4.6: 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 and all land plants. Streptophytes and chlorophytes represent a monophyletic group called Viridiplantae (literally “green plants”). Some green algal lineages have adapted to life on land, either inside of lichens or free-living (see Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): 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.

General Morphology

Similar to red algae, green algae can be unicellular or multicellular. Many unicellular species form colonies.

Unicellular

Unicellular species will have two whiplash flagella.

Figure \(\PageIndex{2}\): 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 .

Figure \(\PageIndex{3}\): Volvox is a genus of unicellular freshwater green algae that form spherical colonies. Each individual beats its flagella to the exterior of the ball-like formation. Within the colony, new colonies can be formed by asexual reproduction. These internal colonies beat their flagella to the interior. When sexual reproduction occurs, a thick-walled, desiccation-resistant zygote forms, as seen in the image on the right. Photos 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.

Figure \(\PageIndex{4}\): Hydrodictyon reticulatum is another colony-forming species. Individual, unicellular algae form together to create a complex colony structure. These algae are commonly called water nets. Photo by louiselewis, CC-BY-NC.

Multicellular

Figure \(\PageIndex{5}\): 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.

Figure \(\PageIndex{6}\): Chaetomorpha coliformis, a marine green alga formed from chains of cylindrical cells (commonly called sea emeralds). Photo by Svenjah Heesch, CC-BY-NC.

Oedogonium is a genus of filamentous green algae. Some species of Oedogonium are nannandrous. In nannandrous species, the antheridia are small, elongate filaments, usually produced on a different filament than the oogonium. Other species are macrandrous and the antheridia are produced as stacked cells within the same filament as the oogonium.

Figure \(\PageIndex{7}\): An oogonium and two antheridia of a macrandrous Oedogonium. The large, spindle-shaped oogonium is shown on top of the antheridia. This oogonium is not yet fertilized, so no oospore is visible. The antheridia are smaller cells within the filament, below the oogonium. Two can be seen in this image. Note: This is an image of a stained slide, so this normally green alga appears blue. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{8}\): An oogonium of a macrandrous Oedogonium with a large, fertilized oospore inside. In an unfertilized oogonium, the inside looks evenly granular, as seen in Figure \(\PageIndex{7}\) above. Once it is fertilized, the contents resolve into a distinguishable, spherical structure: the oospore. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{9}\): A nannandrous Oedogonium sp. There are several oogonia occuring in a row on a single filament. Many small antheridia are reaching toward those oogonia. The fourth of the five oogonia is fertilized and has a large, dark oospore inside. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{10}\): A close up of the sexual structures of a nannandrous Oedogonium sp. The oogonium is located at the end of the filament and, in this case, is almost lemon-shaped. It is unfertilized, still appearing evenly granular throughout. Many small antheridia are reaching up to try to fuse with the oogonium to fertilize it. These can be seen on the sides of the filament below the oogonium and look like upside down blue bowling pins. Photo by Maria Morrow, 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. 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.

Figure \(\PageIndex{11}\): 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, inducing formation of a tube on the cells in the other colony. The conjugation tubes from each colony fuse together.

Figure \(\PageIndex{12}\): 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.

Figure \(\PageIndex{13}\): Spirogyra conjugation tubes meet. These two colonies are both forming conjugation tubes toward each other. Two sets of cells near the top of the image have successfully fused conjugation tubes, forming a connection between the two different organisms. 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 \(\PageIndex{14}\): Movement of the cytoplasm from one colony to another in Spirogyra. The cytoplasm from one of the cells in the colony on the right has almost completely transferred through the conjugation tube and into the colony on the left. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{15}\): 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.

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.

Figure \(\PageIndex{16}\): Completed conjugation in two sets of cells. There are two cells in the colony on the left with fully formed, thick-walled zygotes. The colony on the right now has two empty cells, because they have transferred their cytoplasmic contents into the colony on the left. Photo by Maria Morrow, CC BY-NC.

When conditions are right, the zygote undergoes meiosis to produce another vegetative colony of haploid cells.

Full Life Cycle Diagram

Figure \(\PageIndex{17}\): 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.