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

Abstract

Ocean acidification is perceived to be a major threat for many calcifying organisms, including scleractinian corals. Here we investigate (1) whether past exposure to low pH environments associated with CO2 vents could increase corals tolerance to low pH and (2) whether zooxanthellate corals are more tolerant to low pH than azooxanthellate corals. To test these hypotheses, two Mediterranean colonial corals Cladocora caespitosa (zooxanthellate) and Astroides calycularis (azooxanthellate) were collected from CO2 vents and reference sites and incubated in the laboratory under present-day (pH on the total scale, pHT 8.07) and low pH conditions (pHT 7.70). Rates of net calcification, dark respiration and photosynthesis were monitored during a six-month experiment. Monthly net calcification was assessed every 27 to 35 d using the buoyant weight technique, whereas light and dark net calcification was estimated using the alkalinity anomaly technique during 1 h incubations. Neither species showed any change in net calcification rates, respiration, and photosynthesis regardless of their environmental history, pH treatment and trophic strategy. Our results indicate that C. caespitosa and A. calycularis could tolerate future ocean acidification conditions for at least 6 months. These results will aid in predicting species' future responses to ocean acidification, and thus improve the management and conservation of Mediterranean corals.

Long-term effects of ocean acidification and its interaction with warming on calcifying organisms and their associated microbiome: bryozoans as emerging sentinels of global change?

“…In the marine realm, two of the major threats are ocean warming and ocean acidification. The combination of these two facets of global environmental change, in particular, threatens marine calcifiers, which build carbonate structures and often are habitat-forming species, such as corals (Carbonne et al, 2021; Cornwall et al, 2021). Some calcifying species may be impacted with increased skeletons/shells fragility and/or extra energetic cost for maintaining their carbonate structures.…”

“…The Ischia Island (Naples, Italy) presents shallow underwater volcanic CO 2 vents, which cause local acidification of seawater at pH levels predicted by 2100, being thus natural analogues to explore long-term responses of natural populations of marine calcifiers and their associated microbes to future ocean acidification conditions (Biagi et al, 2020; Carbonne et al, 2021; Teixidó et al, 2020). These CO 2 vent systems are also particularly valuable for studying the combined effects of warming and acidification as they are located in the Mediterranean Sea, which is warming 20% faster than the global average (Lionello and Scarascia, 2018).…”

Elevated heterotrophic capacity as a strategy for Mediterranean corals to cope with low pH at CO2 vents

“…At CO 2 vents in Papua New Guinea and Ischia, Italy, species richness, habitat complexity, and functional diversity decrease with decreasing pH along natural gradients [ 17 – 19 ], with calcifying species being most affected [ 16 – 20 ]. In Ischia, two species of coral, the symbiotic Cladocora caespitosa and the asymbiotic Astroides calycularis , have recently been found growing near CO 2 vents [ 18 , 21 ]. Although colony size is smaller at vent sites compared to nearby ambient sites due to increased bioerosion [ 10 , 18 , 21 ], these colonies continue to grow, suggesting local adaptation or acclimatization in response to low pH.…”

“…In Ischia, two species of coral, the symbiotic Cladocora caespitosa and the asymbiotic Astroides calycularis , have recently been found growing near CO 2 vents [ 18 , 21 ]. Although colony size is smaller at vent sites compared to nearby ambient sites due to increased bioerosion [ 10 , 18 , 21 ], these colonies continue to grow, suggesting local adaptation or acclimatization in response to low pH. Recent work shows that C .…”

Nutrition of Corals and Their Trophic Plasticity under Future Environmental Conditions

“…To gain metabolic energy, scleractinian corals are able to shift from heterotrophy (catching particulate food) [26,27] to autotrophy (through photosynthesis by endosymbionts) [28]. Depending on the species-specific trophic strategy [29,30], corals exhibit the ability to collect food particles (e.g. zooplankton) as a heterotrophic source of energy.…”