**2. Global chemistry and coral reef biostructures**

### **2.1 Healthy coral reefs are efficient ecosystems**

Coral reefs are marine structures made of biogenic calcium carbonate mineralized from dissolved bicarbonate and calcium ions. Scleractinian corals are the largest contributors to the formation of huge and solid limestone scaffolds that are home to countless communities of marine benthos and fish. Often equated to primary tropical rainforests in terms of overall biodiversity, coral reefs concentrate an estimated quarter of all identified marine species in only about 0.1% of the total oceanic surface. Restricted to pan-tropical and subtropical zones where shallow waters are warm enough throughout the year to allow photosynthesisassisted biomineralization to occur, reef-forming scleractinian corals thrive in clear nutrientlimited waters. Optimal efficiency in nutrient cycling is achieved when overall equilibrium between all trophic levels, i.e. from higher predators to microbes, is reached. The higher the biodiversity, the more efficient the carbon and nitrogen fluxes, and the lower the probability of epidemics of bacterial pathogens or of predators upsetting the equilibrium between compartments. Azam & Malfatti (2007) stress on the importance of studying the nano and micro scale microbial structuring of marine ecosystems in order to better understand food webs and biogeoclimatic cycling. Coral reefs are of various types in relation to their location relative to land (fringing reefs, lagoon reefs, barrier reefs) or to geology (remote atoll reefs). Coral reef biodiversity hotspots occur where optimal equilibrium benefits from landassociated ecosystems and the fluxes they generate. More generally, the association of coral reefs with coastal ecosystems is both beneficial and also a source of problems when the above equilibrium is broken (Dinsdale et al., 2008). Like forests, reefs represent a substantial source of revenue to humans in goods and services (Moberg & Folke, 1999), estimated at a global 29.8 billion dollars per year in 2003 (Cesar et al., 2003), but this revenue is already being heavily compromised due to overexploitation (Cesar, 2002; Lough, 2008). The preservation of coral reef biodiversity is becoming a central ecological concern, and an economic issue to hundreds of millions of islanders and coast-dwellers living off the associated resources.

#### **2.2 Climate change and human interference affect natural equilibria**

Production of carbon dioxide and other greenhouse effect promoters, like methane and nitrogen oxide, together with water vapor, contribute to shielding nocturnal infra-red reemissions that are necessary for the cooling of surface seawater and land. This phenomenon, known as the greenhouse effect or global warming, is generated by natural phenomena and by human activities. Natural production of greenhouse gases (by volcanism in particular) can usually be buffered by carbon and nitrogen fixing organisms, mainly bacteria, phytoplankton and forest trees, albeit at the expense of temporary and localized loss of biodiversity, as witnessed by coral skeleton biomarkers. Atmospheric enrichment of greenhouse gases by agricultural and industrial practices is accelerating at an alarming rate and its effects are now perceivable and measurable, but there is currently no means of accurately predicting how much reef biodiversity will be lost in 20, 50 or 100 years from now. In all of the proposed scenarios, estimates of recovery of existing taxa vs. replacement by more resistant or more adaptable taxa are highly speculative.

#### **2.2.1 Greenhouse effects on coral reef biota**

The shielding effect of greenhouse gases leads to three categories of damage to coral reefs:

Coral reefs are marine structures made of biogenic calcium carbonate mineralized from dissolved bicarbonate and calcium ions. Scleractinian corals are the largest contributors to the formation of huge and solid limestone scaffolds that are home to countless communities of marine benthos and fish. Often equated to primary tropical rainforests in terms of overall biodiversity, coral reefs concentrate an estimated quarter of all identified marine species in only about 0.1% of the total oceanic surface. Restricted to pan-tropical and subtropical zones where shallow waters are warm enough throughout the year to allow photosynthesisassisted biomineralization to occur, reef-forming scleractinian corals thrive in clear nutrientlimited waters. Optimal efficiency in nutrient cycling is achieved when overall equilibrium between all trophic levels, i.e. from higher predators to microbes, is reached. The higher the biodiversity, the more efficient the carbon and nitrogen fluxes, and the lower the probability of epidemics of bacterial pathogens or of predators upsetting the equilibrium between compartments. Azam & Malfatti (2007) stress on the importance of studying the nano and micro scale microbial structuring of marine ecosystems in order to better understand food webs and biogeoclimatic cycling. Coral reefs are of various types in relation to their location relative to land (fringing reefs, lagoon reefs, barrier reefs) or to geology (remote atoll reefs). Coral reef biodiversity hotspots occur where optimal equilibrium benefits from landassociated ecosystems and the fluxes they generate. More generally, the association of coral reefs with coastal ecosystems is both beneficial and also a source of problems when the above equilibrium is broken (Dinsdale et al., 2008). Like forests, reefs represent a substantial source of revenue to humans in goods and services (Moberg & Folke, 1999), estimated at a global 29.8 billion dollars per year in 2003 (Cesar et al., 2003), but this revenue is already being heavily compromised due to overexploitation (Cesar, 2002; Lough, 2008). The preservation of coral reef biodiversity is becoming a central ecological concern, and an economic issue to hundreds of millions of islanders and coast-dwellers living off the

**2. Global chemistry and coral reef biostructures** 

**2.2 Climate change and human interference affect natural equilibria** 

by more resistant or more adaptable taxa are highly speculative.

**2.2.1 Greenhouse effects on coral reef biota** 

Production of carbon dioxide and other greenhouse effect promoters, like methane and nitrogen oxide, together with water vapor, contribute to shielding nocturnal infra-red reemissions that are necessary for the cooling of surface seawater and land. This phenomenon, known as the greenhouse effect or global warming, is generated by natural phenomena and by human activities. Natural production of greenhouse gases (by volcanism in particular) can usually be buffered by carbon and nitrogen fixing organisms, mainly bacteria, phytoplankton and forest trees, albeit at the expense of temporary and localized loss of biodiversity, as witnessed by coral skeleton biomarkers. Atmospheric enrichment of greenhouse gases by agricultural and industrial practices is accelerating at an alarming rate and its effects are now perceivable and measurable, but there is currently no means of accurately predicting how much reef biodiversity will be lost in 20, 50 or 100 years from now. In all of the proposed scenarios, estimates of recovery of existing taxa vs. replacement

The shielding effect of greenhouse gases leads to three categories of damage to coral reefs:

**2.1 Healthy coral reefs are efficient ecosystems** 

associated resources.

