25.2A: Streptophytes and Reproduction of Green Algae
- Discuss the general similarities of green algae and land 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 CO 2 , 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.
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.
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