Coral Reefs & Algae

Coral reefs contain many types of algae, most of which are beneficial.

Coral reefs contain many different types of algae, most of which are beneficial and even necessary to life in the reef. If a coral reef is not healthy, invasive algae overgrowth is possible and could cause damage to corals. According to the Reef Research Centre, "Algae . . . range from single celled plankton algae to large seaweeds that can reach lengths of" over 100 feet. Algae produce their own energy via photosynthesis and are the main reason shallow-water coral reefs need sunlight for survival.

Blue-Green Algae

The red color is due to excess cyanobacteria growth.

Blue-green algae (referred to as "cyanobacteria") are some of the oldest algae in the ocean, states University of Hawaii. They form slimy mats over sand or rocks and may be green, purple, brown or red in color. More closely related to bacteria than algae, according to the Reef Research Center, blue-green algae are important because they serve the purpose of making nitrogen available to other organisms in the ocean--through a process known as "nitrogen fixing."

Green Algae

Caulerpa algae are a favorite food of marine herbivores and provide a safe habitat for beneficial filter feeders.

The green algae group is the most diverse and abundant group of algae in the ocean. The University of California at Berkeley reports there are over 7,000 species of green algae. Tiny green algae known as "microalgae" and "plankton" are one of the most important food sources to reef inhabitants. Larger green algae species, known as "macroalgae," also provide an important food source and habitats for many tiny filter-feeders. The filter-feeders living in the algae remove toxins from the ocean and are a food source to many reef organisms. Since green algae absorb minerals from the ocean as they grow, when the algae die, the minerals break down to replenish the reef sand bed.

Brown Algae

Brown algae growing on the outskirts of an inshore reefs help protect the habitat.

Brown algae are generally known as "kelp" and "seaweed." According to the Reef Research Center and the University of California at Berkeley, brown algae colonies can grow up to 100 feet in length, and the largest brown algae are most abundant in cooler climates. In warmer climates, small colonies of brown algae commonly grow on the fringes of inshore reefs. If these inshore reefs are not healthy, they are susceptible to overgrowth of brown algae, which further weakens the reef. In healthy, balanced reef environments, brown algae colonies do not cause problems; rather, they help protect the reef by trapping sediment that would otherwise block sunlight.

Red Algae

The red and purple patches on the rocks are colonies of encrusting coralline algae.

Red algae are hard, calcareous algae, the most common of which are encrusting algae known as "coralline algae." Layers of red, pink and purple coralline algae encrust rocks and many other surfaces to form living reef structures. Coralline algae are very strong and make up one of the building blocks of coral reefs. Red algae found in lower-light environments look similar to leafy brown algae and do not encrust surfaces.

Zooxanthellae

This coral is colonized by brightly colored zooxanthellae algae.

According to the National Oceanic and Atmospheric Administration (NOAA), zooxanthellae algae are microscopic, unicellular algae that colonize the tissue of coral polyps, forming a symbiotic relationship. Zooxanthellae algae are brightly colored organisms, and the colonies living within the translucent coral polyps give corals their distinctive coloration. According to NOAA, "Several million zooxanthellae live and produce pigments in just one square inch of coral." The hard skeletons of the corals protect the zooxanthellae algae, and the algae provide energy to and cleanse the coral polyps.

Herbivores

Butterfly fish eat the zooxanthellae algae living in the tissue of reef corals.

The coral reef is home to many herbivores, and Cornell University reports that algae-eating reef inhabitants include invertebrates such as urchins, crabs, limpets, and worms and vertebrate such as reef fishes (including butterfly fish, tangs, wrasses and gobies) and sea turtles. Herbivores are important for maintaining the balance of reef ecosystems. By eating algae, which is a primary producer of carbon (the basic energy source for all life), herbivores transfer the carbon to other parts of the food web.

References

• Botany Department, University of Hawaii at Manoa; Hawaiian Reef Algae
• Cornell University Department of Microbiology; Herbivory
• National Oceanic and Atmospheric Administration (NOAA) Coral Reef Conservation Program
• Reef Research Centre; Reef Algae
• University of California at Berkeley Museum of Paleontology (UCMP); Introduction to the Green Algae