4.5: Red Algae

The red and green algae are descendents of the primary endosymbiosis event that resulted in the first chloroplast. Their plastids have two membranes, an inner membrane that was the cyanobacterial cell wall and an outer membrane from the organism that first engulfed it. The story of these algal groups is the evolutionary history of all plants.

Figure \(\PageIndex{1}\): The red algae are a fascinating group that have evolved a diversity of morphologies and strategies. This red alga, Asterocolax gardneri, is a parasite on other red algae. Note that it lacks the color characteristic of the Rhodophyta. Because it feeds off other algae, it does not need to make its own food via photosynthesis and so does not require photosynthetic pigments. Photo by Chloe and Trevor, CC-BY-NC.

Red Algae, Phylum Rhodophyta

Morphology

Red algae have a diverse range of morphologies. They can be unicellular or multicellular. Unicellular forms may live solitarily or as colonies. Multicellular forms can be filamentous, leafy, sheet-like, coralloid, or even crust-like (some examples in Figure \(\PageIndex{2}\) and Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{2}\): These images are all of multicellular red algae, which can range from filamentous (left) to "leafy" (center) to sheet-like (right). The red color is due to an abundance of the red pigment phycoerythrin, which gives this group red chloroplasts. Image on the left by Melissa Ha CC-BY-NC. Image on the right by Maria Morrow CC-BY-NC.

Figure \(\PageIndex{3}\): Above are examples of the strange coralline red algae. These have calcerous deposits in the cell walls that make the thallus hard, like a coral. These can take a variety of forms and are able to live at depths other algae cannot (over 500 feet deep for some!). The first image shows the thallus of a Calliarthon tuberculosum alga that has washed up on the beach. Having lost its characteristic pink color, the white, calcerous walls are more obvious. The second photo shows a crustose coralline red alga (species unknown to the author) forming pinkish blotches on a rock. There are a pair of forceps pointing at one of these blotches. First photo by Jennifer Rycenga, CC-BY-NC. Second photo by by Gsaunders, CC-BY-NC.

Cells of multicellular species are connected via incomplete cytokinesis, resulting in pit connections.

Figure \(\PageIndex{4}\): A pit connection between Polysiphonia cells. The images shows a channel connecting two adjacent cells, indicated by an arrow. This channel passes through the cell walls of both cells, as well as the middle lamellae. Photo by Maria Morrow CC-BY-NC.

Polysiphonia Life Cycle

Red algae have an alternation of generations life cycle that has an extra diploid stage: the carposporophyte. Polysiphonia is the model organism for Rhodophyta. The gametophytes of Polysiphonia are isomorphic (iso- meaning same, morph- meaning form), meaning they have the same basic morphology.

Figure \(\PageIndex{5}\): All stages of the Polysiphonia life cycle have the same basic morphology. If you were to see them without magnification, they would all look more or less like this: a small, red, finely branching thallus. The reproductive structures are used to differentiate the life stages: does it have spermatangia, cystocarps, or tetrasporangia? Any difference you see in coloration of the images in this section is due to staining. They would all appear this deep red color in an unstained slide. Photo by Gsaunders, CC-BY-NC.

Male Gametophyte

The male gametophyte has elongated structures that emerge from the tips of the thallus branches. These are spermatangia, where spermatia are produced by mitosis.

Figure \(\PageIndex{6}\): A Polysiphonia male gametophyte. In the image on the left, branches of the male gametophyte each terminate with several elongated structures that look almost like ears of corn. Each of these structures is a spermatangium. In the image on the right, a spermatangium is shown by itself, detached from the gametophyte. Cells in the spermatangum undergo mitosis to produce haploid, non-motile, unicellular gametes called spermatia. The image is not clear enough to distinguish individual spermatia. Photos by Maria Morrow, CC-BY-NC.

Female Gametophyte and Carposporophyte

The female gametophyte produces an egg that is contained within a structure called the carpogonium. This structure has a long, thin projection called a trichogyne (trich- meaning hair, -gyne meaning female). During fertilization, a spermatium fuses with the trichogyne and the nucleus of the spermatium travels down the tube to the egg. When the nucleus of the spermatium fuses with the egg, a zygote is produced. This zygote is retained and nourished by the female gametophyte as it grows.

Figure \(\PageIndex{7}\): A female gametophyte from a prepared slide. During the slide-making process, the gametophyte broke into several pieces and was stained a deep blue-green. Normally, the thallus would appear red (see the tetrasporophyte thallus). There are many large, cup-like structures attached to the branches, indicating that the eggs in these regions were fertilized and a cystocarp developed. Photo by Maria Morrow, CC-BY-NC.

The globose structures you see growing from the female gametophyte thallus are called cystocarps. A cystocarp is composed of both female gametophyte tissue (n) and carposporophyte tissue (2n). The outer layer of the cystocarp, the pericarp (peri- meaning around) is derived from the female gametophyte and is haploid. The interior of the cystocarp consists of the carposporophyte, which is diploid, and produces structures called carposporangia, inside of which it produces carpospores by mitosis. All of these--carposporophyte, carposporangia, and carpospores--are diploid.

Figure \(\PageIndex{8}\): The image shows branches of the female gametophyte thallus on the left side. In the center, a globose cystocarp emerges from one of those branches. The cystocarp is composed of a haploid pericarp that forms the outside of the structure. The cells of the pericarp look blocky, almost scale-like. Within the pericarp, the tissues are diploid and belong to the carposporophyte. The carposporophyte is composed of many elongated carposporangia. Photos by Maria Morrow, CC-BY-NC.

Tetrasporophyte

The diploid carpospores are released into the ocean waters, where they will be carried on currents to another location. If a carpospore lands in an appropriate environment, it will grow by mitosis into a tetrasporophyte (2n).

Figure \(\PageIndex{9}\): On the left, a full, unstained thallus of a tetrasporophyte is visible. It looks quite similar to both the male and female gametophytes, except the presence of round, darkened regions within the branches. These are the tetrasporangia, where meiosis will occur to produce haploid tetraspores. On the right, we see a magnified image of that same thallus and a more detailed view of the tetrasporangia within the branches. Photos by Maria Morrow, CC-BY-NC.

Figure \(\PageIndex{10}\): The image shows branches of the tetrasporophyte. Each of the compartments within the branches is filled with a globose tetrasporangium. In most of these, clear delineations can be seen where the tetrasporangium is dividing by meiosis into four distinct cells. These cells are haploid tetraspores. Photo by Maria Morrow, CC-BY-NC.

The tetrasporophyte produces tetrasporangia (2n) within the branches of the thallus. Each tetrasporangium produces four unique, haploid tetraspores by meiosis. Tetraspores (n) are released and will grow by mitosis into either male or female gametophytes, completing the life cycle.

Full Life Cycle Diagram

Figure \(\PageIndex{11}\): The alternation of generations life cycle of Polysiphonia. On the left side, in the center, there are four haploid tetraspores. These tetraspores grow by mitosis into haploid gametophytes, either "male" or "female". The male gametophyte produces spermatangia at the tips of its branches and these spermatangia produce haploid spermatia by mitosis. The female gametophyte produces carpogonial branches, which have an egg at the base and a long filament called a trichogyne that extends from the egg chamber. A spermatium fuses with a trichogyne and its nucleus travels down the trichogyne to fertilize the egg, making a diploid zygote. The zygote grows, still attached to the gametophyte, within a structure called the cystocarp. The cystocarp has an external layer called the pericarp that is formed from the female gametophyte's tissue (meaning it is haploid). Within the pericarp, the zygote has grown by mitosis into a carposporophyte making elongated carposporangia. Inside each carposporangium, diploid carpospores are produced by mitosis. Carpospores are released and grow by mitosis into tetrasporophytes. Within the branches of the tetrasporophyte, tetrasporangia are formed and undergo meiosis to produce four haploid tetraspores each. These tetraspores are released and we arrive back where we started. Diagram by Nikki Harris CC-BY-NC with labels added by Maria Morrow.