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Responses of corals to seawater acidification have been extensively studied. Sensitivity varies widely between species, highlighting the need to avoid extrapolation from one to another to get an accurate understanding of coral community responses. We tested the responses of seven coral species (Acropora cytherea, Acropora hyacinthus, Acropora pulchra, Leptastrea pruinosa, Montipora grisea, Pavona cactus, Pocillopora verrucosa) from the Mo'orea lagoon to a 48-day exposure to three pH scenarios (pH 7.95, 7.7 and 7.3). Tissue necrosis, mortality, growth rates, photophysiological performances and colour index were recorded. Few significant differences were noted between pH 7.95 and 7.7, but species-specific responses were observed at pH 7.3. While our data do not allow identification of the mechanisms behind this diversity in response between species inhabiting the same environment, it can exclude several hypotheses such as local adaptation, skeletal type, corallum morphology or calcification rate as sole factors determining coral sensitivity to pH.

This study assesses whether secondary calcification is driven by a contrasting seasonal pattern (rainy vs dry) that occurs in the Eastern Tropical Pacific (ETP). Secondary calcifiers net calcification rates and coverage were measured in two reefs: the semi-enclosed Bahía Tiburón reef (BT [21°52′30 \"N, 105°54/54 \"W]) and the open Las Monas fringing reef (LM [21°51ʹ00ʹʹN, 105°52ʹ45ʹʹW]). Measurements were made from 2013 to 2016 using Calcification Accretion Units (CAUs). Seawater temperature, illuminance, pCO2, pH, ΩCa, and ΩAr were also measured. Low means of pCO2, and high means of ΩCa and ΩAr, were measured during the rainy season. At Las Monas, the composition of the calcifier community differed between seasons. A seasonal effect on net calcification was recorded in the semi-enclosed reef and in the exposed microhabitat of both reefs. Overall, net calcification (mean ± SD) was 1.17 ± 1.13 g·CaCO3·m−2·day−1. Calcification in the open fringing reef (1.51 ± 1.32 g·CaCO3·m−2·day−1) was almost double that in the semi-enclosed reef (0.83 ± 0.78 g·CaCO3·m−2·day−1). Calcification also decreased dramatically between 2014 (1.57 g·CaCO3·m−2·day−1) and 2016 (0.99 g·CaCO3·m−2·day−1). The ENSO event of 2015 raised the water temperature almost 1 °C above the decadal average, which led to a mass coral bleaching in both reefs. That thermal stress might explain the calcification decline in 2015–2016, but probably also obscured a clearer seasonal pattern in net calcification. This study is the first to show that anomalous and persistent high seawater temperatures can affect carbonate production by secondary calcifiers.

Reef encrusting calcifiers (non-scleractinian species) constitute assemblages that participate in the carbon cycle at coral reefs. Despite their apparent secondary role in building the reef framework, they contribute to the reef consolidation binding sediments and inducing larval recruitment from other epilithic invertebrates. The contribution of encrusting calcifiers on reef accretion was examined by the assessment of their rate of carbonate deposition on four different simulated reef microhabitats using calcification accretion units (CAUs) during 12 months at Playa Las Gatas and Islote Zacatoso, two coral communities from the coast of the Mexican Pacific. Encrusting calcifiers from Playa Las Gatas, the most impacted site, showed a rate of carbonate deposition (mean ± SD) four times higher than at Islote Zacatoso (10.02 ± 3.22 g CaCO3 m−2 d−1 vs 2.48 ± 1.01 g CaCO3 m−2 d−1). Overall, the rate of carbonate deposition on surfaces protected from sedimentation and light was up to 1.8 times higher than on exposed ones (11.40 ± 4.35 g CaCO3 m−2 d−1 vs 6.18 ± 3.13 g CaCO3 m−2 d−1). Carbonate deposition by calcareous algae was higher on the well-lit exposed surfaces while filter-feeding invertebrates showed the major contribution on the shaded cryptic surfaces. Although rate of carbonate deposition by encrusting calcifiers seems to be lower than hermatypic corals, it seems to be relevant on coral reefs affected by anthropogenic impacts where coral calcification is low. Under global demise of coral reefs by environmental degradation and climate change, encrusting calcifiers may become relevant for the process of carbonate deposition.

Reef-dwelling calcifiers face numerous environmental stresses associated with anthropogenic carbon dioxide emissions, including ocean acidification and warming. Photosymbiont-bearing calcifiers, such as large benthic foraminifera, are particularly sensitive to climate change. To gain insight into their responses to near-future conditions, Amphistegina lobifera from the Gulf of Aqaba were cultured under three pCO2 conditions (492, 963, 3182 ppm) crossed with two temperature conditions (28 °C, 31 °C) for two months. Differential protein abundances in host and photosymbionts were investigated alongside physiological responses and microenvironmental pH gradients assessed via proton microsensors. Over 1000 proteins were identified, of which > 15% varied significantly between treatments. Thermal stress predominantly reduced protein abundances, and holobiont growth. Elevated pCO2 caused only minor proteomic alterations and color changes. Notably, pH at the test surface decreased with increasing pCO2 under all light/dark and temperature combinations. However, the difference between [H+] at the test surface and [H+] in the seawater—a measure of the organism’s mitigation of the acidified conditions—increased with light and pCO2. Combined stressors resulted in reduced pore sizes and increased microenvironmental pH gradients, indicating acclimative mechanisms that support calcite test production and/or preservation under climate change. Substantial proteomic variations at moderate-pCO2 and 31 °C and putative decreases in test stability at high-pCO2 and 31 °C indicate cellular modifications and impacts on calcification, in contrast to the LBFs’ apparently stable overall physiological performance. Our experiment shows that the effects of climate change can be missed when stressors are assessed in isolation, and that physiological responses should be assessed across organismal levels to make more meaningful inferences about the fate of reef calcifiers.

Coralline algae provide important ecosystem services. In situ observations of how they respond to different environmental conditions can help us to understand i) their ability to adapt to their local environment and ii) their capacity to acclimatize to a novel thermal regime. Here, individuals of the tropical coralline algae, Lithophyllum kotschyanum, were translocated on a coral reef from thermally stable areas to areas characterised by natural temperature variability. Changes in their photosynthetic efficiency were determined using pulse amplitude modulation (PAM) chlorophyll fluorescence. Despite an initial stress response, algae exposed to increases in thermal variation recovered within 24h, indicating a rapid, short-term acclimatisation capacity. Algae naturally inhabiting thermally variable areas of the reef showed no change in photosynthetic efficiency throughout the study suggesting longer-term adaptation to living in a variable environment also occurs. However, analysis indicated that coralline algae living in thermally stable reef areas were abundant and marginally larger, suggesting physiological trade-offs are used to survive in variable environments. Thus, our results suggest that while coralline algae can survive in environmentally variable conditions, there may be structural and ecosystem costs.