Charophyceae
Charophyceae, commonly known as charophytes, are a class of green algae primarily found in freshwater environments, though some species can also thrive in moist terrestrial habitats. This diverse group can exist as single cells, colonies, or filamentous structures, displaying a variety of shapes. Key characteristics that define charophytes include the presence of flagellated cells, a unique mitotic process involving phragmoplasts, and the storage of starch in plastids, linking them closely to land plants. They have robust, decay-resistant cell walls enriched with phenolic compounds and sporopollenin, which contribute to their resilience.
Charophytes undergo a two-stage life cycle, predominantly exhibiting a haploid stage that develops reproductive structures. The class is classified into six orders: Mesostigmatales, Chlorokybales, Klebsormidiales, Zygnematales, Coleochaetales, and Charales, with Zygnematales being the most abundant. The evolutionary significance of charophytes is notable, as they are considered closely related to the ancestors of land plants, with ongoing research suggesting that certain members of this group likely contributed to the evolutionary emergence of embryophytes. Their long evolutionary history is evidenced by fossils dating back over 400 million years, highlighting their importance in understanding plant evolution.
Subject Terms
Charophyceae
Categories: Algae; microorganisms; Protista; taxonomic groups; water-related life
Most Charophyceae, like Spirogyra, live in freshwater habitats, but some also occur in moist soil in terrestrial habitats. Charophyceae can live as single cells, colonies, or branched and unbranched filaments and come in a variety of shapes. The characteristics that unite members of the class—and which link them with the embryophytes—include flagellated cells (similar to sperm cells in vascular plants), a nuclear envelope that breaks down during mitosis, mitotic spindles that persist as phragmoplasts (a type of cytoskeletal scaffolding) through cell division either by furrowing or by forming a cell plate, the presence of chlorophylls a and b and phytochrome, and the storage of starch inside plastids.
Charophytes possess decay-resistant cell walls made of phenolic compounds as well as lignins or ligninlike compounds. Cell walls made of similar compounds are found in bryophytes and vascular plants as well. Likewise, all three groups of plants also contain sporopollenin, the substance in the walls of spores and pollen grains that makes them virtually indestructible. Communication channels between cells are similar, too. Plasmodesmata similar to that seen in embryophyte cells allow between-cell communications in charophytes and embryophytes.
Life Cycle
Charophytes have a two-stage life cycle involving a dominant haploid stage, upon which develops the sex organs; antheridia, which produce sperm cells; and oogonia, which produce egg cells. Typically, individual charophytes produce both antheridia and oogonia, but in some species an individual will produce only one or the other. Fertilization—which in one group, the Zygnematales, takes place via conjugation—produces a diploid zygote, which quickly undergoes meiosis. If the environment is unfavorable, the zygote will go dormant and remain so for a long period of time. Dormancy ends when the environment improves.
Classification
Genetic analysis supports the recognition of six orders within the Charophyceae: the Mesostigmatales, Chlorokybales, Klebsormidiales, Zygnematales, Coleochaetales, and Charales.
Of these, the most abundant group is the Zygnematales, a large order which consists of more than three thousand species, including Spirogyra and the desmids, a group of mostly single-celled organisms with a constriction across the middle which nearly divides the cells in two. Zygnematales live primarily in freshwater habitats as phytoplankton, as benthic dwellers, or attached to other aquatic plants. Some species live on snow and ice.
The Charales, commonly called stoneworts or brittleworts, are a large group of filamentous charophytes that feature complex branching patterns. Some can reach lengths of more than a meter. The branching pattern—branches reach out from nodes along the filament—is similar to that of higher plants. Charales reside primarily in freshwater habitats, but some can be found in brackish water as well as on land. The stems of stoneworts and brittleworts can be encrusted with calcium and magnesium carbonates. As a result, their hard bodies are well known from the fossil record. The lineage extends back to more than 400 million years ago. Two current genera, Chara and Nitella, date back about 200 million years.
The Coleochaetales are a small group of complex, microscopic filamentous algae that can be found only in freshwater habitats. Klebsormidiales are a small group of unbranched, filamentous charophytes that occur in both freshwater and terrestrial environments. Mesostigmatales and Chlorokybales are two groups of rare algae. The Coleochaetales and Charales are more closely related to bryophytes and vascular plants than the other groups.
Evolutionary Significance
For decades, structural similarities led plant biologists to suspect that the Embryophyta evolved from charophytes. Recently, cladistic analyses of chemical, structural, and genetic characteristics have opened up research on the topic. In cladistics, characteristics among a number of organisms are analyzed statistically in the hopes of developing a classification system for the group which will reveal evolutionary relationships. Cladistic analyses support the notion that embryophytes are monophyletic; in other words, bryophytes and vascular plants descend from a common ancestor.
Furthermore, several recent analyses support the notion that a member of the Charophyceae gave rise to embryophytes. Cladistic analyses were somewhat unclear, however, about the relationships of the charophyte orders with one another and with other green algae (Chlorophyta), bryophytes, and land plants.
One of the latest analyses of mitochondrial, chloroplast, and nuclear genes helps resolve some of the confusion. The research indicates that the Mesostigmatales were probably the most ancient group of charophytes, followed by the Chlorokybales, Klebsormidiales, Zygnematales, Coleochaetales, and Charales. The work also supports earlier suggestions that the Charales are the closest living relatives to extant embryophytes and that the charophytes descended from other green algae.
Bibliography
Graham, Linda E., and Lee W. Wilcox. Algae. New York: Prentice-Hall, 2000. An overview of the biology of algae, with emphases on systematics, biogeochemistry, and environmental and economic effects.
Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York: W. H. Freeman/Worth, 1999. Includes chapter on protists, such as green algae, with plantlike characteristics.