Could the acid–base status of Antarctic sea urchins indicate a better‐than‐expected resilience to near‐future ocean acidification?

Abstract

Increasing atmospheric carbon dioxide concentration alters the chemistry of the oceans towards more acidic conditions. Polar oceans are particularly affected due to their low temperature, low carbonate content and mixing patterns, for instance upwellings. Calcifying organisms are expected to be highly impacted by the decrease in the oceans' pH and carbonate ions concentration. In particular, sea urchins, members of the phylum Echinodermata, are hypothesized to be at risk due to their high-magnesium calcite skeleton. However, tolerance to ocean acidification in metazoans is first linked to acid-base regulation capacities of the extracellular fluids. No information on this is available to date for Antarctic echinoderms and inference from temperate and tropical studies needs support. In this study, we investigated the acid-base status of 9 species of sea urchins (3 cidaroids, 2 regular euechinoids and 4 irregular echinoids). It appears that Antarctic regular euechinoids seem equipped with similar acid-base regulation systems as tropical and temperate regular euechinoids but could rely on more passive ion transfer systems, minimizing energy requirements. Cidaroids have an acid-base status similar to that of tropical cidaroids. Therefore Antarctic cidaroids will most probably not be affected by decreasing seawater pH, the pH drop linked to ocean acidification being negligible in comparison of the naturally low pH of the coelomic fluid. Irregular echinoids might not suffer from reduced seawater pH if acidosis of the coelomic fluid pH does not occur but more data on their acid-base regulation are needed. Combining these results with the resilience of Antarctic sea urchin larvae strongly suggests that these organisms might not be the expected victims of ocean acidification. However, data on the impact of other global stressors such as temperature and of the combination of the different stressors needs to be acquired to assess the sensitivity of these organisms to global change.

Benthic species of the Kerguelen Plateau show contrasting distribution shifts in response to environmental changes

“…Physiological responses of marine species to changes in seawater temperature, salinity, and more acidic conditions have been assessed by ecophysiological experiments (Collard, De Ridder, David, Dehairs, & Dubois, 2015;Karelitz et al, 2016;Peck, Souster, & Clark, 2013;Suckling et al, 2015), which are usually confronted with practical limitations due to substantial technical and funding issues (Suckling et al, 2015). Species geographic responses to the multiple effects of climate change may include resilience, distribution range shift toward the pole, where they would find more suitable conditions, and local extinction (Doney et al, 2011;Walther et al, 2002).…”

Benthic marine calcifiers coexist with CaCO3‐undersaturated seawater worldwide

“…Consequently, it has been hypothesized that organisms with Mg-calcite structures exceeding these mole % MgCO 3 values could be the most vulnerable to ocean acidification [Morse et al, 2006;Andersson et al, 2008]. Some experimental results support this hypothesis and show that skeletal mineral solubility impacts calcifiers' relative susceptibility to ocean acidification [Kuffner et al, 2008;Martin and Gattuso, 2009] while others do not [Kroeker et al, 2010;Thomsen et al, 2010;Collard et al, 2015]. In an experiment that investigated the impact of ocean acidification on 18 species of marine calcifiers spanning a range of CaCO 3 polymorphs [Ries et al, 2009], five of the six species that exhibited net dissolution under the highest-CO 2 treatment produced a skeleton from the relatively more soluble aragonite and high Mg-calcite polymorphs of CaCO 3 , rather than from the less soluble low Mg-calcite polymorph.…”

The importance of inter‐individual variation in predicting species' responses to global change drivers

“…As shown, P. lividus , in common with other echinoids (e.g., Spicer et al, ; Spicer & Widdicombe, ; Stumpp, Trübenbach, et al, ; Catarino et al, ), develop an extracellular acidosis when seawater p CO 2 increases. This results to some extent in the increase of bicarbonate ions concentration ([HCO3 − ]) (see Farmanfarmaian, ; Miles et al, ; Stumpp, Trübenbach, et al, ; Collard et al, ; Collard, Ridder, David, Dehairs, & Dubois, ). However, the inter‐individuals approach we adopted revealed a pronounced effect of temperature on the acid–base status of P. lividus as individuals’ physiological parameters were all positively affected in urchins exposed at 20°C.…”

“…As hypometabolic and osmoconformers echinoids are considered to be particularly vulnerable to climate and global change drivers (see Dupont & Thorndyke, 2013), as also evidenced by a growing number of studies investigating the effects of OW and OA on urchin's acid-base status (e.g., Miles et al, 2007Spicer & Widdicombe, 2012), calcification (e.g., Stumpp, Trübenbach, et al, 2012), growth (e.g., Albright et al, 2012) increases. This results to some extent in the increase of bicarbonate ions concentration ([HCO3 − ]) (see Farmanfarmaian, 1966;Miles et al, 2007;Stumpp, Trübenbach, et al, 2012;Collard et al, 2013;Collard, Ridder, David, Dehairs, & Dubois, 2015). However, the inter-individuals approach we adopted revealed a pronounced effect of temperature on the acid-base status of P. lividus as individuals' physiological parameters were all positively affected in urchins exposed at 20°C.…”