Coral Calcification Under Ocean Acidification and Global Change

Jonathan Erez, Stéphanie Reynaud, Jacob Silverman, Kenneth Schneider, and Denis Allemand

Abstract Coral reefs are unique marine ecosystems that form huge morphological structures (frameworks) in today's oceans. These include coral islands (atolls), barrier reefs, and fringing reefs that form the most impressive products of CaC0

3 biomineralization. The framework builders are

mainly hermatypic corals, calcareous algae, foraminifera, and mollusks that together are responsible for almost 50% of the net annual CaCO} precipitation in the oceans. The reef ecosystem acts as a huge filtration system that ex tracts plankton from the vast fluxes of ocean water that flow through the framework. The existence of these wave resis­tant structures in spite of chemical, biological, and physical erosion depends on their exceedingly high rates of calcifica­tion. Coral mortality due to bleaching (caused by global warming) and ocean acidification caused by atmospheric CO2 increase are now the major threats to the existence of these unique ecosystems. When the rates of dissolution and erosion bec orne higher th an the rates of precipitation, the entire coral ecosystem starts to collapse and will eventually be reduced to piles of rubble while its magnificent and high diversity fauna will vanish. The loss to nature and to human­ity would be unprecedented and it may occur within the next 50 years. In this chapter, we discuss the issue of ocean acidi­fication and its major effects of corals from the cell level to the reef communities. Based on the recently published literature, it can be generalized that calcification in corals is strongly reduced when seawater become slightly acidified. Ocean acidification lowers both the pH and the CO}2- ion

J. Erez(i8J), J. Silverman, and K. Schneider Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel e-mail: erez@vms.huji.ac.il

S. Reynaud and D. Allemand Centre Scientifique de Monaco, Avenue Saint-Martin, MC-98000, Monaco, Principality of Monaco e-mail: allemand@centrescientifique.mc

J. Silverman and K. Schneider Carnegie Institute, Stanford, USA

concentration in the surface ocean, but calcification at the organism level responds mainly to CO/- and not to pH. Most reports show that the symbiotic algae are not sensitive to changes in the carbonate chemistry. The potential mec ha­nisms responsible for coral sensitivity to acidification are either direct input of seawater to the biomineralization site or high sensitivity of the enzymes involved in calcification to pH and/or CO2 concentrations. Increase in pH at the biomineralization site is most probably the most energy demanding process that is influenced by ocean acidification. While hermatypic corals and other calcifiers reduce their rates of calcification, chemical and biological dissolution increase and hence net calcification of the entire coral reef is decreasing dramatically. Community metabolism in several sites and in field enclosures show in sorne cases net dissolu­tion. Using the relations between aragonite saturation (Qarag) and community calcification, it is possible to predict that coral reefs globally may start to dissolve when atmospheric CO2 doubles.

Keywords Barrier reefs • fringing reefs • ocean acidifica­tion • enzymes • bio-fieters

1 Introduction

1.1 The Eco/ogicallmportance of Coral Calcification

Coral colonies with their myriad of calcifying polyps are responsible for producing the reef bioherm, that is, the large­scale physical structure that is the essence of coral reef func­tionality. Coral reefs function essentially as huge bio-filters, which filter out plankton from the vast amounts of seawater that flow through the reef systems. In doing so, coral reefs are obtaining nutrients and carbon from the particulate phase that supports their high gross production, their ecological

Z. Dubinsky and N. Stambler (eds.), Coral Reefs: AI! Ecosystem il! Transition, 151 DOl 10.1007/978-94-007-0114-4_10, © Springer Science+Business Media B.Y. 2011

170 J. Erez et al.

1 .~.~ t· ..... ~ ~ .t~tï~"" ..... . ~=.. i...'"\.i..... .. ., . .... - . ..--.... ~ . . ' \. .. .

~

.~ ... , l.: .. r. . ... ' k~.· . ..~'W~./ ... ..... . ...... ~

'~ .. . -. . . . . . . . , . \. .. .

% ofPIR TG gross

o 20 40 60 80 100

Fig.13 Reduction in gross community calcification of coral reefs in percentage relative ta pre industrial rates (PIR, 280 ppm) for different atmo~

spheric pCO, scenarios. The relations between community calc ification and Û ""g are based on the relations shown in Fig. 12

(Silverman et al. 2007a) or incubations over variable tropical substrates (Yates and Halley 2006) , the net rates of calcifica­tion become 0 at values of n

arag which are significantly

higher than 1. Using the relations between Gand narag

, we can calculate G for values of n above 1 for the situation of

arag

N = 0 and this should be equal to the rate of dissolution . In principle, we can also derive the relations between D and n . As an example we can use the equation given by

arag

Silverman et al. (2007a) for low nitrate conditions to calcu-

late D values for N = 0 as follows:

D = G = A8S(n _1) 3.2 arag

(where A is the proportion of live coral coyer) For A= 1 when n

arag= 1.5,2,2.5, D will be l , 10, and

30 mmol CaCO} m-2 d- I, respectively. If A is lower, it will

decrease these values accordingly. This rough calculation does not take into account the dependence of D on n

arag

which may also be important, but there is no information on it that we are aware of. It should be noted that most of this dissolution is probably occurring in sediment or rock inter­stitial water within live or dead coral reef framework sub-

strate as a result of microbial respiration and/or the metabolic

activity of the corals or other boring organisms. In this context, it is important to note that in the Moloki reef flats N=O was achieved in ail the sub-environments sam pIed

(Yates and Halley 2006). The average nar •• value for these sites was -2.5 suggesting considerably high"dissolution rates at the range of 3-6 mmol CaCO} m-2 d- I with A = 0 . 1 and

A = 0.2, respectively. Another approach to the dissolution of CaC0

3 in coral

reefs (which may yield an upper limit to the dissolution process) is to make the extreme assumption that the differ­ence between light and dark calc ification (both for individual corals and on the community level) is caused by diffe rences in n

ar," in the calcifying fluid (or seawater) at the biominer­

alization site . This fluid becomes more bas ic due to the

photosynthetic activity of the symbionts (and the rest of the primary producers on the community level) , and more acidic due to respiration of the corals and the ir assoc iated boring organisms and microbial communities. As sugges ted by Schneider and Erez (2006) this difference may be in the range of 0.3-0.4 pH units , mos t of it due to respiration.