Modelling coral polyp calcification in relation to ocean acidification

S Hohn,A Merico

doi:10.5194/bg-9-4441-2012

Abstract

Abstract. Rising atmospheric CO2 concentrations due to anthropogenic emissions induce changes in the carbonate chemistry of the oceans and, ultimately, a drop in ocean pH. This acidification process can harm calcifying organisms like coccolithophores, molluscs, echinoderms, and corals. It is expected that ocean acidification in combination with other anthropogenic stressors will cause a severe decline in coral abundance by the end of this century, with associated disastrous effects on reef ecosystems. Despite the growing importance of the topic, little progress has been made with respect to modelling the impact of acidification on coral calcification. Here we present a model for a coral polyp that simulates the carbonate system in four different compartments: the seawater, the polyp tissue, the coelenteron, and the calcifying fluid. Precipitation of calcium carbonate takes place in the metabolically controlled calcifying fluid beneath the polyp tissue. The model is adjusted to a state of activity as observed by direct microsensor measurements in the calcifying fluid. We find that a transport mechanism for bicarbonate is required to supplement carbon into the calcifying fluid because CO2 diffusion alone is not sufficient to sustain the observed calcification rates. Simulated CO2 perturbation experiments reveal decreasing calcification rates under elevated pCO2 despite the strong metabolic control of the calcifying fluid. Diffusion of CO2 through the tissue into the calcifying fluid increases with increasing seawater pCO2, leading to decreased aragonite saturation in the calcifying fluid. Our modelling study provides important insights into the complexity of the calcification process at the organism level and helps to quantify the effect of ocean acidification on corals.

Highlights

Rising atmospheric CO2 concentrations due to fossil fuel emissions and land use changes are well known perturbations to environmental scientists as well as to the general public (IPCC, 2007)
Even though tropical surface waters will not become undersaturated with respect to aragonite or calcite within the 100 yr (Orr et al, 2005; Hoegh-Guldberg et al, 2007), the changes in carbonate chemistry are expected to cause a decrease in coral calcification rates of up to 30%, most probably turning many coral reefs into non-reef coral communities with zero or even negative calcium carbonate accumulation (Kleypas et al, 2001)
Since CO2 produced in the tissue by dark respiration diffuses into the calcifying fluid, a net increase in total dissolved inorganic carbon (DIC) is observed in this compartment during the dark phase (Fig. 5d)

Summary

Introduction

Rising atmospheric CO2 concentrations due to fossil fuel emissions and land use changes are well known perturbations to environmental scientists as well as to the general public (IPCC, 2007). CO2 reacts with water to produce carbonic acid that further dissociates by releasing hydrogen ions ( called protons in the following). This causes a drop in ocean pH, a process termed “ocean acidification” (OA) (Caldeira and Wickett, 2003). How these changes in seawater chemistry affect the life of marine organisms is a matter of debate (Ridgwell et al, 2009; Ries et al, 2009). Even though tropical surface waters will not become undersaturated with respect to aragonite or calcite within the 100 yr (Orr et al, 2005; Hoegh-Guldberg et al, 2007), the changes in carbonate chemistry are expected to cause a decrease in coral calcification rates of up to 30%, most probably turning many coral reefs into non-reef coral communities with zero or even negative calcium carbonate accumulation (Kleypas et al, 2001)

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Conclusion