**1. Introduction**

74 Biodiversity Loss in a Changing Planet

McFarlane GA, King JR & Beamish RJ. (2000) Have there been recent changes in climate?

Tsonis AA, Swanson K & Kravtsov S. (2007) A new dynamical mechanism for major climate

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2007GL030288, ISSN 0094-8276

Ask the fish. *Progress in Oceanography,* Vol. 47, No. 2, pp. 147-169, ISSN 0097-

shifts. *Geophysical Research Letters*, Vol. 34, No. 13, L13705, doi: 10.1029/

Loss of marine biodiversity seems inevitable in the 21st century. In benthic marine systems, survivors will have to acclimatize to seawater constantly increasing in temperature and evolving chemically, while also needing to out-compete new opportunistic neighbors and possibly facing increased predation pressure. Last but not the least, with a metabolism already pushed to its limits, survivors will have to fight against emerging diseases.

Recent studies have shown that thermal stresses on coral reef scleractinians (Vega Thurber et al., 2009), sponges (Webster et al., 2008) and coralline algae (Webster et al., 2010a) induce changes from a balanced, functional associated microflora to a pathogen-dominated one, causing disease before the physiological tolerance limits of the hosts are reached. Changes in water chemistry (loss of bioavailable calcium carbonate due to acidification) and other stress factors (temperature, salinity, oxygen, sedimentation, etc., due to the greenhouse effect or to its climatic consequences) will affect biodiversity and community structure, and in the long term induce disaggregation of limestone scaffolds.

The first section of this chapter is devoted to a presentation of the mechanisms involved in the climate-driven loss of coral reef biodiversity predicted within the next few decades in response to increasing anthropogenic pressure. Most of the arguments developed in the following sections reflect recent work published on reef-building corals, sponges and algae and their associated macro- and micro- biota, as major reef "engineers" (Wild et al., 2011). *Biodiversity* and *chemodiversity* have always been linked in the history of our planet. Both have undergone explosively creative periods, and at other times suffered dramatic losses or even extinctions followed by the emergence of better-adapted forms of life. Coral reefs have existed for many millions of years and are no exception to this. The final part of this chapter is a reflection on how a few generations of humans have been able to overexploit the planet's biodiversity for their own immediate benefit, and harm it by producing and disseminating freak molecules and genomes for which the ocean is the final depository. The threat to coral reefs comes more from effluents of highly industrialized nations than from the daily activities of low-revenue populations living on site (Donner & Potere, 2007). We now need to apply our creativity or *"intello-diversity*" to preserving existing natural equilibriums to make the planet safe for future generations.

Coral Reef Biodiversity in the Face of Climatic Changes 77




Bleaching has been defined as the loss of integrity of the photosymbiont – host relationship (hermatypic corals and some sponges) or the loss of photosynthetic pigments by the photosymbiont (zooxanthellae). Following the first large-scale bleaching events, Brown (1997) classified the causes of bleaching in corals as (i) elevated/decreased seawater temperature, (ii) solar irradiation, (iii) reduced salinity, and (iv) microbial infection. Longterm bleaching leading to mortality of entire expanses of shallow-water reefs was clearly identified as pathological in contrast to short-term episodes of occasional bleaching that allow corals to renew their resident zooxanthellae with better adapted *clades* (Suggett & Smith, 2011), some of which, e.g. *Symbiodinium* clade D may be regarded as indicators of habitat degradation more than agents of adaptation to warming (Stat & Gates, 2011). Research over the last decade has benefited from two major analytical developments: (i) functional genomics and transcriptomics that allow exploration of stress responses at cellular and whole-organism levels (Reitzel et al., 2008), and (ii) microbial metagenomics that allow culture-independent comparative analyses of bacterial and viral (Vega Thurber et al., 2008) profiles of impacted vs. healthy organisms (Vega Thurber et al., 2009), based on robust database on the former (e.g. Wegley et al., 2007). Various scenarios have been proposed to account for coral bleaching, leading to debate as to the respective importance of causative factors of mortality of corals (Bourne et al., 2009; Leggatt et al., 2007; Rosenberg et al., 2007; Rosenberg et al. 2007b), while the functional importance of bacteria in essential coral life processes is emerging from multiple examples (Mouchka et al., 2010) that also

Decalcification is the decrease or loss of the ability of marine invertebrates and of calcifying algae and plankton to perform accretion of calcium into adapted and functional skeletal

**2.2.2 Man has colonized most reef environments, denaturing them in the process**  Human influence on coral reefs is enormous, multifaceted and expanding at a fast rate. Apart from the generation of gases producing the greenhouse effect, "contact" influences result from (i) natural landscape remodeling, (ii) industrial dumping, and (iii) household pollution. All have direct and readily observable effects on marine biota, with alien molecules killing sensitive species and microbial pathogens plaguing entire populations to

scleractinian corals, coralline algae, foraminiferans, and calcifying sponges,

by wave action and by osmotic damage to polyps by abundant rainfall,

diminishing the chances of recovery of coral colonies.

**2.3 Bleaching of shallow water photosymbiotic systems** 

reveal their evolutionary significance (Fraune et al., 2010).

**2.4 Decalcification of reef-structuring biomineralizers** 

extinction.
