Ocean acidification causes variable trait‐shifts in a coral species

Abstract

High pCO 2 habitats and their populations provide an unparalleled opportunity to assess how species may survive under future ocean acidification conditions, and help to reveal the traits that confer tolerance. Here we utilize a unique CO 2 vent system to study the effects of exposure to elevated pCO2 on trait-shifts observed throughout natural populations of Astroides calycularis, an azooxanthellate scleractinian coral endemic to the Mediterranean. Unexpected shifts in skeletal and growth patterns were found. Colonies shifted to a skeletal phenotype characterized by encrusting morphology, smaller size, reduced coenosarc tissue, fewer polyps, and less porous and denser skeletons at low pH. Interestingly, while individual polyps calcified more and extended faster at low pH, whole colonies found at low pH site calcified and extended their skeleton at the same rate as did those at ambient pH sites. Transcriptomic data revealed strong genetic differentiation among local populations of this warm water species whose distribution range is currently expanding northward. We found excess differentiation in the CO 2 vent population for genes central to calcification, including genes for calcium management (calmodulin, calcium-binding proteins), pH regulation (V-type proton ATPase), and inorganic carbon regulation (carbonic anhydrase). Combined, our results demonstrate how coral populations can persist in high pCO 2 environments, making this system a powerful candidate for investigating acclimatization and local adaptation of organisms to global environmental change.

Genetic and physiological traits conferring tolerance to ocean acidification in mesophotic corals

“…In fact, corals from both environments possess low tissue biomass (i.e., protein content per coral surface area), which in OA-resistant species results in a higher potential for energy production (Agostini et al, 2021) and a higher flexibility in allocating energy toward skeletal growth (Agostini et al, 2021;Wall et al, 2017). Moreover, the regulation of the expression of carbonic anhydrases seems to be a crucial mechanism to control inorganic carbon concentrations within cells in both mesophotic corals and corals from naturally acidic areas (Kenkel et al, 2018;Malik et al, 2020;Teixidó et al, 2020). The existence of such features suggests there is potential for mesophotic corals to better adjust to future OA conditions.…”

Combined responses of primary coral polyps and their algal endosymbionts to decreasing seawater pH

“…Furthermore, a recent study shows that genes related to Ca 2+ management and to inorganic carbon regulation (i.e. carbonic anhydrases) have a major role in the persistence of coral populations adapted to naturally low pH environments (34). In light of the predicted decrease of oceanic pH, the ability of corals to modulate the Ca 2+ transport and the STPCA-2 activity, especially during the delicate early life stages, constitutes a crucial area of further research, that could disclose the susceptibility of different coral species to ocean acidification.…”

Two temperate corals are tolerant to low pH regardless of previous exposure to natural CO2 vents

“…Cladocora caespitosa occurs near a CO2 vent site adjacent to rocky reefs at about 10 m depth (Chiane del Lume, hereafter Vent 1), whereas A. calycularis occurs in another CO2 vent site between 1 -2 m depth (Grotta del Mago, a semi-submerged cave hereafter Vent 2, where CO2 vents occur at 5 m depth). Vent gas compositions at Vent 1 and Vent 2 were predominantly CO2 (mean ± SE: 93.9 ± 0.6 for Vent 1, 93.4 ± 1.0 for Vent 2), with undetectable levels of hydrogen sulfide (< 0.0002 %), and did not elevate the temperature (for more information on gas measurements, see Teixidó et al 2020). The reference sites without any visible vent activity and exhibiting ambient pH were located: i) approximately 100 m west from Vent 1 for C.caespitosa (hereafter Ambient 1, 10 m depth) and ii) 1.5 km away from Vent 2 for A. calycularis (San Pancrazio, an overhang, hereafter Ambient 2, 1-2 m depth) (Fig.…”

“…The survey's depth for C. caespitosa and A. calycularis corresponded to the depth range at which colonies were sampled for the experiment (see below). Abundance and sizefrequency data for A. calycularis have been described previously in Teixidó et al (2020).…”

“…These two scleractinian corals naturally occur at two newly discovered CO2 vent systems along the coast of Ischia, Italy. In situ data obtained by Teixidó et al (2020) showed variable responses in skeletal structure and growth patterns of A. calycularis, with colonies from the CO2 vent site characterized by smaller size, fewer polyps, and less porous, denser skeletons. Thus, these two scleractinian coral species grown under elevated pCO2 environments are great model systems for assessing tolerance to acidification.…”