ABSTRACT
Algae are thallophytes (plants without roots, stem and leaves) that have chlorophyll a as their primary photosynthetic pigment and lack a sterile covering around the reproductive cells. This definition includes many of the plant forms that are not necessarily closely related (e.g. Cyanobacteria, are closer in evolution to the bacteria than to algae, or, on the contrary, the green algae are closer in evolution to the higher plants). Algae vary from small, single-celled forms to complex multicellular forms. They exhibit a wide range of reproductive strategies from simple, asexual cell division to complex forms of sexual reproduction. They occur in most habitats, ranging from marine and freshwater to desert sands and from hot boiling springs to snow and ice. They are extremely important ecologically, since they function in most habitats as the primary producers in the food chain, producing organic material from sunlight, carbon dioxide and water. Besides forming the basic food source for these food chains, they also produce the oxygen necessary for the metabolism of the consumer organisms. The algae probably account for more than half the total primary production
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worldwide, virtually all aquatic organisms are dependent on this production. Photoautotrophic organisms often develop severe competition for resource, space, light or nutrients. The release of allelopathic compounds which can interfere with settlement and growth of competitors, affecting population dynamics (Pratt, 1966; Rojo et al, 2000), is an adaptive strategy adopted by primary producers and microorganisms (Gross, 2003). This hypothesis was reported for the first time by Schreiter (1928), who cited a case, where the phytoplankton population of a pond was lowest in years, when the large aquatic plants were most dense.
