Annual response of two Mediterranean azooxanthellate temperate corals to low-pH and high-temperature conditions

Ocean acidification (OA) and warming related to the anthropogenic increase in atmospheric CO2 have been shown to have detrimental effects on several marine organisms, especially those with calcium carbonate structures such as corals. In this study, we evaluate the response of two Mediterranean shallow-water azooxanthellate corals to the projected pH and seawater temperature (ST) scenarios for the end of this century. The colonial coral Astroides calycularis and the solitary Leptopsammia pruvoti were grown in aquaria over a year under two fixed pH conditions, control (8.05 pHT units) and low (7.72 pHT units), and simulating two annual ST cycles, natural and high (+3 °C). The organic matter (OM), lipid and protein content of the tissue and the skeletal microdensity of A. calycularis were not affected by the stress conditions (low pH, high ST), but the species exhibited a mean 25 % decrease in calcification rate at high-ST conditions at the end of the warm period and a mean 10 % increase in skeletal porosity under the acidified treatment after a full year cycle. Conversely, an absence of effects on calcification and skeletal microdensity of L. pruvoti exposed to low-pH and high-ST treatments contrasted with a significant decrease in the OM, lipid and protein content of the tissue at high-ST conditions and a 13 % mean increase in the skeletal porosity under low-pH conditions following a full year of exposure. This species-specific response suggests that different internal self-regulation strategies for energy reallocation may allow certain shallow-water azooxanthellate corals to cope more successfully than others with global environmental changes.

Impacts of groundwater discharge at Myora Springs (North Stradbroke Island, Australia) on the phenolic metabolism of eelgrass, Zostera muelleri, and grazing by the juvenile rabbitfish, Siganus fuscescens

Myora Springs is one of many groundwater discharge sites on North Stradbroke Island (Queensland, Australia). Here spring waters emerge from wetland forests to join Moreton Bay, mixing with seawater over seagrass meadows dominated by eelgrass, Zostera muelleri. We sought to determine how low pH / high CO2 conditions near the spring affect these plants and their interactions with the black rabbitfish (Siganus fuscescens), a co-occurring grazer. In paired-choice feeding trials S. fuscescens preferentially consumed Z. muelleri shoots collected nearest to Myora Springs. Proximity to the spring did not significantly alter the carbon and nitrogen contents of seagrass tissues but did result in the extraordinary loss of soluble phenolics, including Folin-reactive phenolics, condensed tannins, and phenolic acids by ?87%. Conversely, seagrass lignin contents were, in this and related experiments, unaffected or increased, suggesting a shift in secondary metabolism away from the production of soluble, but not insoluble, (poly)phenolics. We suggest that groundwater discharge sites such as Myora Springs, and other sites characterized by low pH, are likely to be popular feeding grounds for seagrass grazers seeking to reduce their exposure to soluble phenolics.

Effects of Ocean Acidification and Warming on Sperm Activity and Early Life Stages of the Mediterranean Mussel (Mytilus galloprovincialis)

Larval stages are among those most vulnerable to ocean acidification (OA). Projected atmospheric CO2 levels for the end of this century may lead to negative impacts on communities dominated by calcifying taxa with planktonic life stages. We exposed Mediterranean mussel (Mytilus galloprovincialis) sperm and early life stages to pHT levels of 8.0 (current pH) and 7.6 (2100 level) by manipulating pCO2 level (380 and 1000 ppm). Sperm activity was examined at ambient temperatures (16-17 °C) using individual males as replicates. We also assessed the effects of temperature (ambient and = 20 °C) and pH on larval size, survival, respiration and calcification of late trochophore/early D-veliger stages using a cross-factorial design. Increased pCO2 had a negative effect on the percentage of motile sperm (mean response ratio R= 71%) and sperm swimming speed (R= 74%), possibly indicating reduced fertilization capacity of sperm in low concentrations. Increased temperature had a more prominent effect on larval stages than pCO2, reducing performance (RSize = 90% and RSurvival = 70%) and increasing energy demand (RRespiration = 429%). We observed no significant interactions between pCO2 and temperature. Our results suggest that increasing temperature might have a larger impact on very early larval stages of M. galloprovincialis than OA at levels predicted for the end of the century.

Acid-base regulation in the Dungeness crab (Metacarcinus magister)

Homeostatic regulation allows organisms to secure basic physiological processes in a varying environment. To counteract fluctuations in ambient carbonate system speciation due to elevated seawater pCO2 (hypercapnia), many aquatic crustaceans excrete/accumulate acid-base equivalents through their gills; however, not much is known about the role of ammonia in this response. The present study investigated the effects of hypercapnia on acid-base and ammonia regulation in the Dungeness crab, Metacarcinus magister on the whole animal and isolated gill levels. Hemolymph pCO2 and [HCO3]- increased in M. magister acclimated to elevated pCO2 (330 Pa), while pH remained stable. Additionally, hemolymph [Na+], [Ca2+], and [SO4]2- were significantly increased. When challenged with varying pH during gill perfusion, the pH of the artificial hemolymph remained relatively unchanged. Overall, ammonia production and excretion, as well as oxygen consumption, were reduced in crabs acclimated to elevated pCO2, demonstrating that either (amino acid) oxidation is reduced in response to this particular stress, or nitrogenous wastes are excreted in an alternative form.

Responses of the metabolism of the larvae of Pocillopora damicornis to ocean acidification and warming

Ocean acidification and warming are expected to threaten the persistence of tropical coral reef ecosystems. As coral reefs face multiple stressors, the distribution and abundance of corals will depend on the successful dispersal and settlement of coral larvae under changing environmental conditions. To explore this scenario, we used metabolic rate, at holobiont and molecular levels, as an index for assessing the physiological plasticity of Pocillopora damicornis larvae from this site to conditions of ocean acidity and warming. Larvae were incubated for 6 hours in seawater containing combinations of CO2 concentration (450 and 950 µatm) and temperature (28 and 30°C). Rates of larval oxygen consumption were higher at elevated temperatures. In contrast, high CO2 levels elicited depressed metabolic rates, especially for larvae released later in the spawning period. Rates of citrate synthase, a rate-limiting enzyme in aerobic metabolism, suggested a biochemical limit for increasing oxidative capacity in coral larvae in a warming, acidifying ocean. Biological responses were also compared between larvae released from adult colonies on the same day (cohorts). The metabolic physiology of Pocillopora damicornis larvae varied significantly by day of release. Additionally, we used environmental data collected on a reef in Moorea, French Polynesia to provide information about what adult corals and larvae may currently experience in the field. An autonomous pH sensor provided a continuous time series of pH on the natal fringing reef. In February/March, 2011, pH values averaged 8.075±0.023. Our results suggest that without adaptation or acclimatization, only a portion of naïve Pocillopora damicornis larvae may have suitable metabolic phenotypes for maintaining function and fitness in an end-of-the century ocean.

Calcification rate of massive Porites spp. and Porites rus in the experiment of Moorea

This study tested the hypothesis that the response of corals to temperature and pCO2 is consistent between taxa. Juvenile massive Porites spp. and branches of P. rus from the back reef of Moorea were incubated for 1 month under combinations of temperature (29.3 °C and 25.6 °C) and pCO2 (41.6 Pa and 81.5 Pa) at an irradiance of 599 µmol quanta/m/s. Using microcosms and CO2 gas mixing technology, treatments were created in a partly nested design (tanks) with two between-plot factors (temperature and pCO2), and one within-plot factor (taxon); calcification was used as a dependent variable. pCO2 and temperature independently affected calcification, but the response differed between taxa. Massive Porites spp. was largely unaffected by the treatments, but P. rus grew 50% faster at 29.3 °C compared with 25.6 °C, and 28% slower at 81.5 Pa vs. 41.6 Pa CO2. A compilation of studies placed the present results in a broader context and tested the hypothesis that calcification for individual coral genera is independent of pH, [HCO3]-, and [CO3]2-. Unlike recent reviews, this analysis was restricted to studies reporting calcification in units that could be converted to nmol CaCO3/cm**2/h. The compilation revealed a high degree of variation in calcification as a function of pH, [HCO3]-, and [CO3]2-, and supported three conclusions: (1) studies of the effects of ocean acidification on corals need to pay closer attention to reducing variance in experimental outcomes to achieve stronger synthetic capacity, (2) coral genera respond in dissimilar ways to pH, [HCO3]-, and [CO3]2-, and (3) calcification of massive Porites spp. is relatively resistant to short exposures of increased pCO2, similar to that expected within 100 y.