Chlorophyta, Green Algae
Ulva Linnaeus, 1753
Chlorophyta, Order: Ulvales; Family: Ulvaceae.
The scientific name of the alga, Ulva, is the ancient name originally given to all algae. Since the days of Linnaeus, the name has been dedicated to this alga, one of the most dominant in nature.
The Ulva is a green, flattened alga, irregularly shaped and comprising of two layers of cells. This is one of the most common algae here and around the world. The Ulva connects to the substrate through a flat holdfast or thin rhizoids.
Ulvae reach a size of 50 cm; however, most individuals found in nature are usually smaller. There is no fundamental difference between the length and the width. Often the algal size is a result of biotic influence, since the alga is a favourite of many herbivores.
The Ulva is shiny green, but the algal colour is influenced by environmental conditions, the thallus’ age, exposure to light and dehydration processes the alga has undergone. Differences in shade are also evident in various species of Ulva.
It is not rare to confuse Enteromorpha and Ulva, although microscopic cross-section near the base of the holdfast reveals that the Ulva has two layers of cells, while the Ulva (Enteromorpha) has only one and is hollow inside. Formerly, the two genera were united under the name Ulva. A taxonomy of species cannot be executed in the field without a microscope. An identification key of species follows.
The Ulva plants usually develop in the upper regions of the intertidal zone and are often exposed to the air. Ulva can also be found in the subtidal zone, usually in winter and often in large quantities. Although the Ulva is normally a marine alga, it can subsist in fresh water and even in sewage. The Ulva at sea develops upon any kind of substrate, but it can also be cultivated free-floating in water.
Biology and reproduction
Nearly all family members have an isomorphic sexual system, and most species are marine. Many of the species develop in the intertidal zone, and they are often exposed to the air. For most algae immersed in seawater, the level of carbon dioxide (CO2) in the water is a growth impediment. The Ulva, like the Enteromorpha, can absorb bicarbonate (HCO3 -), which is copious in seawater, and use it as a source of carbon for photosynthesis. However, in spite of this ability,
which provides the Ulva with a clear advantage over other underwater species, they are rarely found during the year in the water. The reason for this may be biotic rather than physiological, since the alga is a preferred food for alga- eating fish.
Seasonality and distribution
The Ulva species are found throughout nearly the entire globe, in tropical and sub-tropical seas. They can be found throughout the year. In winter there is a significant rise in the algal biomass, and it occupies nearly the entire intertidal zone. The rise in biomass continues, barring any catastrophe. This may occur after a winter storm, especially if the thallus is developed, since it is then torn away from the substrate and hurled on to the shore. Several days of low tide without regular waves that are characteristic of the easterly winds (‘hamsin’), may cause dehydration, whitening and the destruction of a colony. Such occurrences cause a drastic reduction and almost total destruction of the colony. Its regeneration is very slow, and a long period of time elapses before it regains its position upon the rocks.
Ulva are tubular or flat, and have two layers of cells that probably developed from primordial tubular shapes. The algal shape and variety are influenced by marine environmental conditions. As a result, in spite of their abundance and human interest, family members are difficult to identify, and species are hard to distinguish. Definition is according to thallus shape, cellular arrangement and size (see below). Absolute identification is impossible without a microscope.
Some species of Ulva and Enteromorpha have been noticed along the Eastern Mediterranean coasts. Three species of Ulva identified by Professor G. Giaccone are:
Ulva fasciata Delile Ulva laetevirens Areschoug = Ulva rigida C. Agardh Ulva olivascens P. Dang.
Regarding Ulva cfr lactuca Linnaeus, the availability of the species has been repeatedly reported in this area; however, the reports seem to have been a mistake in definition, since the species cannot be found in the Mediterranean. The thallus is flat, and usually one thallus per holdfast (if there is branching, it is very proliferate). Often holes can be discerned in the thallus. An overview reveals cells arranged in rows, and the upper lip is smooth. The cross section is 80-90 cm. Pyrenoids 1, sometimes 2 or 3. The alga can be found in the Northern Sea.
Key to species is available!
Algae belonging to Ulva are an important ingredient in the fish and marine invertebrate diet. Many species are cultivated in marine agriculture as fodder for fish and crabs. Previously, they have also been used in water purification plants as a source of oxygen. The Ulva is used for human consumption in many countries, in salad (dressed with oil, vinegar and seasoned), in soup, dried as a source for iodine, or as a salad seasoning. Ulva can be added to fish or rice dishes. The plant is also used in research, in Israel as well, since its photosynthetic abilities are very high. It is readily available and easy to grow and supervise.