Macroalgae grows profusely amongst the branches of this Acropora sp. coral colony. If conditions shift the competitive balance, macroalgae may outcompete the coral

Macroalgal species are divided among three large groups that are named according to the colour of their dominant photosynthetic and accessory pigments: red (Rhodophyta), green (Chlorophyta) and brown (Phaephyta).

Red algae are the largest and most diverse group, and are extremely important reef-building organisms. Green algae contain chlorophyll and often favour stressful environments where nutrients are high and herbivory is low. Others are calcified and contribute heavily to the sandy sediments of reef areas. Brown algae range in colour from beige to almost black and are more abundant in temperate seas.

Algae can also display a variety of forms including: fleshy macroaglae (which generally has a canopy height of over 1cm), turfing (which form a mat less than 1 cm high on the substratum), and crustose coralline algae (which form flat plates on the substratum and may cement coral fragments together).

Fleshy macroalgae are larger, erect algae which quite often possess chemical or structural deterrents to protect against herbivory. Although macroalgae are often more resilient to physical and biological disturbances than coralline and turf algae, grazing by certain herbivores and high wave energy can inhibit macroalgal growth. An overabundance of macroalgae can also impede coral recruitment and reduce overall coral survival.

Macroalgae have important ecological and economic roles on coral reefs. They are primary producers that form the basis of many marine food chains and provide habitat and refuge for a range of organisms. They are also commercially important for food (e.g. sushi), science (e.g. agar culture mediums) and for their compounds (e.g alginate - used in a variety of products from toothpaste to ice cream).

Macroalgae are also often used as indicators of water quality and reef health. Many species of macroalgae have the ability to assimilate the surrounding water and store it over a short period. These species can therefore be used to monitor changes in the nutrient concentration in the water, particularly nitrogen. In addition, the abundance of macroalgae responds to changes in nutrient concentrations, with increased abundances often indicating elevated levels of nutrients.

Macroalgae can also be major contributors to reef degradation via ecological 'phase-shifts' whereby the dominant taxa on the coral reef shifts from hard, reef-building corals to fleshy macroalgae. The phase shift is generally initiated by a disturbance such as coral bleaching, outbreaks of the coral-eating crown-of-thorns starfish (COTS), coral disease or storm damage. In a healthy system, corals would be able to slowly recover and recolonise the disturbed area. If, however, the number of herbivores in the area has been reduced by overfishing, or the area is affected by elevated levels of nutrients or sediment, a permanent shift to an algal dominated system and an overall reduction in the aesthetic value of coral reefs might occur, which could have flow-on economic for tourism in highly visited areas.