Coralline algae are a globally distributed group of calcified red algae found in marine environments, from polar waters to tropical reefs. As primary producers, they create food through photosynthesis and form hard, pink or purplish crusts on underwater surfaces. Their presence and health are intimately tied to the overall structure and function of diverse marine ecosystems. Understanding their unique biology and ecological functions is fundamental to comprehending the stability of environments like coral reefs.
Biological Identity and Forms
Coralline algae belong to the division Rhodophyta, or red algae, and are characterized by a unique process called biomineralization. This biological process involves the deposition of calcium carbonate within their cell walls, which gives them a hard, stony texture. The specific mineral they precipitate is primarily high-magnesium calcite, a form of calcium carbonate that is less stable than the aragonite used by corals.
These calcified algae present in two main growth forms: articulated and non-geniculate, or crustose. Articulated coralline algae are upright and branching, often resembling small, pink bushes. Crustose Coralline Algae (CCA) are non-geniculate and grow as thin, tightly adhering layers on rock, rubble, and dead coral skeletons.
CCA are the forms of coralline algae most significant to reef ecology. The calcified thallus, or body, is composed of cells that deposit high-magnesium calcite, organized into radial crystals within the cell walls. This hard structure enables them to survive in turbulent, wave-swept environments.
The Role as Ecological Cement
The calcified structure of CCA allows it to perform a major function in the marine environment by acting as a biological glue. As the algae grow, they encrust and reinforce dead coral skeletons and other fragments of rubble on the seafloor. This process of encrustation fills in small crevices and binds together the loose components of the reef structure.
This stabilization helps the entire reef maintain its structural integrity, a function observed in fossil records for at least 150 million years. On surf-pounded reefs, coralline algae are often the primary builders of algal ridges, which are robust carbonate frameworks that absorb the energy of oceanic waves. Without this constant cementation, the reef would be vulnerable to erosion and collapse from wave action.
The continuous accretion and binding of material by CCA are essential for creating the three-dimensional scaffolding that defines a healthy reef. They transform a pile of loose debris into a solid, resilient structure capable of withstanding storms. This structural reinforcement is a foundational service that supports the vast biodiversity of the entire ecosystem.
Facilitating Coral Reef Growth
Beyond physical cementation, a specific ecological function of Crustose Coralline Algae (CCA) is facilitating the recruitment of new corals. When reef-building coral larvae transition from a free-swimming stage to a stationary polyp, they must locate a suitable substrate. CCA provides the necessary cue for this settlement and metamorphosis to occur.
The algae produce and exude specific chemical compounds, such as glycoglycerolipids and polysaccharides, which induce coral larvae to attach and transform into a young coral polyp. Larvae actively sense these biochemical signals, using them to identify a healthy and suitable habitat on which to begin their adult lives. This reliance means that the presence of healthy CCA is a prerequisite for the successful replenishment of coral populations.
If the cover of CCA declines, the number of successful coral larval settlements is severely reduced, hindering the natural recovery and maintenance of the reef. This dependency links the health of the coralline algae directly to the long-term resilience and survival of the coral reef ecosystem.
Sensitivity to Environmental Change
Coralline algae are particularly sensitive to shifts in ocean chemistry, making them a significant indicator species for environmental stress. Their high-magnesium calcite skeleton is inherently more susceptible to dissolution than the aragonite skeletons of stony corals. As the ocean absorbs increasing amounts of atmospheric carbon dioxide, the water’s pH decreases, a process known as ocean acidification.
This decrease in pH makes it harder for coralline algae to perform calcification, the process of building and maintaining their calcium carbonate structures. Experimental studies show that under predicted future acidification scenarios, the net calcification rate of CCA declines. Furthermore, the lowered pH can cause existing high-magnesium calcite to dissolve more easily, leading to a net loss of skeletal material.
The combined stressors of ocean acidification and warming negatively affect the algae’s ability to grow, calcify, and survive. Since the algae’s functions as cement and larval settlement cue are so fundamental, their vulnerability directly threatens the ability of coral reefs to persist. Monitoring the health and calcification rates of coralline algae provides scientists with a measure of the ocean’s overall health and the future prospects for other calcifying organisms.