Ulvophyceae

Categories: Algae; microorganisms; Protista; taxonomic groups; water-related life

The name Ulvophyceae is derived from a Latin word designating plants that live in marshes. Carolus Linnaeus referred to algae with the taxonomic term Ulva in his Species plantarum (1753). In 1978 Kenneth D. Stewart and Karl R. Mattox identified Ulvophyceae, or the ulvophytes, as a distinct phyletic line in the division Chlorophyta. The classification of Ulvophyceae was modified as botanists gained new insights into its ultrastructure.

Using electron microscopes, botanists detected that Ulvophyceae algae exhibit unique flagellar apparatus and root systems. Ulvophyceae taxonomy was developed according to these physical characteristics. During the 1980’s, botanists assigned six orders to class Ulvophyceae: Ulotrichales, Ulvales, Cladophorales, Dasycladales, Caulerpales, and Siphonocladales.

By 1990 cladistic analysis of Ulvophyceae genetic material caused researchers to reconsider the evolutionary relationships and lineages of Ulvophyceae orders. Molecular comparisons suggest that ulvophytes might be basal to (evolutionary predecessors of) those green algae classified in Chlorophyceae and Pleurastrophyceae. Investigations continue to advance comprehension of Ulvophyceae‘s role in plant evolution.

Structure

The ultrastructure of species of Ulvophyceae usually is radially symmetrical and consists of motile cells with flagella, basal bodies positioned in a counterclockwise rotation, cruciate microtubular roots, and persistent mitotic spindles that do not collapse during teleophase. Many ulvophytes are macroscopic and appear as filamentous or as flat sheets of cells. Others are microscopically small. Some are scaly or slippery. The sea lettuce (Ulva) and Ventricaria are the most familiar representatives of Ulvophyceae.

In the order Ulotrichales, species in the genus Ulothrix consist of single-nucleus cells arranged to form unbranched filaments. A cell at each filament’s basal end is a rootlike holdfast that adheres plants to surfaces. Plants in the order Siphonocladales often resemble bubbles and are found on shells or stones in tropical waters. The plants in order Ulvales have a flat or hollow thallus (body). Members of Enteromorpha in this order consist of tubular strands that can attain widths of 5 centimeters (2 inches). Some species in the genus Ulva are as large as 65 centimeters (25.6 inches).

Members of Codium magnum in order Caulerpales sometimes reach lengths of 8 meters (26.25 feet). Order Cladophorales includes both branched and unbranched filamentous algae. Members of Cladophora (also sometimes classified in the order Siphonocladales) have branching filaments, which often grow tightly together and are formed by multinucleate cells with thick walls. Some members of order Dasycladales are extinct. Fossils indicate that these ulvophytes first lived during the Middle Silurian period, about 420 million years ago.

Sexual reproduction in Ulvophyceae is significant because ulvophytes are the only members of Chlorophyta that reproduce by an alternation of generations, with haploid and diploid thalli. Instead of zygotes, spores are the site of meiosis. Species also form dense stands as a result of vegetative reproduction of fragments, which usually sink instead of float. Temperature and sunlight regulate the rate of growth.

Distribution

Ulvophytes are usually found in marine environments, especially along coasts and in harbors, although freshwater species thrive in shallow parts of lakes and streams. Other habitats include rocks and soil. These algae live primarily in small clumps but occasionally form mats near the water surface as dense as five tousand fronds per square meter (11 square feet). They occasionally live at maximum depths of 10 meters (33 feet), although some species have been discovered as deep as 99 meters (325 feet). Ulvophytes prefer still, clear water but can survive in brackish and polluted environments.

Plants in class Ulvophyceae are indigenous to tropical regions in the Pacific, Indian, and Atlantic Oceans and the Red Sea. Some species are unintentionally transported on ships to other regions. If conditions are suitable, ulvophyte colonies multiply by as much as 28 percent daily, invading territory and competing with native plants. In 1984 Caulerpa taxifolia was discovered in the Mediterranean Sea. By 1996 that species had spread from covering an area of one square meter (11 square feet) to invading 3,096 hectares (7,647 acres) in seventy-seven places in the Mediterranean. Ulvophytes often adapt to new environments by deviating from their original characteristics—for example, lengthening and increasing the number of fronds to adjust to lower water temperatures.

Uses and Cultural Impact

Several Ulvophyceae species are valued by humans for their nutritional qualities, texture, and flavor. Rich in vitamins and minerals, especially iron and iodine, Ulva are sources of protein, sugar, starch, and roughage. Ulva lactuca, commonly called sea lettuce, is collected for salad, soup, and sauce ingredients. Ulva is also cultivated as livestock feed and fish bait.

Many ulvophytes have medicinal characteristics and are able to soothe burns. Soaps, oils and shower gels, and nutritional supplements often include ingredients from ulvophytes. Some animals symbiotically rely on Ulvophyceae. For example, sea slugs gather chloroplasts from Codium, which continue to undergo photosynthesis in the slugs’ respiratory chambers, enhancing their oxygen supply. Ulva is cultivated to absorb nitrogen and phosphorus in water at abalone farms. Ulvophytes indicate environmental quality and are used to detect metal contamination and eutrophication, because they quickly accumulate in areas where large amonts of polluting substances cause those conditions.

Ulvophytes also can be toxic. Caulerpa taxifolia emits terpene caulerpenyne, which discourages predators from eating algae. Cephaleuros causes red rust, a fungus that can be economically disastrous for tea, pepper, and citrus growers. Sometimes considered a pest or pollutant, filamentous algae can destroy water quality necessary to sustain life, interfere with commercial and recreational uses, and deplete oxygen, causing fish kills. Codium can destroy oyster beds. Algae decomposition, especially when plants wash onto beaches and rot, results in unbearable stenches caused by the ulvophytes’ sulfur content. Green tides resulting from an overabundance of ulvophytes thriving on nutrients in agricultural effluent are problematic. These thick clumps of algae prevent other plant and animal organisms from having access to sufficient quantities of crucial sunlight and oxygen.

Bibliography

Canter-Lund, Hilda, and John W. G. Lund. Freshwater Algae: Their Microscopic World Explored. Bristol, England: Biopress, 1995. Includes information about ulvophytes in an accessible text about algae and its parasites and predators. Includes color and black-and-white photographs.

Lee, Robert E. Phycology. 3d ed. New York: Cambridge University Press, 1999. Thorough description of the orders classified in Ulvophyceae and their characteristics and life cycle. Includes bibliographical references, figures, illustrations, and index.

Lobban, Christopher S., and Paul J. Harrison. Seaweed Ecology and Physiology. New York: Cambridge University Press, 1997. Focuses on the filamentous ulvophytes. Includes bibliographical references, illustrations, and index.

South, G. Robin, and A. Whittick. Introduction to Phycology. Oxford, England: Blackwell Scientific, 1987. A basic text that incorporates information about Ulvophyceae. Includes illustrations, bibliography, and index.

Sze, Philip. A Biology of the Algae. 3d ed. San Francisco: McGraw-Hill, 1998. Discusses how Ulvophyceae compare with other algae classified in Chlorophyta. Includes color illustrations, tables, glossary, bibliographical references, and index.