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Brown algae

The Phaeophyceae or brown algae (singular: alga), is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments both as food, and for the habitats they form. For instance Macrocystis, a member of theLaminariales or kelps, may reach 60 m in length, and forms prominent underwater forests. Another example is Sargassum, which creates unique habitats in the tropical waters of theSargasso Sea. Many brown algae such as members of the order Fucales are commonly found along rocky seashores. Some members of the class are used as food for humans.

Worldwide there are about 1500-2000 species of brown algae. Some species are of sufficient commercial importance, such as Ascophyllum nodosum, that they have become subjects of extensive research in their own right.

Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among heterokonts in developing into multicellular forms with differentiatedtissues, but they reproduce by means of flagellatespores and gametes, which closely resemble other heterokont cells. Genetic studies show their closest relatives to be the yellow-green algae.

Morphology

Brown algae exist in a wide range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlike cells no more than a few centimeters long. Some species even have a stage in their life cycle that consists of only a few cells, making the entire alga microscopic. Other groups of brown algae grow to much larger sizes. The leathery kelps and rockweeds are often the most conspicuous algae in their habitats. Kelps can range in size from the two-foot-tall sea palm (Postelsia) to the giant kelpMacrocystis pyrifera, which grows to over 45 m (150 ft) long and is the largest of all the algae. In form, the brown algae range from small crusts or cushions to leafy free-floating mats formed by species ofSargassum. They may consist of delicate felt-like strands of cells, as inEctocarpus, or of foot-long flattened branches resembling a fan, as inPadina.

Reagrdless of size or form, two visible features set the Phaeophyceae apart from all other algae. First, members of the group possess a characteristic color that ranges from an olive green to various shades of brown. The particular shade depends on the amount of fucoxanthin present in the alga. Second, all brown algae are multicellular. There are no known species that exist as single cells or as colonies of cells, and the brown algae are the only major group of seaweeds that does not include such forms. However, this may be the result of classification rather than evolutionary, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms.

Visible structures

Whatever their form, the body of all brown algae is termed a thallus, indicating that it lacks the complexxylem and phloem of vascular plants. This does not mean that brown algae completely lack specialized structures. But, because some botanists define "true" stems, leaves, and roots by the presence of these tissues, their absence in the brown algae means that the stem-like and leaf-like structures found in some groups of brown algae must be described using different terminology. Although not all brown algae are structurally complex, those that are typically possess one or more characteristic parts.

A holdfastis a rootlike structure present at the base of the alga. Like a root system in plants, a holdfast serves to anchor the alga in place on the substrate where it grows, and thus prevents the alga from being carried away by the current. Unlike a root system, the holdfast generally does not serve as the primary organ for water uptake, nor does it take in nutrients from the substrate. The overall physical appearance of the holdfast differs among various brown algae and among various substrates. It may be heavily branched, or it may be cup-like in appearance. A single alga typically has just one holdfast, although some species have more than one stipe growing from their holdfast.

A stipeis a stalk or stemlike structure present in an alga. It may grow as a short structure near the base of the alga (as inLaminaria), or it may develop into a large, complex structure running throughout the algal body (as inSargassumor


From Yahoo Answers

Question:What are the common characteristics of algae, fungi and protozoa?

Answers:All three are Eukaryotes, so they have in common (among other things): a membrane bound nucleus linear chromosomal DNA histones mitochondria various membrane-bound organelles 80S ribosomes microtubules RNA processing ester-linked lipids

Question:What are some uses of algae?

Answers:The major characteristic is that they are all photosynthetic with cell walls but lacking differentiated tissues. A few groups have primitive vascular tissues, fronds, stipe and holdfast (Phaeophyta) or specialized reproductive organs. Each phylum or division of algae is characterized by the pigments present for photosynthesis, food reserves stored within the cells, and life cycle pattern. If you've have eaten sushi, you likely have consumed an algae in the form of the wrapper around the contents. Extracts from algae are used as emulsifiers, thickening agents, and flavoring in foods. If you see the word, "alginates" or "carrageen," you are eating an alga extract. The silica cell walls of diatoms have accumulated in vast deposits called diatomaceous earth. Diatomaceous earth is used as an abrasive, as an insecticide, as a filtration medium, and formily was mixed with nitroglycerin to form dynamite explosives. Composted seaweeds have been used in coastal communites to increase soil fertility. Liquid seaweed is sprayed on plants both as a fertilizer and an organic pesticide. Have you thanked a photosynthetic organism today?

Question:What is a characteristic of sexual reproduction in green algae? A. Zoospores are formed. B. Male and female gametes are formed. C. Meiosis produces gametes. D. Meiosis occurs after a zygote is formed. I am thinking that the answer would maybe be letter C but I am not a 100% sure, any help would be great.

Answers:Meiosis does produce gametes so c is a correct answer. Marine Chlorophyta (green algae) is commonly isogamous where the same size gametes are both motile. They cannot be strictly classed as male or female because they look alike, they are just classed as "+" or "-" mating types. Both a plus and a minus are required to produce a zygote but this is not yet distinctly male and female http://www.iemss.org/iemss2004/pdf/ecol/togarepr.pdf

Question:

Answers:What type of algae? There is one phylum of animals, one phylum of plants and several phyla of algae. plants use cellulose to give them structure, some algae uses cellulose but not all, they use other sugars for structure. honestly you asked a question that could be a large volume of books. There are so many different kinds of algae that i didn't even know it had a characteristic shape. Algae can be a single celled organism or 60 foot tall kelp and everything in between. Some have air sac like structures to help them float. Color is a little bit simpler, this just relates back to what pigment the algae uses for photosynthesis. If it uses chlorophyll, it will be greenish. honestly I am just typing answers to a question that doesn't make sense. There are so many species of algae that if they had a characteristic shape or shapes it would be even more extreme than every plant and fungi looking all like and having a characteristic shape.