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2 Coral Reef Ecosystem
Corals from the phylum Cnidaria necessitate light for survival and growth, with
salinities in the range 32–40 parts per thousand (‰) and seawater temperatures
ranging from 18 to 30 °C (Veron 1986; Hoegh-Guldberg 1999; Souter and Lindén
2000). Most corals are colonial organisms, consisting of thousands of individual
coral polyps (Souter and Lindén 2000), which secrete limestone/calcium carbonate
(CaCO 3 ) that functions as the skeleton and over time builds the 3D-structure of the
coral reef.
It is well documented that the calcium carbonate skeletons of corals record variations of the environmental conditions in which they live and these geochemical
records have been used extensively in paleoclimatic and paleoceanographic research
(Goreau 1977; Guzman and Jimenez 1992; Bastidas and Garcia 1999; Esslemont
1999; Fallon et al. 2002; David 2003; Runnalls and Coleman 2003; Prouty et al.
2008; Wei et al. 2009; McGregor et al. 2013; Zinke et al. 2014; Chen et al. 2015). For
instance, Li et al. (2020) stated that the ability of corals to record upwelling, riverine
runoff, and pollution from various sources depends upon the concentration of heavy
metals in the coral aragonite matrix, which precisely reveals the concentrations of
heavy metals in the marine environment.
2.2 Zooxanthellae in Corals
A symbiotic relationship exists between the coral polyp and the single celled algae
(zooxanthellae) Symbiodinium spp; which live within the cells of the polyp (Muscatine and Porter 1977; Shah 2008; Pogoreutz et al. 2017; Bielmyer-Fraser et al. 2018)
and are in extremely high densities (greater than 10
6 cm
−2 ) (Muscatine and Porter
1977; Allemand and Osborn 2019). This mutualistic symbiosis between the coral
polyp and Symbiodinium spp is highly complex (Belda-Baillie et al. 2002) and forms
both the trophic and structural foundation of coral reef ecosystems (Muscatine 1990;
Ganot et al. 2011; Hume et al. 2015). The success of this symbiosis is evident in the
geological persistence of coral reefs of more than 200 million years (Veron 1995)
and the geographical area occupied by modern coral reefs in tropical and subtropical
waters of over 280,000 km
2 (Spalding et al. 2001).
The photosynthetic symbionts (zooxanthallae) fix large quantities of carbon
dioxide thus producing reduced organic carbon which is translocated to the host
as mobile compounds (photosynthates) such as glycerol, glucose, lipids and amino
acids (Venn et al. 2008; Ganot et al. 2011; Tremblay et al. 2012; Muller-Parker
et al. 2015). These compounds provide the coral with energy for respiration, growth,
reproduction and the deposition of its calcium carbonate skeleton (Muscatine et al.
1983; Muscatine 1990; Lesser 2004; Tremblay et al. 2012).
In return, the zooxanthellae are nourished with nutrients such as inorganic
nitrogen, phosphorus and inorganic carbon (Venn et al. 2008; Ganot et al. 2011)
and gets shelter. The coral-zooxanthellae symbiosis is very sensitive to increases in
temperature however; and changes of as little as 1 °C above the average summer
2 Coral Reef Ecosystem
Corals from the phylum Cnidaria necessitate light for survival and growth, with
salinities in the range 32–40 parts per thousand (‰) and seawater temperatures
ranging from 18 to 30 °C (Veron 1986; Hoegh-Guldberg 1999; Souter and Lindén
2000). Most corals are colonial organisms, consisting of thousands of individual
coral polyps (Souter and Lindén 2000), which secrete limestone/calcium carbonate
(CaCO 3 ) that functions as the skeleton and over time builds the 3D-structure of the
coral reef.
It is well documented that the calcium carbonate skeletons of corals record variations of the environmental conditions in which they live and these geochemical
records have been used extensively in paleoclimatic and paleoceanographic research
(Goreau 1977; Guzman and Jimenez 1992; Bastidas and Garcia 1999; Esslemont
1999; Fallon et al. 2002; David 2003; Runnalls and Coleman 2003; Prouty et al.
2008; Wei et al. 2009; McGregor et al. 2013; Zinke et al. 2014; Chen et al. 2015). For
instance, Li et al. (2020) stated that the ability of corals to record upwelling, riverine
runoff, and pollution from various sources depends upon the concentration of heavy
metals in the coral aragonite matrix, which precisely reveals the concentrations of
heavy metals in the marine environment.
2.2 Zooxanthellae in Corals
A symbiotic relationship exists between the coral polyp and the single celled algae
(zooxanthellae) Symbiodinium spp; which live within the cells of the polyp (Muscatine and Porter 1977; Shah 2008; Pogoreutz et al. 2017; Bielmyer-Fraser et al. 2018)
and are in extremely high densities (greater than 10
6 cm
−2 ) (Muscatine and Porter
1977; Allemand and Osborn 2019). This mutualistic symbiosis between the coral
polyp and Symbiodinium spp is highly complex (Belda-Baillie et al. 2002) and forms
both the trophic and structural foundation of coral reef ecosystems (Muscatine 1990;
Ganot et al. 2011; Hume et al. 2015). The success of this symbiosis is evident in the
geological persistence of coral reefs of more than 200 million years (Veron 1995)
and the geographical area occupied by modern coral reefs in tropical and subtropical
waters of over 280,000 km
2 (Spalding et al. 2001).
The photosynthetic symbionts (zooxanthallae) fix large quantities of carbon
dioxide thus producing reduced organic carbon which is translocated to the host
as mobile compounds (photosynthates) such as glycerol, glucose, lipids and amino
acids (Venn et al. 2008; Ganot et al. 2011; Tremblay et al. 2012; Muller-Parker
et al. 2015). These compounds provide the coral with energy for respiration, growth,
reproduction and the deposition of its calcium carbonate skeleton (Muscatine et al.
1983; Muscatine 1990; Lesser 2004; Tremblay et al. 2012).
In return, the zooxanthellae are nourished with nutrients such as inorganic
nitrogen, phosphorus and inorganic carbon (Venn et al. 2008; Ganot et al. 2011)
and gets shelter. The coral-zooxanthellae symbiosis is very sensitive to increases in
temperature however; and changes of as little as 1 °C above the average summer