Green Algae: Precursors of Land Plants

Streptophytes and Reproduction of Green Algae

Land plants and closely-related green algae (charophytes) are classified as Streptophytes; the remaining green algae are chlorophytes.

Learning Objectives

Discuss the general similarities of green algae and land plants

Key Takeaways

Key Points

  • There is a diverse array of green algae including single-celled or multicellular species, which can reproduce both sexually or asexually.
  • The classification of green algae is challenging because they bear many of the structural and biochemical traits of plants.
  • Species of green algae that are closely related to embryophytes are classified as charophytes while the remaining green algae are classified as chlorophytes.
  • Like plants, charophytes have chlorophyll a and b, store carbohydrates as starch, have cell walls consisting of cellulose, and undergo similar cell-division processes.
  • Charophytes have unique reproductive organs that differ considerably from that of other algae.

Key Terms

  • streptophytes: a subphylum consisting of several orders of green algae and embryophytes
  • Charophyta: a division of green algae that includes the closest relatives of the embryophyte plants
  • Chlorophyta: a division of green algae that are considered more distantly related to plants

Streptophytes

Until recently, all photosynthetic eukaryotes were considered members of the kingdom Plantae. The brown, red, and gold algae, however, have been reassigned to the Protista kingdom. This is because, apart from their ability to capture light energy and fix CO2, they lack many structural and biochemical traits that distinguish plants from protists. The position of green algae is more ambiguous. Green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid, and filamentous forms, along with macroscopic seaweeds, all of which add to the ambiguity of green algae classification since plants are multicellular.

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Chara vulgaris: A representative charophyte alga is Chara vulgris, or common stonewort, which is a multicellular branching species that can grow up to 120m long.

Green algae contain the same carotenoids and chlorophyll a and b as land plants, whereas other algae have different accessory pigments and types of chlorophyll molecules in addition to chlorophyll a. Both green algae and land plants also store carbohydrates as starch. Cells in green algae divide along cell plates called phragmoplasts and their cell walls are layered with cellulose in the same manner as the cell walls of embryophytes. Consequently, land plants (embryophytes) and closely-related green algae ( Charophyta ) are now part of a new monophyletic group called Streptophyta. The remaining green algae, which are more distantly related to plants, belong to a group called Chlorophyta that includes more than 7000 different species that live in fresh or brackish water, in seawater, or in snow patches.

The Charophyta are a division of green algae that includes the closest relatives of the embryophyte plants. Charophyta are a small but important group of plants which show marked differences from both the Thallophyta and the Bryophyta. They are all specialized water plants. The reproductive organs consist of antheridia and oogonia, although the structure of these organs differs considerably from the corresponding organs in the Algae.

Charales

Algae in the order Charales live in fresh water and are often considered the closest-living relatives of embryophytes.

Learning Objectives

Identify the principle features of charophyte algae

Key Takeaways

Key Points

  • The structure of charophyte algae consists of a thallus, which is the main stem, and branches that arise from nodes which bear both male and female reproductive structures.
  • Although charophyte algae do not exhibit alteration of generations, they share a number of adaptations to life on land with embryophytes, including the encasement of eggs in protective enclosures.
  • As new DNA sequence analysis techniques develop, revisions may need to be made in our understanding of plant evolution, such as indications that green algae in the order of Zygnematales may be more-closely related to embryophytes than is Charales.

Key Terms

  • Charales: green algae in the division Charophyta which are green plants believed to be the closest relatives of the green land plants
  • sporopollenin: a combination of biopolymers observed in the tough outer layer of the spore and pollen wall

Charales

Green algae in the order Charales, and the coleochaetes, microscopic green algae that enclose their spores in sporopollenin, are considered the closest-living relatives of embryophytes. The Charales can be traced as far back as 420 million years. They live in a range of fresh water habitats and vary in size from as small as a few millimeters to as large as a meter in length. A representative species of Charales is Chara, which is often called muskgrass or skunkweed because of its unpleasant smell.

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Charophyte algae: A representative charophyte alga, Chara, is a noxious weed in Florida, where it clogs waterways.

In Charales, large cells form the thallus: the main stem of the alga. Branches arising from the nodes are made of smaller cells. Male and female reproductive structures are found on the nodes; the sperm have flagella. Unlike land plants, Charales do not undergo alternation of generations in their lifecycle. Like embryophytes, Charales exhibit a number of traits that are significant in their adaptation to land life. They produce the compounds lignin and sporopollenin. They form plasmodesmata, which are microscopic channels that connect the cytoplasm of adjacent cells. The egg and, later, the zygote, form in a protected chamber on the parent plant.

New information from recent, extensive DNA sequence analysis of green algae indicates that the Zygnematales are more closely-related to the embryophytes than the Charales. The Zygnematales include the familiar genus Spirogyra. As techniques in DNA analysis improve and new information on comparative genomics arises, the phylogenetic connections between species will probably continue to change. Clearly, plant biologists have yet to solve the mystery of the origin of land plants.