Ocean warming and acidification synergistically increase coral mortality

Abstract

Organisms that accumulate calcium carbonate structures are particularly vulnerable to ocean warming (OW) and ocean acidification (OA), potentially reducing the socioeconomic benefits of ecosystems reliant on these taxa. Since rising atmospheric CO2 is responsible for global warming and increasing ocean acidity, to correctly predict how OW and OA will affect marine organisms, their possible interactive effects must be assessed. Here we investigate, in the field, the combined temperature (range: 16–26 °C) and acidification (range: pHTS 8.1–7.4) effects on mortality and growth of Mediterranean coral species transplanted, in different seasonal periods, along a natural pH gradient generated by a CO2 vent. We show a synergistic adverse effect on mortality rates (up to 60%), for solitary and colonial, symbiotic and asymbiotic corals, suggesting that high seawater temperatures may have increased their metabolic rates which, in conjunction with decreasing pH, could have led to rapid deterioration of cellular processes and performance. The net calcification rate of the symbiotic species was not affected by decreasing pH, regardless of temperature, while in the two asymbiotic species it was negatively affected by increasing acidification and temperature, suggesting that symbiotic corals may be more tolerant to increasing warming and acidifying conditions compared to asymbiotic ones.

Reproduction of an azooxanthellate coral is unaffected by ocean acidification

“…Specimens of L. pruvoti transplanted along the same p CO 2 gradient as in the present study show a decrease in net calcification rates with decreasing pH, and a significant increase in polyp mortality rate, but only when average temperatures is high 38 .…”

“…The high investment of energy that L. pruvoti places to keep the gametogenesis constant along the pH gradient may leave little energy for the coral to sustain net calcification in the face of ocean acidification 45 . Confirming this hypothesis, a parallel study on L. pruvoti transplanted along the same natural p CO 2 gradient shows decreased net calcification rate with decreasing pH 38 , even under pH values projected for the end of this century. Thus, under future pH conditions, L. pruvoti may allocate more energy to maintain constant the reproduction process, at the expense of net calcification, which significantly decreases under acidified conditions.…”

“…Along this gradient, four sampling sites were selected: the control site (site 1: mean Total Scale (TS) pH TS 8.07), located about 34 m away from the center of the crater, two intermediate pH sites (site 2 and 3: mean pH TS respectively 7.87 and 7.74), representing the intermediate and the most pessimistic IPCC scenario for the end of this century and the extreme pH site (site 4: mean pH TS 7.40), situated in proximity of the vents, representing the projection for 2500. The experimental site has stable hydrothermal–chemical properties and only p CO 2 concentration differed significantly among the four sites (see Supplementary information of  38 for detailed seawater carbonate chemistry in each transplantation site).…”

Host and Symbionts in Pocillopora damicornis Larvae Display Different Transcriptomic Responses to Ocean Acidification and Warming

“…Concurrently, the surface ocean is absorbing about one-third of anthropogenic CO 2 (Sabine et al, 2004), causing OA. Although OA and warming cause stress through different mechanisms, OA and warming may have synergistic effects for reef-building corals: increased physiological sensitivity to increased temperature and/or broader fitness-related consequences (Metzger et al, 2007;Walther et al, 2009;Pörtner et al, 2017;Prada et al, 2017).…”

Coral responses to temperature, irradiance and acidification stress: linking physiology to satellite remote sensing

“…Changes in light level received by reefs as a result of changes in cloud cover and turbidity could impact on the coral-dinoflagellate symbiotic relationship (Mumby et al 2001b). Importantly, changes in sea temperature, light levels, and ocean pH are expected to interact, synergistically enhancing the negative effects on corals of the individual stressors (Vogel et al 2015;Prada et al 2017). …”