End of the Century pCO2 Levels Do Not Impact Calcification in Mediterranean Cold-Water Corals

Abstract

Ocean acidification caused by anthropogenic uptake of CO2 is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO2 (pCO2) that ranged between 380 µatm for present-day conditions and 930 µatm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO2 levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO2 investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO2. An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO2 level projected at the end of the century.

Resistance of Two Mediterranean Cold-Water Coral Species to Low-pH Conditions

“…No significant differences between treatments were observed among the different sampling times for both species (ANOVA, F 4,18 = 0.110, p = 0.739 for L. pertusa and F 4,30 = 0.138, p = 0.406 for M. oculata; Figure 1a,b). Our results are in accordance with two previous mid-term studies assessing the effects of OA on these same species, where no differences were observed after 6 months in L. pertusa [13] or 9 months of exposure in L. pertusa and M. oculata [21]. This suggests that, at least at mid-term, CWC are able to counteract the a priori more negative environment that lowered-pH oceans should create for calcifying organisms.…”

“…These growth rates are comparable to those previously measured on the same CWC species using different techniques [19][20][21][22]. However, the average growth rate of L. pertusa observed in the experiment of Form and Riebesell [13] was about one order of magnitude lower than in previous works, probably due to the lower incubation temperature used by these authors (7.5 °C) compared to other studies (between 10 and 13 °C).…”

“…This suggests that, at least at mid-term, CWC are able to counteract the a priori more negative environment that lowered-pH oceans should create for calcifying organisms. Regarding possible effects of time on calcification, in contrast with changes observed by Maier et al [21], no significant differences were detected neither for L. pertusa (ANOVA, However, it is important to remark that a different growth pattern was observed between both species depending on the initial size of the nubbins. While all specimens of M. oculata showed similar growth rates (calcification rates measured for the whole duration of the experiment), a correlation between the calcification rate and the initial weight of the fragments was observed in L. pertusa (Figure 1c), where smaller nubbins exhibited greater calcification rates than those with a higher initial weight.…”

“…For instance, the first short-term studies based on short incubations (between 24 h and 1 week-long) conducted on specimens of Lophelia pertusa and Madrepora oculata, found a reduction in the calcification rate ranging from 30% to 56% when the pH was dropped between 0.1 and 0.3 units [13,19,20]. On the contrary, in the most recent short-term (between 24 h and 21 days) and in the two medium-term experiments (between 6 months and 9 months) conducted to date with the same species, no effects were observed when rearing corals at pH values similar to those expected by the end of the century [13,[20][21][22]. Thus, despite the different results observed in the first short-term experiments, the outcomes so far from medium-term experiments evidence that some CWC species could be well adapted to possible changes in the chemical conditions of seawater.…”

Intra-Specific Variation Reveals Potential for Adaptation to Ocean Acidification in a Cold-Water Coral from the Gulf of Mexico

“…Moreover, these species naturally exhibit slow growth and calcification rates, which range between ∼2 to 9 mm year −1 (Brooke and Young, 2009;Larcom et al, 2014;Lartaud et al, 2014) making them particularly vulnerable to the effects of future ocean acidification. Nevertheless, recent evidence from experimental studies suggests that cold-water corals can be resistant to low aragonite saturation states, and may even be able to calcify in undersaturated conditions (Form and Riebesell, 2012;Maier et al, 2013;Hennige et al, 2015), though at a reduced rate (Maier et al, 2009;Lunden et al, 2014a). The ability to maintain calcification under lowpH conditions has been explained by the capacity of cold-water corals to upregulate the pH of their internal calcifying fluid by up to 0.8 units (McCulloch et al, 2012;Wall et al, 2015).…”