Algae are simple chlorophyll-containing organisms composed of one cell, or grouped together in colonies, or as organisms with many cells, sometimes forming simple tissues (Note: algae is plural, alga is singular). They differ from seagrrasses in that they lack true roots, stems, and leaves. Algae are classified into phyla based on their dominant photosynthetic pigments and include green algae (Chlorophyta), brown algae (Phaeophyta), and red algae (Rhodophyta). These three phyla are macroscopic and easily visible with the naked eye
For green algae, colors range from pale green to bright green, as well as yellowish to brownish green and originates from chlorophylls a and b. They grow in a variety of shapes and calcareous forms of green algae (especially Halimeda spp.) contribute significant amounts of calcium carbonate to marine sediments found in seagrrass beds, and on coral reefs and beaches. Brown algae have colors ranging from brown to yellow-green brown resulting from the brown pigment fucoxanthin. Extractions from brown algae are commonly used in toothpaste and ice cream. Red algae are the most diversified of the algae with colors ranging from pale pink to dark burgundy red that are derived primarily from the red-pink pigment phycoerythrin. Calcareous red algae play an important role in the reef building process by adding calcium carbonate to the reef and aiding in cementation.
Tropical reef algae can be divided into three key functional algal groups: crustose coralline algae, algal turfs, and macroalgae. Using a functional grouping approach is useful for understanding broader generalizations on the ecology of algae as well as predicting changes in algal community structure. Crustose coralline algae are often calcified, usually pink in color and adhere to the substrate; although some species have different characteristics. Algal turfs consist mostly of tiny filaments with canopy heights of less than 10 mm. Macroalgae are usually larger, canopy-formers, including brown algae (Dictyota, Sargassum), red algae (Gracilaria, Laurencia); green algae (Caulerpa, Microdictyon) and can be either fleshy (e.g., Lobophora) or calcareous (e.g., Halimeda).
Crustose corallines are calcified, often encrusting algae usually pink to dark burgundy in color (e.g., Porolithon, Peyssonnelia, Lithophyllum). Crustose coralline algae play two important roles in the coral reef community, first by contributing calcium carbonate to reef structure and second by possibly facilitating settlement of coral recruits. The ecology of crustose corallines is complex often interrelated with the presence of macro and turf algae, grazing intensity by herbivores, and productivity.
Turf algae are a multispecific assemblage of diminutive,
often filamentous, algae that attain a canopy height of only 1 to 10 mm (see
Steneck 1988 for review). These microalgal species have a high diversity
(>100 species in western
Macroalgae are larger (canopy height usually >10mm) erect algae often with anatomically complex forms. Most macroalgae possess some from of deterrent against herbivory, either through chemical deterrents or structural resistance. Although macroalgae are often more resistant to physical and biological disturbances than corallines and turfs, grazing by certain herbivores and high wave action can inhibit macroalgal growth. High macroalgal biomass can interfere with coral recruitment and reduce coral survival.
Algal community structure is complex often interrelated with the presence of or competition with other algae, intensity of herbivory, and productivity (and productivity-related environmental factors such as light, nutrients, and water motion). Most herbivore groups are able to feed on turfs, while fewer feed on macroalgae and even fewer on corallines (Steneck 1988). Crustose corallines are able to withstand or resist most intensive grazing due to the mechanical properties of their calcified thallus (Steneck 1988); many macroalgae have structural or chemical defenses to deter grazing; but turf algae are not as effective in resisting or deterring herbivory, although they are capable of rapid growth and regeneration. Increased herbivory reduces algal biomass and shifts algal community dominance from macroalgae to turf algae and eventually to encrusting corallines.
After the mass mortality of the herbivorous sea urchin, Diadema antillarum, in 1983 (Lessios 1988), a shift was observed from coralline to macroalgal dominance in shallow reefs and from turf to macroalgae in deep forereefs of Jamaica and St. Croix (Steneck 1994, 1997). Similar declines of corallines have been observed after the reduction of other herbivores such as fish (Lewis 1986), limpets (Steneck and Dethier 1994), and sea urchins (Hay and Taylor 1985). Generally, when macroalgal biomass is high, coral recruitment declines (Birkeland 1977) and coral abundance declines as well (Hughes 1994).
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