SYNOPSIS. Photosynthesis and calcification in zooxanthellate scleractinian corals and coral reefs are reviewed at several scales: cellular (pathways and transport mechanisms of inorganic carbon and calcium), organismal (interaction between photosynthesis and calcification, effect of light) and ecosystemic (community primary production and calcification, and air-sea CO 2 exchanges).The coral host plays a major role in supplying carbon for the photosynthesis by the algal symbionts through a system similar to the carbon-concentrating mechanism described in free living algal cells. The details of carbon supply to the calcification process are almost unknown, but metabolic CO 2 seems to be a significant source. Calcium supply for calcification is diffusional through oral layers, and active membrane transport only occurs between the calicoblastic cells and the site of calcification. Photosynthesis and calcification are tightly coupled in zooxanthellate scleractinian corals and coral reef communities. Calcification is, on average, three times higher in light than in darkness. The recent suggestion that calcification is dark-repressed rather than light-enhanced is not supported by the literature. There is a very strong correlation between photosynthesis and calcification at both the organism and community levels, but the ratios of calcification to gross photosynthesis (0.6 in corals and 0.2 in reef communities) differ from unity, and from each other as a function of level.The potential effect of global climatic changes (pCO 2 and temperature) on the rate of calcification is also reviewed. In various calcifying photosynthetic organisms and communities, the rate of calcification decreases as a function of increasing pCO 2 and decreasing calcium carbonate saturation state. The calculated decrease in CaCO, production, estimated using the scenarios considered by the International Panel on Climate Change (IPCC), is 10% between 1880 and 1990, and 9-30% (mid estimate: 22%) from 1990 to 2100. Inadequate understanding of the mechanism of calcification and its interaction with photosynthesis severely limits the ability to provide an accurate prediction of future changes in the rate of calcification.

Several experiments have shown a decrease of growth and calcification of organisms at decreased pH levels but relatively few studies have focused on early life stages which are believed to be more sensitive to environmental disturbances such as hypercapnia. Here, we present experimental data demonstrating that the growth of planktonic mussel (Mytilus edulis) larvae is significantly affected by a decrease of pH to a level expected for the end of the century. Even though there was no significant effect of a 0.25–0.34 pH unit decrease on hatching and mortality rates during the first 2 days of development nor during the following 13-day period prior to settlement, final shells were, respectively, 4.5±1.3 and 6.0±2.3% smaller at pH_NBS~7.8 than at a control pH_NBS of ~8.1. Moreover, a decrease of 12.0±5.4% of shell thickness was observed. More severe impacts were found with a decrease of ~0.5 pH_NBS unit during the first 2 days of development which could be attributed to a decrease of calcification due toslight undersaturation of seawater with respect to aragonite. Indeed, important effects on both hatching and D-veliger shell growth were found. Hatching rates were 24±4% lower while D-veliger shells were 12.7±0.9% smaller at pH_NBS~7.6 than at a control pH_NBS of ~8.1. Although these results show that blue mussel larvae are still able to develop a shell in seawater undersaturated with respect to aragonite, decreases of hatching rates and shell growth suggest a negative impact of ocean acidification on the future survival of bivalve populations potentially leading to significant ecological and economical losses

The ocean and the atmosphere exchange massive amounts of carbon dioxide (CO2). The pre-industrial influx from the ocean to the atmosphere was 70.6 Gt C yr –1 , while the flux in the opposite direction was 70 Gt C yr –1 ( IPCC 2007 ). Since the Industrial Revolution an anthropogenic flux has been superimposed on the natural flux. The concentration of CO2 in the atmosphere, which remained in the range of 172–300 parts per million by volume (ppmv) over the past 800 000 years ( Lüthi et al. 2008 ), has increased during the industrial era to reach 387 ppmv in 2009. The rate of increase was about 1.0% yr –1 in the 1990s and reached 3.4% yr –1 between 2000 and 2008 ( Le Quéré et al. 2009 ). Future levels of atmospheric CO2 mostly depend on socio-economic parameters, and may reach 1071 ppmv in the year 2100 ( Plattner et al. 2001 ), corresponding to a fourfold increase since 1750. As pointed out over 50 years ago, ‘human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future’ ( Revelle and Suess 1957 ). Anthropogenic CO2 has three fates. In the years 2000 to 2008, about 29% was absorbed by the terrestrial biosphere and 26% by the ocean, while the remaining 45% remained in the atmosphere ( Le Quéré et al. 2009 ). The accumulation of CO2 in the atmosphere increases the natural greenhouse effect and generates climate changes ( IPCC 2007 ). It is estimated that the surface waters of the oceans have taken up 118 Pg C, or about 25% of the carbon generated by human activities since 1800 ( Sabine et al. 2004 ). By taking CO2 away from the atmosphere, the oceanic and terrestrial sinks mitigate climatic changes. Should their efficiency decrease, more CO2 would remain in the atmosphere, generating larger climate perturbations. This book has four main groups of chapters.

Abstract. The cold-water coral Lophelia pertusa is one of the few species able to build reef-like structures and a 3-dimensional coral framework in the deep oceans. Furthermore, deep cold-water coral bioherms are likely among the first marine ecosystems to be affected by ocean acidification. Colonies of L. pertusa were collected during a cruise in 2006 to cold-water coral bioherms of the Mingulay reef complex (Hebrides, North Atlantic). Calcium-45 labelling was conducted shortly after sample collection onboard. After this method proved to deliver reliable data, the same experimental approach was used to assess calcification rates and the effect of lowered pH during a~cruise to the Skagerrak (North Sea) in 2007. The highest calcification rates were found in youngest polyps with up to 1% d−1 new skeletal growth and average values of 0.11±0.02% d−1(±S.E.). Lowering the pH by 0.15 and 0.3 units relative to ambient pH resulted in a strong decrease in calcification by 30 and 56%, respectively. The effect of changes in pH on calcification was stronger for fast growing, young polyps (59% reduction) than for older polyps (40% reduction) which implies that skeletal growth of young and fast calcifying corallites will be influenced more negatively by ocean acidification. Nevertheless, L. pertusa revealed a positive net calcification (as indicated by 45Ca incorporation) at an aragonite saturation state (Ωa) below 1, which may indicate some adaptation to an environment that is already relatively low in Ωa compared to tropical or temperate coral bioherms.

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The effectiveness, feasibility, duration of effects, co-benefits, disbenefits, cost effectiveness and governability of four ocean-based negative emissions technologies (NETs) are assessed in comparison to eight other ocean-based measures. Their role in revising UNFCCC Parties' future Nationally Determined Contributions is discussed in the broad context of ocean-based actions for both mitigation and ecological adaptation. All measures are clustered in three policy-relevant categories (Decisive, Low Regret, Concept Stage). None of the ocean-based NETs assessed are identified as Decisive at this stage. One is Low Regret (Restoring and increasing coastal vegetation), and three are at Concept Stage, one with low to moderate potential disbenefits (Marine bioenergy with carbon capture and storage) and two with potentially high disbenefits (Enhancing open-ocean productivity and Enhancing weathering and alkalinization). Ocean-based NETs are uncertain but potentially highly effective. They have high priority for research and development.

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Abstract. The uptake of anthropogenic CO 2 by the oceans has led to a rise in the oceanic partial pressure of CO 2 , and to a decrease in pH and carbonate ion concentration. This modification of the marine carbonate system is referred to as ocean acidification. Numerous papers report the effects of ocean acidification on marine organisms and communities but few have provided details concerning full carbonate chemistry and complementary observations. Additionally, carbonate system variables are often reported in different units, calculated using different sets of dissociation constants and on different pH scales. Hence the direct comparison of experimental results has been problematic and often misleading. The need was identified to