Charales

The Charales grow in freshwater and brackish environments worldwide, and have large, macroscopic thalli growing up to 120 cm long, they are branched, multicellular, and use chlorophyll to photosynthesize. Their only diploid stage in the life cycle is the unicellular oospore. They may be called stoneworts,[2] because the plants can become encrusted in lime (calcium carbonate) after some time. The "stem" is actually a central stalk consisting of giant, multinucleated cells. They are unique in having a whorl of small branchlets at each node in the stipe, this gives them a superficial resemblance to the genus Equisetum. In these whorls it is possible to see the phenomenon of cytoplasmic streaming. In fact the streaming in Chara is the fastest recorded of any cell. Cytoplasmic streaming is caused by the microfilaments found inside the cell, as proven by the use of cytochalasin B to stop streaming.

There are about 400 species worldwide, with 33 in Britain and Ireland according to Groves and Bullock-Webster),[3][4] however Stewart and Church (1992) reduce this to 21.[5]

Characeae are the principal plant life of some of the volcanic crater lakes of Nicaragua, and can be found in excess of 20 meters depth in some circumstances. Introduced tilapia (Oreochromis niloticus) consumed all the Characeae in Lake Apoyo.[6]

Most Characeae are found in fresh water, usually in still, clear water where they attach to the substrate by rhizoids. They can be pioneer colonizers or ephemerals.[7] They are usually found in low to medium nutrient-rich water and tend to disappear due to eutrophication. Some stoneworts can survive in brackish or maritime habitats and occur in ephemeral saline lakes in Australia that have twice the salinity of seawater.[8]

The antheridia (or globules[9]) and oogonia (or nucules[9]) are protected by a layer of sterile cells when mature; the oogonium is oblong in shape and consists of a single egg, while the spherical antheridium is packed with threadlike cells that produce spermatia. As a result, the Charales have the most complex structure of all green algae, if indeed they should be so labelled.

The possible sister group of the land plants are also known as brittleworts or skunkweed. These curious labels arise from the fragility of their lime-encrusted stems, and from the foul odor these produce when stepped on.

Many botanists propose that the stoneworts and their relatives be placed in a division, sub-kingdom, or even kingdom by themselves, often named Charophyta.[10][11] Their classification by taxonomists is currently undergoing much cladistic scrutiny. Further DNA and RNA analysis may prove the charophytes to be a crucial evolutionary link in the phylogenetic tree of life, the critical developmental step from the algae toward the non-vascular and then vascular land plants.

• Bryant, J. The stoneworts (Chlorophyta, Charales). In Guiry, M.D., John, D.M., Rindi, F. and McCarthy, T.K. 2007. New Survey of Clare Island. Royal Irish Academy. ISBN 978-1-904890-31-7.
• Schaible, R. and Schubert, H. 2008. The occurrence of sexual Chara canesces populations (Charophyceae) is not related to ecophysiological potentials with respect to salinity and irradiance. Eur. J. Phycol. 43: 309 - 316.
• Desai, Udaysingh and Karande C.T. 2008. "Biodiversity of Charophytes from Kolhapur District, Maharashtra". Shivaji University, Kolhapur.

• Images of Charales
• University California
• Chara curta in Ireland