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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

• 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

• 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 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)

• 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

• 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

• 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)



 

• 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

• 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 photosythetic 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

• 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.