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19.8: Red and Green Algae

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    Learning Objectives
    • Distinguish between different groups of algae using life cycle, morphological features, and cellular composition.
    • Connect adaptations in the red and green algae to habitat characteristics and ecology.
    • Identify structures and phases in the Polysiphonia and Spirogyra life cycles; know the ploidy of these structures.
    A unicellular organism with two large, green cells inside
    Figure \(\PageIndex{1}\): A nonmotile glaucophyte cell asexually reproducing. Two daughter cells are produced within the cell wall of the mother cell. Each daughter cell has a distinct cell wall and is filled with green cyanelles. Photo by James Tran, CC BY 2.0, via Wikimedia Commons
    An oblong cell lacking a cell wall. It has two flagella on the right side and two sets of green organelles inside.
    Figure \(\PageIndex{2}\): A motile cell (zoospore) of Cyanophora paradoxa. The green structures within this cell are cyanelles. Two flagella can be distinguished on the right side. Scale bar indicates 10 µm. From algae culture of University Duisburg-Essen (Germany), pictures were taken with Zeiss Axioplan and Canon 600D, CC BY-NC. Retrieved from


    Red algae descended from the same endosymbiotic event as the Glaucophyta. The red algae are almost exclusively marine. Some are unicellular but most are multicellular. Approximately 6,000 species have been identified. They have true chloroplasts with two membranes (no remnant peptidoglycan) containing chlorophyll a. Like the cyanobacteria, they use phycobilins as antenna pigments - phycoerythrin (which makes them red) and phycocyanin. Red pigment allows the red algae to photosynthesize at deeper depths than the green or brown algae, harnessing more of the blue light waves that penetrate deeper into the water column. Unlike green algae and plants, red algae store carbohydrates as Floridean starch in the cytosol. Some are used as food in coastal regions of Asia. Agar, the base for culturing bacteria and other microorganisms, is extracted from a red alga.

    Selection Pressures and Drivers

    An important aspect of understanding the life history traits of the Rhodophyta is understanding the challenges of living in a marine environment.

    1. Access to sunlight: Most colors of light cannot penetrate into deeper water, as they are scattered by water molecules. The wavelengths of light that reach deepest into the ocean are blue and green. Many fish that live in the deep ocean are red. Because red light does not penetrate to the depths where they live, this makes them virtually undetectable by sight. Remember, we see things because of the light that bounces off of them. Red pigments reflect red light, so no red light, no reflected light. Red algae are using a similar strategy--absorb the wavelengths of light that are not red--with a different goal: to use that absorbed light to make food. The phycoerythrin in their chloroplasts reflects red light, giving them a red appearance, and absorbs the blue light that is able to penetrate to deeper areas in the water column.
    2. Fertilization: The ocean is an expansive environment, often with large areas of open space between populations of organisms. In this environment, successful fertilization of an egg by a nonmotile sperm--red algae lack flagella--presents a challenge. Having multicellular haploid and diploid phases provides red algae more opportunities to produce gametes and spores. A diploid stage that clones the zygote, the carposporophyte, provides more opportunities to do meiosis from each fertilization event.
    3. Salinity: Marine environments are relatively high in salinity. A possible adaptation for this is to have sulfated polysaccharides in the cell wall, such as the galactans present in Rhodophyta. This is a strategy present in (potentially all) marine algae and is inferred to be an adaptation for salinity-tolerance. See this open-access article for further information.
    A small, stellate, pale alga grows attached to a larger dark red alga
    Figure \(\PageIndex{3}\): 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 have a diverse range of morphologies. Unicellular forms may live solitarily or as colonies but, unlike other members of the Archaeplastida, lack flagella. Flagella are absent from the Rhodophyta, lost at some point in their evolutionary history. Multicellular forms can be filamentous, leafy, sheet-like, coralloid, or even crust-like (some examples in Figure \(\PageIndex{4}\) and Figure \(\PageIndex{5}\)). The strange coralline red algae 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!).

    Callithamnion, a filamentous, multicellular red alga with cells forming long, branching chains
    Two different thalli of multicellular red algae that have been pressed and mounted to make herbarium specimens
    Figure \(\PageIndex{4}\): These images are all of multicellular red algae, which can range from filamentous (first image) to "leafy" (second image, left) to sheet-like (second image, right). The red color is due to an abundance of the red pigment phycoerythrin, which gives this group red chloroplasts. First image by Melissa Ha CC-BY-NC. Second image by Maria Morrow CC-BY-NC.
    A bone-white algal thallus composed of segments that look like vertebrae. It is branched into a tree-like shape.
    Crust-forming red algal species making pink blotch-like growths on a rock.
    Figure \(\PageIndex{5}\): Examples of coralline red algae.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
    A close up of red algal cells with an arrow indicating something that looks like cytoplasm pinched between two cells
    Figure \(\PageIndex{6}\): A pit connection between Polysiphonia cells. The images shows a channel (pit connection) connecting two adjacent cells, indicated by an arrow. This pit connection 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 a haplodiplontic (alternation of generations) life cycle that has an extra diploid stage: the carposporophyte. Polysiphonia is the model organism for the Rhodophyta life cycle. The gametophytes of Polysiphonia are isomorphic (iso- meaning same, morph- meaning form), meaning they have the same basic morphology. Any difference you see in coloration of the images in this section is due to staining. They would all appear a deep red color in an unstained slide.

    A red algal thallus next to a coin for size. The thallus is perhaps 5x the length of the coin.
    Figure \(\PageIndex{7}\): 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: presence of spermatangia, cystocarps, or tetrasporangia. 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.

    A Polysiphonia male gametophyte with a spermatangium labeled
    A close up of a spermatangium from a male gametophyte
    Figure \(\PageIndex{8}\): A Polysiphonia male gametophyte. In the first image, 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 second image, 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.

    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.

    A labeled Polysiphonia cystocarp with emerging carposporangia
    Figure \(\PageIndex{9}\): 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.


    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). 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.

    A labeled Polysiphonia tetrasporophyte showing tetrasporangia and tetraspores
    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.

    Full Life Cycle Diagram

    Polysiphonia 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.

    Summary of Characteristics for Red Algae

    • Morphology: Unicellular to multicellular, no flagellated stages. Cells of multicellular species are connected via incomplete cytokinesis, resulting in pit connections.
    • Cell wall composition: Cellulose and galactans
    • Chloroplasts: 2 membranes, pigments are chlorophyll a and phycobilins (primarily phycoerythrin, providing their red color)
    • Storage carbohydrate: Floridean starch
    • Life cycle: Alternation of generations with an extra diploid stage, the carposporophyte
    • Ecology: Primarily marine (97% of species)

    Green Algae

    The most abundant group of algae is the green algae. The nature of the evolutionary relationships between the green algae are still up for debate. As of 2019, genetic data supports splitting the green algae into two major lineages: chlorophytes and streptophytes. The streptophytes include several lineages of green algae (such as the charophytes) and all land plants. Streptophytes and chlorophytes represent a monophyletic group called Viridiplantae (literally “green plants”). The green algae exhibit similar features to the land plants, particularly in terms of chloroplast structure. They have chlorophyll a and b, have lost phycobilins but gained carotenoids, and store carbohydrates as starch inside plastids. Although some of the multicellular forms are large, they never develop more than a few types of differentiated cells and their fertilized eggs do not develop into an embryo.

    Green algae are an important source of food for many aquatic animals. When lakes and ponds are "fertilized" with phosphates and nitrates (e.g., from sewage and the runoff from fertilized fields and lawns), green algae often form extensive algal "blooms". Members of this group can be found in freshwater and marine habitats, and many have adapted to life on land, either inside of lichens or free-living (see Figure \(\PageIndex{12}\)).

    Branches of a tree with no foliage, covered in thick orange fuzz
    Figure \(\PageIndex{12}\): Trentepohlia is a genus of green algae that is found in terrestrial environments. It forms fluffy orange colonies on trees and is a photobiont in many lichens. One might not know they were looking at a green algae, due to the orange pigmentation. However, green algae have carotenoids. These terrestrial green algae produce an abundance of carotenoids, perhaps for protection from sun damage. Photo by Scott Loarie, CC0.

    Selection Pressures and Drivers

    1. Sun Damage. Green algae represent a diverse group of organisms with diverse life history traits, many of which are shared with land plants. The development of carotenoids-- yellow, orange, and red pigments that act in both light harvesting and sun protection--offers this group increased access to sunlight while simultaneously protecting against UV damage. UV rays do not penetrate very far into the water column, so organisms moving into shallower waters or terrestrial environments would need to deal with this new challenge. Many terrestrial species of green algae appear orange, rather than green, due to the production of large amounts of carotenoids.


    These algae exhibit great diversity of form and function. Similar to red algae, green algae can be unicellular or multicellular. Many unicellular species form colonies and some green algae exist as large, multinucleate, single cells. Green algae primarily inhabit freshwater and damp soil, and are a common component of plankton. Chlamydomonas is a simple, unicellular chlorophyte with a pear-shaped morphology and two opposing, anterior flagella that guide it toward light sensed by its eyespot (Figure \(\PageIndex{13}\)). More complex species exhibit haploid gametes and spores that resemble Chlamydomonas.

    Two unicellular green algae from the genus Chlamydomonas
    Figure \(\PageIndex{13}\): This image shows two unicellular green algae from the genus Chlamydomonas. They appear green due to the loss of phycobilins and evolution of chlorophyll b. They each have two whiplash flagella, though these are only visible on one of them in the picture. Photo by Melissa Ha, CC-BY-NC.

    The alga Volvox is one of a colonial organism, which behaves in some ways like a collection of individual cells, but in other ways like the specialized cells of a multicellular organism (Figure \(\PageIndex{14}\)). Volvox colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein secretion. Individual Volvox cells move in a coordinated fashion and are interconnected by cytoplasmic bridges. Only a few of the cells reproduce to create daughter colonies, an example of basic cell specialization in this organism.

    Three images of Volvox colonies
    Figure \(\PageIndex{14}\): Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions. Descriptive text: The micrograph on the left shows a sphere about 400 microns across with round green cells about 50 microns across inside. The middle micrograph shows a similar view at higher magnification. The micrograph on the right shows a broken sphere that has released some of the cells, while other cells remain inside. (credit: Dr. Ralf Wagner)

    Volvox can reproduce both asexually and sexually. In asexual reproduction, the gonidia develop into new organisms that break out of the parent (which then dies). In sexual reproduction, the presence of an inducing chemical causes the following:

    • The gonidia of the males to develop into clusters of sperm.
    • The gonidia of the females to develop into new spheres each of whose own gonidia develops into a pair of eggs.
    • The sperm break out of the male parent and swim to the female where they fertilize her eggs.
    • The zygotes form a resting stage that enables Volvox to survive harsh conditions (Figure \(\PageIndex{15}\)).
    A close up of a Volvox colony with zygotes inside
    Figure \(\PageIndex{15}\): Volvox thick-walled, desiccation-resistant zygote. There is a larger sphere composed of many individuals (shown as blue dots). Within that larger sphere, there are several smaller spheres with thick, warty walls. Photo by Maria Morrow, CC-BY-NC.

    Video \(\PageIndex{1}\): This video shows how sexual reproduction occurs in the colonial green alga Volvox. Sourced from YouTube.

    The genome of Volvox carteri consists of 14,560 protein-encoding genes - only 4 more genes than in the single-celled Chlamydomonas reinhardtii! Most of its genes are also found in Chlamydomonas. The few that are not encode the proteins needed to form the massive extracellular matrix of Volvox.

    Species in the genus Caulerpa exhibit flattened fern-like foliage and can reach lengths of 3 meters (Figure \(\PageIndex{16}\)). Caulerpa species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.

    Fern-like plants growing on the sea bottom.
    Figure \(\PageIndex{16}\): Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of nuclei, much like a plasmodial slime mold. (credit: NOAA)

    True multicellular organisms, such as the sea lettuce, Ulva, are also represented among the green algae (Figure \(\PageIndex{17}\) and Figure \(\PageIndex{18}\)).

    An herbarium specimen of a multicellular green algal thallus that forms flat, ribbon- and sheet-like structures
    Ulva cells under the microscope with nuclei labeled
    Figure \(\PageIndex{17}\): Ulva is a genus of multicellular marine green algae that forms flat sheets of cells. In the image on the left, there is a pressed sample of an Ulva expansa thallus that is serving as an herbarium specimen. In the image on the right, a piece of an Ulva thallus is being viewed through a microscope. Each cell contains green chloroplasts and a large nucleus, two of which are labeled in the image. Photos by Maria Morrow, CC-BY-NC.
    Strings of squishy-looking, bright green, bead-like algae, stacked on top of each other.
    Figure \(\PageIndex{18}\): Chaetomorpha coliformis, a marine green alga formed from chains of cylindrical cells (commonly called sea emeralds). Photo by Svenjah Heesch, (CC-BY-NC).

    Spirogyra Life Cycle

    Though green algae display a diversity of life cycles, many have a haplontic life cycle. A model organism for the green algae is Spirogyra (Figure \(\PageIndex{19}\)). Spirogyra is a unicellular green algae that grows in long, filamentous colonies, making it appear to be a multicellular organism. Even though it is technically unicellular, its colonial nature allows us to classify its life cycle as haplontic. In the haploid vegetative cells of the colony, the chloroplasts are arranged in spirals, containing darkened regions called pyrenoids where carbon fixation happens. Each haploid cell in the filament is an individual, which makes sexual reproduction between colonies an interesting process.

    Spirogyra vegetative cell with the nucleus and chloroplast pyrenoids labeled
    Figure \(\PageIndex{19}\): A vegetative cell in a Spirogyra colony. The nucleus is visible in the center of the cell, including a large, dark nucleolus. The chloroplasts are arranged in spirals around the cell and have dark regions called pyrenoids where carbon dioxide is fixed. Photo by Maria Morrow, CC-BY-NC.

    When two colonies of Spirogyra meet that are of a complementary mating type (+/-), sexual reproduction occurs. The two colonies align, each cell across from a complementary cell on the other filament. A conjugation tube extends from each cell in one colony (Figure \(\PageIndex{20}\)), inducing formation of a tube on the cells in the other colony. The conjugation tubes from each colony fuse together.

    Spirogyra conjugation tube formation
    Figure \(\PageIndex{20}\): Spirogyra forming conjugation tubes. There are two vegetative colonies that are about to interact. The colony on the right has chemically sensed the presence of the colony on the right and has started to grow projections in the cell walls of each cell in the colony, extending them toward the other colony. These are the beginnings of conjugation tubes. Photo by Maria Morrow, CC-BY-NC.

    The contents of one cell will move through the conjugation tube and fuse with the contents of the complementary cell, resulting in a diploid zygote (Figure \(\PageIndex{21}\)). The zygote appears as a large, egg-like structure contained within the complementary cell. It has a thick wall that provides resistance to desiccation and cold, allowing colonies of Spirogyra to overwinter, when needed. The other colony is now a filament of empty cells that will be broken down by some decomposer. When conditions are right, the zygote undergoes meiosis to produce another vegetative colony of haploid cells.

    Spirogyra conjugation and formation of zygotes
    Figure \(\PageIndex{21}\): Cells in various stages of conjugation. Of the cells that have formed conjugation tubes and connected, the one farthest to the left has just recently finished the transfer and fusion of its cytoplasm, but the zygote hasn't fully formed yet. In the cell on the far right, there is a fully formed zygote. It is dark in color and has thick walls. The chloroplasts are not individually distinguishable within it. Photo by Maria Morrow, CC-BY-NC.

    Full Life Cycle Diagram

    Spirogyra Life Cycle Diagram
    Figure \(\PageIndex{22}\): The haplontic life cycle of Spirogyra. Starting from the upper left corner and moving right, there is a single haploid vegetative colony of Spirogyra. The chloroplasts are drawn in as a single ribbon with circles representing pyrenoids. Each cell has a large, dark nucleus. Moving to the right, two colonies of complementary mating types begin to interact with each other through chemical signals and start forming conjugation tubes. In the next frame, the conjugation tubes have connected and the contents of one cell begins to transfer through the conjugation tube into a cell in the other colony. This is plasmogamy. Karyogamy occurs when the two nuclei fuse together and the diploid zygote is formed. This zygote waits for appropriate conditions to germinate, undergo meiosis, and form a new haploid colony. Diagram by Nikki Harris, CC BY-NC with labels added by Maria Morrow.

    Summary of Characteristics for Green Algae


    Glaucophytes, red algae, and green algae are part of the Archaeplastida. These organisms are descended from the same primary endosymbiosis event. Glaucophytes are thought to be one of the earliest lineages to diverge due to the presence of remnant peptidoglycan between the membranes of its chloroplast-like cyanelles. Unsurprisingly, glaucophytes and red algae share the same pigments as Cyanobacteria.

    Red algae (phylum Rhodophyta) are united by several synapomorphies (shared derived characteristics). They lack flagella, have pit connections between cells, and store carbohydrates as Floridean starch. The sulfated galactans in their cell walls allows them increased fitness in marine environments, while the pigment phycoerythrin allows them to photosynthesize deeper in the water column. They have an alternation of generations life cycle with an extra diploid phase, the carposporophyte, that clones the zygote. These characteristics can be connected to the environmental stressors presented by the marine habitats most red algae are found in.

    Green algae represent several distinct lineages. Like plants, they store carbohydrates as starch within their plastids and have the pigments chlorophyll a and b, as well as carotenoids. Organisms in this group have haplontic (e.g. Spirogyra) or haplodiplontic (e.g. Ulva) life cycles. Many green algae are unicellular, forming complex colonies. Green algae can be found in marine, freshwater, and terrestrial environments (including within lichens!).


    Curated and authored by Maria Morrow, CC BY-NC, using the following sources:

    This page titled 19.8: Red and Green Algae is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Teresa Friedrich Finnern.