The effects of thermal and high-CO2 stresses on the metabolism and surrounding microenvironment of the coral Galaxea fascicularis

The effects of elevated temperature and high pCO₂ on the metabolism of Galaxea fascicularis were studied with oxygen and pH microsensors. Photosynthesis and respiration rates were evaluated from the oxygen fluxes from and to the coral polyps. High-temperature alone lowered both photosynthetic and respiration rates. High pCO₂ alone did not significantly affect either photosynthesis or respiration rates. Under a combination of high-temperature and high-CO₂, the photosynthetic rate increased to values close to those of the controls. The same pH in the diffusion boundary layer was observed under light in both (400 and 750 ppm) CO₂ treatments, but decreased significantly in the dark as a result of increased CO₂. The ATP contents decreased with increasing temperature. The effects of temperature on the metabolism of corals were stronger than the effects of increased CO₂. The effects of acidification were minimal without combined temperature stress. However, acidification combined with higher temperature may affect coral metabolism due to the amplification of diel variations in the microenvironment surrounding the coral and the decrease in ATP contents.     

Carbon Dioxide Efflux from Leaves in Light and Darkness

Efflux of carbon dioxide in light and darkness was measured at low ambient CO₂ concentrations in leaves of Rumex acetosa. Light carbon dioxide production (photo-respiration) was found to depend on irradiance and to differ from dark production as to the response to temperature and ambient concentrations of O₂ and CO₂. These observations support previously made suggestions that photorespiration follows a different metabolic pathway to dark respiration.     

Spectral Effects on Symbiodinium Photobiology Studied with a Programmable Light Engine

The spectral light field of Symbiodinium within the tissue of the coral animal host can deviate strongly from the ambient light field on a coral reef and that of artificial light sources used in lab studies on coral photobiology. Here, we used a novel approach involving light microsensor measurements and a programmable light engine to reconstruct the spectral light field that Symbiodinium is exposed to inside the coral host and the light field of a conventional halogen lamp in a comparative study of Symbiodinium photobiology. We found that extracellular gross photosynthetic O₂ evolution was unchanged under different spectral illumination, while the more red-weighted halogen lamp spectrum decreased PSII electron transport rates and there was a trend towards increased light-enhanced dark respiration rates under excess irradiance. The approach provided here allows for reconstructing and comparing intra-tissue coral light fields and other complex spectral compositions of incident irradiance. This novel combination of sensor technologies provides a framework to studying the influence of macro- and microscale optics on Symbiodinium photobiology with unprecedented spectral resolution.     

Temporal patterns in effective quantum yield of individual zooxanthellae expelled during bleaching

Bleaching is a worldwide phenomenon affecting coral reefs. During elevated temperature and light conditions (bleaching), expelled zooxanthellae show distinct patterns in photosynthetic health. An innovative new device was used to collect individual expelled zooxanthellae, when a coral was exposed to bleaching conditions. This has provided new insight into the photosynthetic condition and abundance of expelled zooxanthellae. It has been assumed that expelled zooxanthellae were dead or moribund; however, we have found individual cells can have healthy effective quantum yields (?_PSII) >0.65 after 8 h of bleaching conditions (500 μmol photons m⁻² s⁻¹, 33 °C). The population of expelled zooxanthellae from Cyphastrea serailia and Pocillopora damicornis showed distinct patterns in the frequency distribution of ?_PSII over time and between locations (sun versus shade) within a colony. During the first 4 h of exposure to bleaching conditions, only 5% of expelled individual cells from P. damicornis were photosynthetically inactive (?_PSII<0.05), whereas for C. serailia, this was 30%. The overall photosynthetic health of expelled zooxanthellae from C. serailia was better than P. damicornis (0.53±0.13 and 0.38±0.13 after 8 h, respectively). This was generally reflected by the in hospite measurement of the coral, yet, the in hospite cells always had a higher ?_PSII than expelled cells, suggesting that host tissue provided added photoprotection for the zooxanthellae.     

Carbon translocation from symbiont to host depends on irradiance and food availability in the tropical coral Stylophora pistillata

Reef-building corals live in symbiosis with dinoflagellates that translocate a large proportion of their photosynthetically fixed carbon compounds to their coral host for its own metabolism. The carbon budget and translocation rate, however, vary depending on environmental conditions, coral host species, and symbiont clade. To quantify variability in carbon translocation in response to environmental conditions, this study assessed the effect of two different irradiance levels (120 and 250 μmol photons m^?2 s^?1) and feeding regimes (fed with Artemia salina nauplii and unfed) on the carbon budget of the tropical coral Stylophora pistillata. For this purpose, H¹³CO₃ ^?-enriched seawater was used to trace the conversion of photosynthetic carbon into symbiont and coral biomass and excrete particulate organic carbon. Results showed that carbon translocation (ca. 78 %) and utilization were similar under both irradiance levels for unfed colonies. In contrast, carbon utilization by fed colonies was dependent on the growth irradiance. Under low irradiance, heterotrophy was accompanied by lower carbon translocation (71 %), higher host and symbiont biomass, and higher calcification rates. Under high irradiance, heterotrophy was accompanied by higher rates of photosynthesis, respiration, and carbon translocation (90 %) as well as higher host biomass. Hence, levels of resource sharing within coral–dinoflagellate symbioses depend critically on environmental conditions.     

Photosystem II recovery in the presence and absence of chloroplast protein repair in the symbionts of corals exposed to bleaching conditions

Increased seawater temperature causes photoinhibition due to accumulation of photodamaged photosystem II (PSII) in symbiotic algae (genus Symbiodinium) within corals, and it is assumed to be associated with coral bleaching. To avoid photoinhibition, photosynthetic organisms repair the photodamaged PSII through replacing the PSII proteins, primarily the D1 protein, with newly synthesised proteins. However, in experiments using cultured Symbiodinium strains, the PSII repair of Symbiodinium has been suggested not to be related to the synthesis of the D1 protein. In this study, we examined the relationship between the recovery of PSII photochemical efficiency (F _V/F _M) and the content of D1 protein after high-light and high-temperature treatments using the bleaching-sensitive coral species, Pocillopora damicornis and Acropora millepora, and the bleaching-tolerant coral species, Montipora digitata and Pavona decussata. When corals were exposed to strong light (600 µmol photons m^?2 s^?1) at elevated temperature (32 °C) for 8 h, significant bleaching occurred in bleaching-sensitive coral species although an almost similar extent of reduced PSII function was found across all coral species tested. During a subsequent 15-h recovery under low light (10 µmol photons m^?2 s^?1) at optimal temperature (22 °C), the reduced F _V/F _M recovered close to initial levels in all coral species, but the reduced D1 content recovered only in one coral species (Pavona decussata). D1 content was therefore not strongly linked to chloroplast protein synthesis-dependent PSII repair. These results demonstrate that the recovery of photodamaged PSII does not always correspond with the recovery of D1 protein content in Symbiodinium within corals, suggesting that photodamaged PSII can be repaired by a unique mechanism in Symbiodinium within corals.     

Responses of massive and branching coral species to the combined effects of water temperature and nitrate enrichment

The branching coral species Pocillopora damicornis (Linnaeus) and the massive coral species Porites lobata Dana were exposed for 30 days to different temperatures and nitrate concentrations to study the response of the coral-zooxanthella symbiosis. Results suggest that the effect of nitrate enrichment on the polyp-zooxanthella symbiosis varies according to the coral morphology. After the experimental period only 30% of P. damicornis colonies remained healthy, in contrast to 90% of P. lobata. The branching P. damicornis was significantly affected by the addition of nitrate, whereas P. lobata was significantly influenced by water temperature. The two species showed enhanced zooxanthella volume, and chlorophyll contents per cell under high nitrate concentrations. The reduced zooxanthellae density in both species indicated a detrimental influence of the interaction of high nitrate and high temperature. Tissue soluble proteins in P. lobata were significantly reduced by elevated temperature. Results showed that tissue soluble proteins and chlorophylls in P. lobata were from two- to three-fold higher than in P. damicornis. The number of zooxanthellae in P. lobata was double that of P. damicornis. Therefore, we suggest that the slow-growing species P. lobata is better able to cope with changing environmental conditions than the fast-growing coral P. damicornis.     

Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

Ocean acidification (OA) and its associated decline in calcium carbonate saturation states is one of the major threats that tropical coral reefs face this century. Previous studies of the effect of OA on coral reef calcifiers have described a wide variety of outcomes for studies using comparable partial pressure of CO₂ (pCO₂) ranges, suggesting that key questions remain unresolved. One unresolved hypothesis posits that heterogeneity in the response of reef calcifiers to high pCO₂ is a result of regional-scale variation in the responses to OA. To test this hypothesis, we incubated two coral taxa (Pocillopora damicornis and massive Porites) and two calcified algae (Porolithon onkodes and Halimeda macroloba) under 400, 700 and 1000 μatm pCO₂ levels in experiments in Moorea (French Polynesia), Hawaii (USA) and Okinawa (Japan), where environmental conditions differ. Both corals and H. macroloba were insensitive to OA at all three locations, while the effects of OA on P. onkodes were location-specific. In Moorea and Hawaii, calcification of P. onkodes was depressed by high pCO₂, but for specimens in Okinawa, there was no effect of OA. Using a study of large geographical scale, we show that resistance to OA of some reef species is a constitutive character expressed across the Pacific.     

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.     

A coral polyp model of photosynthesis, respiration and calcification incorporating a transcellular ion transport mechanism

A numerical simulation model of coral polyp photosynthesis, respiration and calcification was developed. The model is constructed with three components (ambient seawater, coelenteron and calcifying fluid), and incorporates photosynthesis, respiration and calcification processes with transcellular ion transport by Ca-ATPase activity and passive transmembrane CO₂ transport and diffusion. The model calculates dissolved inorganic carbon and total alkalinity in the ambient seawater, coelenteron and calcifying fluid, dissolved oxygen (DO) in the seawater and coelenteron and stored organic carbon (CH₂O). To reconstruct the drastic variation between light and dark respiration, respiration rate dependency on DO in the coelenteron is incorporated. The calcification rate depends on the aragonite saturation state in the calcifying fluid (Ωa _cal). Our simulation result was a good approximation of “light-enhanced calcification.” In our model, the mechanism is expressed as follows: (1) DO in the coelenteron is increased by photosynthesis, (2) respiration is stimulated by increased DO in the light (or respiration is limited by DO depletion in the dark), then (3) calcification increases due to Ca-ATPase, which is driven by the energy generated by respiration. The model simulation results were effective in reproducing the basic responses of the internal CO₂ system and DO. The daily calcification rate, the gross photosynthetic rate and the respiration rate under a high-flow condition increased compared to those under the zero-flow condition, but the net photosynthetic rate decreased. The calculated calcification rate responses to variations in the ambient aragonite saturation state (Ωa _amb) were nonlinear, and the responses agreed with experimental results of previous studies. Our model predicted that in response to ocean acidification (1) coral calcification will decrease, but will remain at a higher value until Ωa _amb decreases to 1, by maintaining a higher Ωa _cal due to the transcellular ion transport mechanism and (2) the net photosynthetic rate will increase.     

Temperature-Regulated Bleaching and Lysis of the Coral Pocillopora damicornis by the Novel Pathogen Vibrio coralliilyticus

Coral bleaching is the disruption of symbioses between coral animals and their photosynthetic microalgal endosymbionts (zooxanthellae). It has been suggested that large-scale bleaching episodes are linked to global warming. The data presented here demonstrate that Vibrio coralliilyticus is an etiological agent of bleaching of the coral Pocillopora damicornis. This bacterium was present at high levels in bleached P. damicornis but absent from healthy corals. The bacterium was isolated in pure culture, characterized microbiologically, and shown to cause bleaching when it was inoculated onto healthy corals at 25°C. The pathogen was reisolated from the diseased tissues of the infected corals. The zooxanthella concentration in the bacterium-bleached corals was less than 12% of the zooxanthella concentration in healthy corals. When P. damicornis was infected with V. coralliilyticus at higher temperatures (27 and 29°C), the corals lysed within 2 weeks, indicating that the seawater temperature is a critical environmental parameter in determining the outcome of infection. A large increase in the level of the extracellular protease activity of V. coralliilyticus occurred at the same temperature range (24 to 28°C) as the transition from bleaching to lysis of the corals. We suggest that bleaching of P. damicornis results from an attack on the algae, whereas bacterium-induced lysis and death are promoted by bacterial extracellular proteases. The data presented here support the bacterial hypothesis of coral bleaching.     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Prior illumination and the respiration of maize leaves in the dark

The course of respiration of attached maize (Zea mays L.) leaves was measured by infrared gas analysis of CO₂ efflux in the dark following illumination in atmospheres of 300 microliters of CO₂ per liter of air, CO₂-free air, and CO₂-free N₂ containing 400 microliters of O₂ per liter. CO₂ efflux from control leaves started 3 to 4 minutes after darkening, increased to a maximum after about 20 minutes, and returned to a steady minimum after 2 to 3 hours. Respiration was quantitatively related to prior illumination, independent of net CO₂ fixation in the light, and depressed by N₂. Light, but not air, was required to produce a substrate for respiration in the subsequent dark period; air was required for oxidation of the substrate to CO₂. The stimulation of respiration by prior illumination in maize leaves differs in its slower onset and greater duration from the postillumination burst of photorespiration.     

Photosynthetic activity of a temperate coral Acropora pruinosa (Scleractinia, Anthozoa) with symbiotic algae in Japan

Physiological properties of the temperate hermatypic coral Acropora pruinosa Brook with symbiotic algae (zooxanthellae) on the southern coast of the Izu Peninsula, Shizuoka Prefecture, central Japan, were compared between summer and winter. Photosynthesis and respiration rates of the coral with symbiotic zooxanthellae were measured in summer and winter under controlled temperatures and irradiances with a differential gasvolumeter (Productmeter). Net photosynthetic rate under all irradiances was higher in winter than in summer at the lower range of temperature (12–20°C), while lower than in summer at the higher range of temperature (20–30°C). The optimum temperature for net photosynthesis was apt to fall with the decrease of irradiance both in summer and winter, whereas it was higher in summer than in winter under each irradiance. At 25/ 50/100 μmol photons nr² s^?1, it was nearly the sea‐water temperature in each season. Dark respiration rate was higher in winter than in summer, especially in the range from 20–30°C. In both seasons the optimum temperature for gross photosynthesis was 28°C under 400 μmol photons nr² s^?1 and lowered with decreasing irradiance up to 22°C under 25 μmol photons nr² s^?1 in summer, while 20°C under the same irradiance in winter. The optimum temperature for production/respiration (P/R) ratio was higher in summer than in winter under each irradiance. Results indicated that metabolism of coral and zooxanthellae is adapted to ambient temperature condition under nearly natural irradiance in each season.     

Paul Henry Latimer (1925–2011): discoverer of selective scattering in photosynthetic systems

The response of marine phytoplankton to the ongoing increase in atmospheric pCO₂ reflects the consequences of both increased CO₂ concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO₂ and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H₂ ¹⁸O. In long-term experiments (~6-h) at saturating light intensity, we observed no effects of pH or pCO₂ on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of pCO₂ or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO₂-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H⁺ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO₂ predicted for the twenty-first century.     

Separation and measurement of direct and indirect effects of light on stomata

Conductance for water vapor, assimilation of CO₂, and intercellular CO₂ concentration of leaves of five species were determined at various irradiances and ambient CO₂ concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO₂ concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO₂ concentration was constant, and when CO₂ concentration changed and irradiance was constant. On leaves of Xanthium strumarium L., Gossypium hirsutum L., Phaseolus vulgaris L., and Perilla frutescens (L.), Britt., the stomatal response to light was determined to be mainly a direct response to light and to a small extent only a response to changes in intercellular CO₂ concentration. This was also true for stomata of Zea mays L., except at irradiances < 150 watts per square meter, when stomata responded primarily to the depletion of the intercellular spaces of CO₂ which in turn was caused by changes in the assimilation of CO₂.     

Increased temperature mitigates the effects of ocean acidification on the calcification of juvenile Pocillopora damicornis,but at a cost

This study tested the interactive effects of increased seawater temperature and CO₂ partial pressure (pCO₂) on the photochemistry, bleaching, and early growth of the reef coral Pocillopora damicornis. New recruits were maintained at ambient or high temperature (29 or 30.8 °C) and pCO₂ (~ 500 and ~ 1100 μatm) in a full-factorial experiment for 3 weeks. Neither a sharp decline in photochemical efficiency (Fv/Fm) nor evident bleaching was observed at high temperature and/or high pCO₂. Furthermore, elevated temperature greatly promoted lateral growth and calcification, while polyp budding exhibited temperature-dependent responses to pCO₂. High pCO₂ depressed calcification by 28% at ambient temperature, but did not impact calcification at 30.8 °C. Interestingly, elevated temperature in concert with high pCO₂ significantly retarded the budding process. These results suggest that increased temperature can mitigate the adverse effects of acidification on the calcification of juvenile P. damicornis, but at a substantial cost to asexual budding.

     

Diversity in skeletal architecture influences biological heterogeneity and Symbiodinium habitat in corals

Scleractinian corals vary in response to rapid shifts in the marine environment and changes in reef community structure post-disturbance reveal a clear relationship between coral performance and morphology. With exceptions, massive corals are thought to be more tolerant and branching corals more vulnerable to changing environmental conditions, notably thermal stress. The typical responses of massive and branching coral taxa, respectively, are well documented; however, the biological and functional characteristics that underpin this variation are not well understood. We address this gap by comparing multiple biological attributes that are correlated with skeletal architecture in two perforate (having porous skeletal matrices with intercalating tissues) and two imperforate coral species (Montipora aequituberculata, Porites lobata, Pocillopora damicornis, and Seriatopora hystrix) representing three morphotypes. Our results reveal inherent biological heterogeneity among corals and the potential for perforate skeletons to create complex, three-dimensional internal habitats that impact the dynamics of the symbiosis. Patterns of tissue thickness are correlated with the concentration of symbionts within narrow regions of tissue in imperforate corals versus broad distribution throughout the larger tissue area in perforate corals. Attributes of the perforate and environmentally tolerant P. lobata were notable, with tissues ～5 times thicker than in the sensitive, imperforate species P. damicornis and S. hystrix. Additionally, P. lobata had the lowest baseline levels of superoxide and Symbiodinium that provisioned high levels of energy. Given our observations, we hypothesize that the complexity of the visually obscured internal environment has an impact on host–symbiont dynamics and ultimately on survival, warranting further scientific investigation.     

Leaf paraheliotropism in Styrax camporum confers increased light use efficiency and advantageous photosynthetic responses rather than photoprotection

Styrax caporum is a native shrub from the Brazilian savanna. Most of its leaves are diaheliotropic, whereas some are paraheliotropic, mainly at noon. A previous study of this species revealed higher stomatal conductance (gs) and transpiration rates (E) in para- compared to diaheliotropic leaves, and a rise in CO₂ assimilation rates (A) with an increase of irradiance for paraheliotropic leaves. We hypothesized that this species exploits the paraheliotropism to enhance the light use efficiency, and that it is detected only if gas exchange is measured with light interception by both leaf surfaces. Gas exchange was measured with devices that enabled light interception on only one of the leaf surfaces and with devices that enabled light interception by both leaf surfaces. Water relations, relative reflected light intensity, leaf temperature (T_l), and leaf anatomical analyses were also performed. When both leaf surfaces were illuminated, a higher A, E, and gs were observed in para- compared to diaheliotropic leaves; however, A did not depend on gs, which did not influence CO₂ accumulation in the stomatal cavity (Ci). When only the adaxial leaf surface was illuminated, a greater A was detected for para- than for diaheliotropic leaves only at 11:00 h; no differences in T_l were observed between leaf types. Light curves revealed that under non-saturating light the adaxial side of paraheliotropic leaves had higher A than the abaxial side, but they showed similar values under saturating light. Although the abaxial leaf side was highly reflective, both surfaces presented the same response pattern for green light reflection, which can be explained by the compact spongy parenchyma observed in the leaves, increasing light use efficiency in terms of CO₂ consumption for paraheliotropic leaves. We propose that paraheliotropism in S. camporum is not related to leaf heat avoidance or photoprotection.