Throughout the world’s oceans, during the late/early to
early/late Miocene, while many phototrophs declined
globally, coralline red algae radiated and diversified
greatly. In fact, corallines have shown a long-term overall
increase in species richness throughout most of their history. Despite this, coralline species diversification was
not sustained after the Miocene (Aguirre et al., 2000)
and coralline algae suffered a series of extinction events,
each of which eliminated at least 20% of the species. In
fact, during the two largest extinction events of the late
Cretaceous and late Miocene-Pliocene, about 66% of all
coralline species were lost. Each extinction event was
followed by substantial episodes of rapid radiation and
speciation, particularly in the early Paleocene and
Pleistocene.
The conspicuous Miocene shift from coral- to
rhodolith-dominated carbonate communities has been
attributed to global climate changes. Halfar and Mutti
(2005) argued that the prevalence of rhodolith facies,
which developed under broader nutrient and temperature
ranges than did the coral-reef facies, was initially triggered
by a global enhancement of nutrient resources. In the middle Miocene, nutrient availability was apparently augmented by increased upwelling- and erosion-related
nutrient inputs into coastal ecosystems. These nutrient
increases together with declining temperatures, following
the early to middle Miocene climatic optimum, led to further expansion of rhodalgal facies. Hence, a global phase
shift occurred in nearshore shallow-water carbonate producers from corals to domination by coralline species that
were tolerant of the higher levels of nutrification. Geological deposits of coralline algae contain organic carbon
compounds such as cellulose (due to photosynthesis) as
well as massive calcareous compounds due to calcification (precipitation of inorganic CaCO 3 ). The resultant carbon stores may be among the most important in the
biosphere as neutralizers of global oceanic acidification
and as reservoirs of excess greenhouse CO 2 .
A striking geological pattern is the occurrence of an
inverse relationship in the species richness of two of the
families of Corallinales. From the Cretaceous to Pleistocene, Corallinaceae (both warm- and cold-water species)
increased, while the tropical Sporolithaceae declined. This
climatic link was supported for Sporolithaceae and
Corallinaceae by both direct and inverse correlations,
respectively, between species diversity and documented
Cretaceous to Cenozoic paleotemperatures. Although,
remaining to be more firmly established, coralline diversification since the Cretaceous (Steneck, 1983, 1985) may
have been driven by simultaneous coevolutionary
increases in herbivores (mainly limpets, echinoids, and
grazing fishes) that physically scrape and remove fleshy
algal competitors.
Distribution patterns and functional significance
Crustose (nongeniculate) coralline algae occur worldwide
from polar to tropical regions, reaching their highest
diversity in tropical reef environments. Three subgroups
of Corallinales show characteristic distributions (Aguirre
et al., 2000): (1) Sporolithoideae, low latitude/mainly
deepwater; (2) Melobesioideae, high latitude/shallow
waters
to
low
latitude/deep
waters;
and
(3) Lithophylloideae/Mastophoroideae, mid to low
latitude/shallow waters.
The abundant occurrence of crustose corallines (and
corals) in Indo-Pacific reefs was initially recognized by
Darwin (1842); however, their important role as reef
builders was first emphasized by Agassiz (1888). Most
often, corals (Cnidaria) supply the bulk building blocks;
whereas, coralline algae do much of the cementing
together of debris. Barrier, fringing, and atoll reefs are
complex ecosystems that depend on calcareous coralline
algae for the development and maintenance of waveresistant outer fronts (Figure 5). The crustose coralline
algae, which deposit a type of calcium carbonate (calcite
limestone) that is harder and denser than the aragonite of
corals, build this raised “algal ridge” (Dawson, 1961).
The intertidal outer algal-ridge habitat, in which crustose
Algae, Coralline, Figure 5 An example of a well-developed
algal ridge from windward Guam, with close-up of the
head-forming Lithophyllum kotschyanum and the pink crustose
Hydrolithon (Porolithon) onkodes.
ALGAE, CORALLINE
25
early/late Miocene, while many phototrophs declined
globally, coralline red algae radiated and diversified
greatly. In fact, corallines have shown a long-term overall
increase in species richness throughout most of their history. Despite this, coralline species diversification was
not sustained after the Miocene (Aguirre et al., 2000)
and coralline algae suffered a series of extinction events,
each of which eliminated at least 20% of the species. In
fact, during the two largest extinction events of the late
Cretaceous and late Miocene-Pliocene, about 66% of all
coralline species were lost. Each extinction event was
followed by substantial episodes of rapid radiation and
speciation, particularly in the early Paleocene and
Pleistocene.
The conspicuous Miocene shift from coral- to
rhodolith-dominated carbonate communities has been
attributed to global climate changes. Halfar and Mutti
(2005) argued that the prevalence of rhodolith facies,
which developed under broader nutrient and temperature
ranges than did the coral-reef facies, was initially triggered
by a global enhancement of nutrient resources. In the middle Miocene, nutrient availability was apparently augmented by increased upwelling- and erosion-related
nutrient inputs into coastal ecosystems. These nutrient
increases together with declining temperatures, following
the early to middle Miocene climatic optimum, led to further expansion of rhodalgal facies. Hence, a global phase
shift occurred in nearshore shallow-water carbonate producers from corals to domination by coralline species that
were tolerant of the higher levels of nutrification. Geological deposits of coralline algae contain organic carbon
compounds such as cellulose (due to photosynthesis) as
well as massive calcareous compounds due to calcification (precipitation of inorganic CaCO 3 ). The resultant carbon stores may be among the most important in the
biosphere as neutralizers of global oceanic acidification
and as reservoirs of excess greenhouse CO 2 .
A striking geological pattern is the occurrence of an
inverse relationship in the species richness of two of the
families of Corallinales. From the Cretaceous to Pleistocene, Corallinaceae (both warm- and cold-water species)
increased, while the tropical Sporolithaceae declined. This
climatic link was supported for Sporolithaceae and
Corallinaceae by both direct and inverse correlations,
respectively, between species diversity and documented
Cretaceous to Cenozoic paleotemperatures. Although,
remaining to be more firmly established, coralline diversification since the Cretaceous (Steneck, 1983, 1985) may
have been driven by simultaneous coevolutionary
increases in herbivores (mainly limpets, echinoids, and
grazing fishes) that physically scrape and remove fleshy
algal competitors.
Distribution patterns and functional significance
Crustose (nongeniculate) coralline algae occur worldwide
from polar to tropical regions, reaching their highest
diversity in tropical reef environments. Three subgroups
of Corallinales show characteristic distributions (Aguirre
et al., 2000): (1) Sporolithoideae, low latitude/mainly
deepwater; (2) Melobesioideae, high latitude/shallow
waters
to
low
latitude/deep
waters;
and
(3) Lithophylloideae/Mastophoroideae, mid to low
latitude/shallow waters.
The abundant occurrence of crustose corallines (and
corals) in Indo-Pacific reefs was initially recognized by
Darwin (1842); however, their important role as reef
builders was first emphasized by Agassiz (1888). Most
often, corals (Cnidaria) supply the bulk building blocks;
whereas, coralline algae do much of the cementing
together of debris. Barrier, fringing, and atoll reefs are
complex ecosystems that depend on calcareous coralline
algae for the development and maintenance of waveresistant outer fronts (Figure 5). The crustose coralline
algae, which deposit a type of calcium carbonate (calcite
limestone) that is harder and denser than the aragonite of
corals, build this raised “algal ridge” (Dawson, 1961).
The intertidal outer algal-ridge habitat, in which crustose
Algae, Coralline, Figure 5 An example of a well-developed
algal ridge from windward Guam, with close-up of the
head-forming Lithophyllum kotschyanum and the pink crustose
Hydrolithon (Porolithon) onkodes.
ALGAE, CORALLINE
25