Red algae are mostly found in a marine environment. A number of species is unicellular, but most species are multicellular. The thallus can be either thread-like, like in Porphyra where branching arrays assemble, or blade-shaped, like in the genus Porphyra (to which the edible nori, known from sushi belong). Red algae possess pigments of the phycoerythrine (red) and Phycocyanine (blue) group, involved in photosynthesis. Phycoerythrine reflects red light; hence the perception of the red color for us. The pigment absorbs rays in the blue-green-yellow range of the spectrum. These colors can penetrate deeper in the seawater than rays of other colors, and that is an advantage for the Red algae.
Meer data about algae and a large colletion of photographs can be found under www.algaebase.org/
The growth of Red algae occurs in some species through from a group of cells. In other species, growth occurs by mean of a tip cell, giving rise to a thallus consisting of a single or multiple axis (example here: Polysiphonia). In such a thallus differentiation occurs and distinction can be made between a cortical region providing rigidity and containing photosynthetic pigments.
Triphasic life cycle: example of Polysiphonia
Red algae are haplo-diplonts with a complicated life cycle that often involves three phases. Characteristical for Red algae is that no motile stage occurs during the entire life cycle. The spores and gametes are transported by the water in a passive manner. In the sexual reproduction only oogamy is observed. oogamy is a type of anisogamy (unegual gamets) in whch the egg cell is large and non-motile, in contrast to the sperms. In red algae the egg cell develops in a female gametangium, called carpogonium. It is here that fertilization occurs. The sperms are produced in a own spermatangium (male gametangium), but lack an own motility apparatus.
Levencyclus van Polysiphonia
During the life cycle of Polysiphonia, the three following phases can be distinguished (scheme here above):
A. (Orange background)
Starting from spores, male and female heterothallic (different thalli) gametophytes develop after germination. On a fertile side-trichoblast spermatangia develop which eventually give rise to spermatia (male gametes). On (short) trichoblasts on the female part, carpogonia are formed that give rise to egg cells.
B. (Green background)
The egg cell is fertilized in the carpogonium and also the outgrowth of the zygote happens in the female gametophyte. The thus formed carposporophyte remains on the gametophyte! On the carposporophyte sporangiae surrounded by sterile "tissue" develop: the cystocarp. Through mitotic divisions spores are released from these carposporangia in the cystocarp. These spores are called carposporen.
C. (Blue background)
The carpospores don't develop to a gametophyte (which should be the case if the alternation of generation was diphasic), but they grow to form so-called tetrasporophytes. On this second kind of sporophyte tetrasporangia are formed which go through meiosis and produce tetraspores. From the four (= tetra) spores in the average two male and two female gametophyte develop. In tetraspore heterospory occurs.
Microscopical and macroscopical views of Polysiphonia
Spermatangia producing spermatia develop on fertile "side shoots" (not real shoots) of the male gametophyte. 1 spermatangium
Carposporangium of a red alga: A carpospore and B carposporangium. The female gametophyte forms carpogonia in which the egg cell can be found. After fertilization by a spermatium the zygote grows out into a carposporophyte, which forms carposporangia with inside the carpospores. 1 thallus, 2 pericarp, 3 carposporophyte, 4 carpospores
Upper view: dried Polysiphonia from Roscoff (France)
Left: Tetrasporangia of a red alga. After meiosis in the tetrasporangium 4 haploid tetraspores are formed that grow out to undependent male and female. 1 thallus, 2 early tetraspore, 3 late tetraspore