjacent non-sister cell

  • Function: communication between genetically different cells and structural integrity among filaments
Multinucleate Cells and Polyploid Nuclei
  • Nuclear mitosis without cell division in many higher organized red algae produces long, multinucleate cells

  • Number of nuclei is correlated to number of plastids within cells
  • Endoreduplication: repeated genome duplication without mitotic nuclear division, resulting in polyploid nuclei
  • Polyploidy  is thought to buffer against mutation of essential genes

Cell Growth and Filament Repair
  • Cell growth by elongation: Cells of Ceramium can increase from 4 µm to 420 µm – a 100-fold increase in cell length and a 14,000-fold increase in cell volume; in Lemanea, cells increase from 8 µm to 8 mm, a 1000-fold increase in length and 44,000-fold increase in volume
  • Cell growth occurs only at the base of the cell

  • Cell death in the middle of a filament triggers cell division in adjacent cells, which form daughter cells with rhizoid-like form; these cells grow towards each other and eventuall fuse to repair the filament

  • Wound repair hormons seem to be involved in most studied species (rhodomorphin)
Growth of Red Algae Thalli
  • Apical cell occur in most species as localized division site at the end of a filament
  • Peraxial cells below the apical cell divide several times radially to branch filaments, each branch with a new apical cell; sub-apical cells also produce rhizoids = single filaments for thallus attachement
  • Simple model: apical cell produces a central, axial filament of individual elongate cells. "Primary" branches produced along the central filament
  • Model 2: same as above except many branches that form next to each other along the central filament („secondary" branches); cell fusions make separate branches appear as one structure
  • Equal branch growth: all branches reach same length; adjacent cells can fuse to produce a laminar sheet

  • Note apical cell and central primary filament; cells in the lamina originate from branches that have fused
Internal Structure of Cylindrical Thalli
  • Apical cell produces central filament
  • Central filament produces branches
  • Branches produce further sub-branches, whose cells might fuse laterally
  • Cell size decreases with each successive branch, so that cells are smallest at the surface of the thallus (false epidermis)
  • Differentiation in central medulla and peripheral cortex
  • Multiaxial thalli possess several central filaments, each with an apical cell

Life Cycle of Red Algae
  • Biphasic life cycle occurs in evolutionary early species
  • Triphasic life cycle is unique to evolutionary young red algae
  • Life cycles can change in some species, e.g. Porphyra: monospore, aplanospores, gametophyte (sexual reproduction)

Sexual Reproduction in Red Algae
  • Oogamy occurs in all red algae
  • Carpogonium: larger, non-flagellate female gamete produced in carpogonia on female gametophyte
  • Carpogonia are produced at the tip of special branches (carpogonial branches); typically flask-shaped with long, thin neck called trichogyne
  • Spermatium: non-flagellate male gamete produced in spermatangium on male gametophyte; spermatia move passively (currents) to carpogonia 
  • Fertilization: spermatium fused with tip of trichogyne; a channel is enzymatically opened to allow the spermatium‘s nucleus to enter
  • Carpospores: several diploid spores produced by mitosis of the zygote

Post-Fertilization Development
  • Bangiophyceans 
    • Carpospores are released, settle, and grow into an independent sporophyte
    • Conchocelis phase: filamentous sporophytes were regarded as different species (Conchocelis) living in mollusc shells or greater depths than the gametophytes
    • Conchospores are released by the sporophyte (diploid!) 
    • Meiosis occurs upon germination of conchospores, producing 4 haploid cells, two male, two female
    • Bangiophycean thalli contain mosaic of male and female cells or male and female sectors, arising from mitotic divisions of the original four haploid cells

    left: Bangia; right: Porphyra

    Life cycle of Porphyra
  • Florideophyceans 
    • Fertilized carpogonium produces diploid carposporophyte instead of carpospores:  triphasic life-cycle!
    • Carposporophyte produces and releases carpospores; carposporophytes always live on the female gametophyte and receive nutrients from the gametophyte
    • Cystocarp is the single carposporophyte plus the gametophyte tissues surrounding and protecting it; the gametophyte tissue is separated into a photosy thetic outer layer (pericarp) and a colorless inner layer
    • Each fertilized carpogonium can produce several cystocarps due to nuclear transfers among connected filament cells
    • Carpospores grow into a second, multicellular, diploid generation, the tetrasporophyte
    • Tetrasporophyte produces tetraspores in tetrasporangia
    • Meiosis occurs upon germination of tetraspores, producing haploid gametophytes again

Coralline Red Algae
  • Precipitate calcium carbonate in their outer cell wall layers;
  • Rigid extracellular matrix that contributes to the formation of coral reefs. 
  • Two general types based on their morphology: 
    • Geniculate: branched and flexible due to the presence of genicula between rigid intergeniculate segments.
    • Non-Geniculate: encrusting; some produce small foliose branches, while others are nodular.