There are between 50 and 100 species of green algae that make their homes on coral reefs. A few species are covered with calcium carbonate skeletons very similar to the ones that protect coral animals. These plants—called coralline, or calcareous, algae—play special roles in building reef structure.
Coralline algae occur in a wide variety of shapes and sizes. Some look like masses of fine, threadlike filaments spreading over the reef and rock surfaces. The filaments are able to trap sediments and cement the particles together. In this way, coralline algae strengthen and support the coral reef structure. Even if a storm hits and many coral colonies are broken, coralline algae quickly bind the pieces back together. Unlike the encrusting forms, other species of coralline algae grow upright. They, too, produce calcium carbonate for structural strength and protection. When these algae die, the limestone in their bodies is converted to sand. About 50 percent of the sand found on coral reef beaches originated in coralline algae.
One of the abundant coralline green algae belongs to the genus Halimeda, shown in Figure below. Resembling an underwater cactus, Halimeda can be found in most areas of the reef community. Fronds of Halimeda are modified into flat, calcified segments, each 0.2 to 1.2 inches (0.5 to 3 cm) wide. These segments are joined by very short, uncalcified areas, and they give the plant a branching shape. For Halimeda, the calcium carbonate skeleton is an asset because it provides support for the plant while protecting it from predation.
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| Halimeda is a coralline green alga found on coral reefs. (Courtesy of NOAA, Coral Kingdom Collection) |
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| Halimeda |
Coralline algae such as Halimeda help build and repair the coral reef. When the algae’s hard segments break off, they fall to the reef floor where they are tossed and rolled around by waves and currents. Eventually, the calcium carbonate skeletons erode into particles of sand, much of which washes to the floor of the lagoon and contributes to the sandy bottom. Some is carried by moving water to other parts of the reef and deposited there. The sand settles into cracks and crevices on the reef, filling them. Up to 25 percent of a coral reef may be contributed by Halimeda.
Like all green algae, Halimeda can reproduce sexually and asexually. Asexual reproduction is fairly straightforward: If pieces of Halimeda break off and fall in a quiet place, they will grow there, eventually developing into a new plant. However, Halimeda’s style of sexual reproduction has an unusual twist. Halimeda produces male and female gametes, or sex cells, just like other species of green algae. These cells discharge into the water, where they fuse and form zygotes. On the night before Halimeda’s gametes are ready to be released, the entire plant loses its green color. The only green structures left on the plant are masses of tiny spots scattered on the surfaces of the fronds. These spots, called gametangia, are the organs that make the sex cells. The contents of the plant’s cells have moved into the gametangia, packing them with hundreds of chloroplasts, nuclei, mitochondria, and other cell parts. The next morning, the gametangia rupture, spewing flagellated gametes and cell contents into the seawater. The cells swim around until they encounter a gamete of the opposite sex. The two cells fuse to form a zygote that settles on the bottom and grows into a new Halimeda plant.
Although Halimeda and many other species of green algae help build and support the coral reef, not all do. Some green algae actually damage the reef. The common green bubble alga, Dictyosphaeria cavernosa, is one of those. The fronds of Dictyosphaeria are modified into bright green bubbles that measure about 0.2 inch (5 mm) or larger in diameter. Patches of green bubble algae can form mats that measure 6.6 feet (2 m) across. These attach to the reef and grow across it very quickly. A mat of Dictyosphaeria can move over the reef at a rate of one inch (2.5 cm) a month. Green bubble alga is a fast grower because its mats form hollow chambers where gases and nutrients can get trapped. These raw materials then support the alga’s fast rate of growth.
By overgrowing the coral, the algal mats smother the coral animals, destroying large areas of living reef. In addition, Dictyophaeria weakens the base of the reef structure. Eventually, it causes chunks of coral to break off, but it holds the coral loosely in place with its tendrils. When strong waves occur, they easily rip apart the mat and the coral supported by it.
Several species of another green alga, Caulerpa, are also found on reefs. These fast growing organisms occur in many varieties, some with delicate, fernlike fronds and others characterized by round, grapelike structures. Caulerpa can grow quickly by sending out runners that give rise to new plants. The runners cling securely to the reef floor with holdfasts.