Algae are marine plants that are simple in structure and are different from typical land plants because they lack roots, stems and true leaves. Algae have a large variety of growth forms ranging form single celled plankton algae to large seaweeds that can reach lengths of several metres. In contrast, seagrasses are true flowering plants like many land plants.

Algae play a very important role in the marine ecosystem. They are the primary producers that form the basis of most marine food chains. Phytoplankton is particularly important in this regard, and is a considerable sink for carbon dioxide. Photosynthesis uses energy from sunlight to convert carbon dioxide and water into food and oxygen. On coral reefs, calcareous algae have a significant role in cementing the reef together. Large algae may also provide homes for a wide variety of small organisms.

Major types of algae

Blue-green algae are more appropriately known as cyanobacteria. Although they were originally thought to be a type of algae, they are more closely related to bacteria. Some cyanobacteria form brown, green, red, or purple tufts on the reef. Another common type of cyanobacteria is the planktonic Trichodesmium, which forms sometimes forms blooms in tropical waters. After the blooms die Trichodesmium is visible as a reddish slick on the surface of the water. Cyanobacteria are important to marine ecosystems because they fix atmospheric nitrogen and make it available to the food web.

Red macroalgae. Photo by AIMS

The most common red algae in coral reef environments are the calcareous coralline algae. The algae form hard crusts on the reef surface, cementing together coral and building the reef framework. The algae are particularly common in shaded areas or on the underside of coral rubble. Other red algae are sometimes very attractive leafy or bushy seaweeds, which are found often in deeper waters.

Sargassum bed Photo: L. McCook, AIMS

Brown algae contain an extra pigment, which often masks the green chlorophyll shared by all algae. Brown algae are abundant in cooler climates, such as in kelp beds. In the tropics, a few species such as the large-sized Sargassum and Turbinaria can be very common in summer on inshore reefs.

Halimeda algae: photo by AIMS

The last group - the green algae - occur in a wide variety of shapes and sizes. Sea lettuce (Ulva), sea grapes (Caulerpa) and turtle grass (Chlorodesmis) are some of the most abundant types of green algae on coral reefs. The green alga Halimeda resembles a series of small, connected "cornflakes". This alga is common on many reefs and can also be found in vast beds in deep water. It contributes greatly to the sediment that forms sand banks in the Great Barrier Reef region.

'Golden noodle algae' Chrysocystis fragilis. Photo by Britta Schaffelke

Another type of algae recently found on the Great Barrier Reef are golden algae. Tourism operators in the Great Barrier Reef are concerned about extensive blooms of seabed algae known as 'golden noodle algae', which was first observed in the 1980s.

Scientists have identified the bloom-forming alga as Chrysocystis fragilis, a species that has not been recorded in the Great Barrier Reef region. No-one knows where the alga originated: it was originally seen in Guam, and has been found at Palau, Pohnpei and Hawaii.

Blooms of golden noodle algae, occasionally blanketing tens of square metres of reef, have been found in a few places in the northern part of the Reef. The alga grows on a variety of surfaces in water depths from 5m to at least 20m, but is most abundant on dead coral. The algae may be more obvious due to recent reductions in live coral cover due to coral bleaching and outbreaks of crown-of-thorns starfish.

For more information about CRC Reef Research into golden noodle algae click here.

Effects of algae on the Great Barrier Reef

On the Great Barrier Reef, there are large differences in the amount and type of seaweeds between fringing reefs around inshore islands and reefs further offshore. In general there are more abundant and larger-sized algae on reefs closer to the coast.

Environmental conditions, natural disturbances, the presence of herbivorous fish, human activities and large regional climatic fluctuations all influence the presence of algae. While reefs may be affected by changes in water quality due to increased sediment or nutrient levels, and can become overgrown by large seaweeds, this is not always the case.

Although enhanced nutrients can increase daily algal growth rate, total biomass may remain the same because new growth is consumed by herbivores as rapidly as it is produced. Persistent macroalgal blooms have only occurred on reefs where herbivores are depleted, for example by fishing pressure, or where a major disturbance has diminished the live coral cover. In the Great Barrier Reef region herbivores are generally not fished.

In general, there is little evidence that algae can overgrow healthy, established corals. The recovery of live coral cover after a disturbance, however, can be delayed if a dense algal cover has established in favourable growth conditions for algae, for example under sustained high nutrient availability.