Coralline algae


Coralline algae are red algae in the order Corallinales, characterized by a thallus containing calcareous deposits within its cell walls, giving it hardness. The colors of these algae are typically some hue of pink, or another shade of red, but some species can be purple, yellow, blue, white, or gray-green. Typically, these algae grow in a crustose manner ; in the intertidal zone of rocky shorelines, and within coral reefs, these algae appear as an abundance of colorful patches on rock surfaces. Unattached specimens may form relatively smooth compact balls, or forming warty to fruticose thalli.
The red algae belong to the division Rhodophyta, within which the coralline algae form the order Corallinales. There are over 1600 described species of nongeniculate coralline algae. The corallines are presently grouped into two families on the basis of their reproductive structures. Most are marine, though one species lives in freshwater; Pneophyllum cetinaensis.
Coralline algae play an important role in the ecology of coral reefs. Sea urchins, parrot fish, along with limpets and chitons feed on coralline algae. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef, the Coralligène.

Description

Forms

Corallines have been divided into two groups based on their growth forms, although this division does not conform to taxonomic grouping:
  • the geniculate corallines;
  • the nongeniculate corallines.
Geniculate corallines are branching, tree-like organisms which are attached to the substratum by being crustose or possessing calcified, root-like holdfasts. The organisms are made flexible by having non-calcified sections separating longer calcified sections. Nongeniculate corallines range from a few micrometres to several centimetres thick crusts. They are often very slow growing, and may occur on rock, coral skeletons, shells, other algae or seagrasses. Crusts may be thin and leafy to thick and strongly adherent. Some are parasitic or partly endophytic on other corallines. Many coralline crusts produce knobby protuberances ranging from a millimetre to several centimetres high. Some are free-living as rhodoliths. The morphological complexity of rhodoliths enhances species diversity, and can be used as a non-taxonomic descriptor for monitoring.

Growth

Corallines, especially encrusting forms, are slow growers, and expand by annually. All corallines begin with a crustose stage; some later become frondose.
The thalli can be divided into three layers: the hypothallus, perithallus and epithallus. The epithallus is periodically shed, either in sheets or piecemeal.

Mineralogy

Since coralline algae contain calcium carbonate, they fossilize fairly well. They are particularly significant as stratigraphic markers in petroleum geology. Coralline rock was used as building stone since ancient Greece.
The calcite crystals composing the cell wall are elongated perpendicular to the cell wall. The calcite normally contains magnesium, with the magnesium content varying as a function of species and water temperature. If the proportion of magnesium is high, the deposited mineral is more soluble in ocean water, particularly in colder waters, making some coralline algae deposits more vulnerable to ocean acidification.

Evolutionary history

The fossil record of corallines matches their molecular history, and is complete and continuous.
Stem group corallines are reported from the Ediacaran Doushantuo formation; later stem-group forms include Arenigiphyllum, Petrophyton, Graticula, and Archaeolithophyllum. The corallines were thought to have evolved from within the Solenoporaceae, a view that has been disputed. True corallines are found in rocks of Jurassic age onwards.
The crown group corallines have an excellent fossil record from the Early Cretaceous onwards, consistent with molecular clocks that show the divergence of the modern taxa beginning around this period. The fossil record of nonarticulated forms is better: the unmineralized geniculae of articulated forms break down quickly, scattering the mineralized portions, which then decay more quickly. This said, non-mineralizing coralline algae are known from the Silurian of Gotland, and the earliest known coralline deposits date from the Ordovician, although modern forms radiated in the Cretaceous.
The Sporolithaceae tend to be more diverse in periods of high ocean temperatures; the opposite is true for the Corallinaceae. The group's diversity has closely tracked the efficiency of grazing herbivores; for instance, the appearance of parrotfish in the Eocene marked a spike in coralline diversity, and the extinction of many delicately branched forms.

Taxonomy

The group's internal taxonomy is in a state of flux; for many years, they were included in the order Cryptonemiales as the family Corallinaceae until, in 1986, they were raised to the order Corallinales. Molecular studies are proving more reliable than morphological methods in approximating relationships within the group. Recent advances in morphological classification based on skeletal ultrastructure, however, are promising. Crystal morphology within the calcified cell wall of coralline algae was found to have a high correspondence with molecular studies. These skeletal structures thus provide morphological evidence for molecular relationships within the group.
According to AlgaeBase:
  • Family Corallinaceae J.V. Lamouroux — 186 species
  • Family Cornutulaceae Korde — 8 species
  • Family Epiphytaceae Korde — 119 species
  • Family Hydrolithaceae R.A. Townsend & Huisman — 29 species
  • Family Katavellaceae Korde — 0 species
  • Family Lithophyllaceae Athanasiadis — 286 species
  • Family Ludloviaceae Tchuvashov — 0 species
  • Family Mastophoraceae R.A. Townsend & Huisman — 24 species
  • Family Porolithaceae R.A. Townsend & Huisman — 41 species
  • Family Spongitidaceae Kützing — 50 species
  • Family Tomentulaceae Korde — 4 species
  • Family incertae sedis — 17 species
According to the World Register of Marine Species:
  • Suborder Corallineae
  • Suborder Mesophyllineae
  • Family Hydrolithaceae J.E.Gray
  • Family Mastophoraceae E. Verheij, 1993
  • Family Porolithaceae Lamouroux, 1812
  • Family incertae sedis
According to ITIS:
  • Family Corallinaceae J.V.Lamouroux

    Habitat

Coralline algae are widespread in all of the world's oceans, where they often cover close to 100% of rocky substrata. Corallines live in varying depths of water, ranging from intertidal zones where they are periodically exposed to air to water depth. Some species can tolerate brackish or hypersaline waters, and only one strictly freshwater coralline species exists; Pneophyllum cetinaensis, whose ancestor lived in brackish water, and was already adapted to osmotic stress and rapid changes in water salinity and temperature. A wide range of turbidities and nutrient concentrations can be tolerated.

Biology

Fresh surfaces are generally colonized by thin crusts of coralline algae, which are replaced by thicker or branched forms during succession over the course of one to ten years. However, the transition from crusts to branched form depends on environmental conditions. Crusts may also become detached and form calcareous nodules known as Rhodoliths. Their growth may be also disrupted by local environmental factors.
While coralline algae are present in most hard substrate marine communities in photic depths, they are more common in higher latitudes and in the Mediterranean. Their ability to calcify in low light conditions makes them the some of deepest photosynthetic multicellular organisms in the ocean, having been found as deep as,< and as such a critical base of mesophotic ecological systems.

Ecology

Many corallines produce chemicals which promote the settlement of the larvae of certain herbivorous invertebrates, particularly abalone. Larval settlement is adaptive for the corallines because the herbivores remove epiphytes which might otherwise smother the crusts and preempt available light. Settlement is also important for abalone aquaculture; corallines appear to enhance larval metamorphosis and the survival of larvae through the critical settlement period. It also has significance at the community level; the presence of herbivores associated with corallines can generate patchiness in the survival of young stages of dominant seaweeds. This has been seen this in eastern Canada, and it is suspected the same phenomenon occurs on Indo-Pacific coral reefs, yet nothing is known about the herbivore enhancement role of Indo-Pacific corallines, or whether this phenomenon is important in coral reef communities.
Some coralline algae develop into thick crusts which provide microhabitat for many invertebrates. For example, off eastern Canada, Morton found juvenile sea urchins, chitons, and limpets suffer nearly 100% mortality due to fish predation unless they are protected by knobby and undercut coralline algae. This is probably an important factor affecting the distribution and grazing effects of herbivores within marine communities. Nothing is known about the microhabitat role of Indo-Pacific corallines. However, the most common species in the region, Hydrolithon onkodes, often forms an intimate relationship with the chiton Cryptoplax larvaeformis. The chiton lives in burrows it makes in H. onkodes plants, and comes out at night to graze on the surface of the coralline. This combination of grazing and burrowing results in a peculiar growth form in H. onkodes, in which the coralline produces nearly vertical, irregularly curved lamellae. Coralline algae are part of the diet of shingle urchins.
Nongeniculate corallines are of particular significance in the ecology of coral reefs, where they add calcareous material to the structure of the reef, help cement the reef together, and are important sources of primary production. Coralline algae are especially important in reef construction, as they lay down calcium carbonate as calcite. Although they contribute considerable bulk to the calcium carbonate structure of coral reefs, their more important role in most areas of the reef, is in acting as the cement which binds the reef materials into a sturdy structure.
Corallines are particularly important in constructing the algal ridge's reef framework for surf-pounded reefs in both the Atlantic and Indo-Pacific regions. Algal ridges are carbonate frameworks constructed mainly by nongeniculate coralline algae. They require high and persistent wave action to form, so develop best on windward reefs with little or no seasonal change in wind direction. Algal ridges are one of the main reef structures that prevent oceanic waves from striking adjacent coastlines, helping to prevent coastal erosion.