Herbivory facilitates growth of a key reef‐building Caribbean coral

The decline of reef‐building corals in conjunction with shifts to short‐lived opportunistic species has prompted concerns that Caribbean reef framework‐building capacity has substantially diminished. Restoring herbivore populations may be a potential driver of coral recovery; however, the impact of herbivores on coral calcification has been little studied. We performed an exclusion experiment to evaluate the impact of herbivory on Orbicella faveolata coral growth over 14 months. The experiment consisted of three treatments: full exclusion cages; half cage procedural controls; and uncaged control plates, each with small O. faveolata colonies. We found that herbivorous fish exclusion had a substantial impact on both macroalgal cover and coral growth. Fleshy macroalgae reached 50% cover within some exclusion cages, but were almost absent from uncaged control plates. Critically, O. faveolata calcification rates were suppressed by almost half within exclusion cages, with monthly coral growth negatively related to overgrowth by fleshy macroalgae. These findings highlight the importance of herbivorous fishes for coral growth and the detrimental impact of macroalgal proliferation in the Caribbean. Policy makers and local managers should consider measures to protect herbivorous fishes and reduce macroalgal proliferation to enable coral communities to continue to grow and function.     

Dark calcification and the daily rhythm of calcification in the scleractinian coral, <Emphasis Type="Italic">Galaxea fascicularis</Emphasis>

The rate of calcification in the scleractinian coral Galaxea fascicularis was followed during the daytime using ⁴⁵Ca tracer. The coral began the day with a low calcification rate, which increased over time to a maximum in the afternoon. Since the experiments were carried out under a fixed light intensity, these results suggest that an intrinsic rhythm exists in the coral such that the calcification rate is regulated during the daytime. When corals were incubated for an extended period in the dark, the calcification rate was constant for the first 4 h of incubation and then declined, until after one day of dark incubation, calcification ceased, possibly as a result of the depletion of coral energy reserves. The addition of glucose and Artemia reduced the dark calcification rate for the short duration of the experiment, indicating an expenditure of oxygen in respiration. Artificial hypoxia reduced the rate of dark calcification to about 25% compared to aerated coral samples. It is suggested that G. fascicularis obtains its oxygen needs from the surrounding seawater during the nighttime, whereas during the day time the coral exports oxygen to the seawater.     

Ocean acidification effects on calcification and dissolution in tropical reef macroalgae

Net calcification rates for coral reef and other calcifiers have been shown to decline as ocean acidification (OA) occurs. However, the role of calcium carbonate dissolution in lowering net calcification rates is unclear. The objective of this study was to distinguish OA effects on calcification and dissolution rates in dominant calcifying macroalgae of the Florida Reef Tract, including two rhodophytes (Neogoniolithon strictum, Jania adhaerens) and two chlorophytes (Halimeda scabra, Udotea luna). Two experiments were conducted: (1) to assess the difference in gross (⁴⁵Ca uptake) versus net (total alkalinity anomaly) calcification rates in the light/dark and (2) to determine dark dissolution (⁴⁵CaCO₃), using pH levels predicted for the year 2100 and ambient pH. At low pH in the light, all species maintained gross calcification rates and most sustained net calcification rates relative to controls. Net calcification rates in the dark were ~84% lower than in the light. In contrast to the light, all species had lower net calcification rates in the dark at low pH with chlorophytes exhibiting net dissolution. These data are supported by the relationship (R² = 0.82) between increasing total alkalinity and loss of ⁴⁵Ca from pre-labelled ⁴⁵CaCO₃ thalli at low pH in the dark. Dark dissolution of ⁴⁵CaCO₃-labelled thalli was ~18% higher in chlorophytes than rhodophytes at ambient pH, and ~ twofold higher at low pH. Only Udotea, which exhibited dissolution in the light, also had lower daily calcification rates integrated over 24 h. Thus, if tropical macroalgae can maintain high calcification rates in the light, lower net calcification rates in the dark from dissolution may not compromise daily calcification rates. However, if organismal dissolution in the dark is additive to sedimentary carbonate losses, reef dissolution may be amplified under OA and contribute to erosion of the Florida Reef Tract and other reefs that exhibit net dissolution.

     

Light-enhanced calcification and dark decalcification in isolates of the soft coral Cladiella sp. during tissue recovery

Light-enhanced calcification is a general characteristic of zooxanthellate corals, suggesting a link between calcification by the coral and photosynthesis by the zooxanthellae, but the relationship between zooxanthellae and coral hosts during this process has not been elucidated. We hypothesized that the effects of tissue injury on the coral fragments used in experiments studying calcification might obscure that link. To detect the effects of tissue injury on light-enhanced calcification, we measured calcification rates (sclerite formation) in the soft coral Cladiella sp. by the alkalinity anomaly method during a 36-day experiment following injury associated with coral fragmentation. In the 2 weeks after colony fragmentation, the calcification response did not show a relation with light intensity. The typical light-enhanced calcification pattern was not noticed until day 15 of tissue recovery. The calcification rate of this soft coral increased with light intensity and time of tissue recovery and was comparable to that of hard corals exposed to similar experimental conditions. However, Cladiella sp. decalcified in the dark. The diurnal calcification-decalcification cycles probably control sclerite size and shape.     

Paraformaldehyde-Resistant Starch-Fermenting Bacteria in “Starch-Base” Drilling Mud

Starch-fermenting bacteria were found in each of 12 samples of nonfermenting starch-base drilling mud examined. Of the 12 samples, 3 contained very active starch-fermenting gram-positive spore-bearing bacilli closely resembling Bacillus subtilis. Similar active starch-fermenting bacteria were found in fermenting starch-base drilling mud and in corn starch and slough water used to prepare such mud. The active starch-fermenting microorganisms completely hydrolyzed 1% (w/v) corn starch within 24 hr at 37.5 C. The active starch-fermenting bacteria isolated from fermenting drilling mud were capable of surviving 12 hr of continuous exposure to 0.1% (w/w) paraformaldehyde or 1 hr of continuous exposure to 0.5% (w/w) paraformaldehyde, with no diminution in starch-fermenting ability. The same organisms fermented starch after 3 hr of continuous exposure to 0.5% (w/w) paraformaldehyde, but not after 4 hr of exposure. The phenomenon of rapid disappearance of paraformaldehyde from fermenting drilling mud was observed in the laboratory using a modified sodium sulfite test. Paraformaldehyde, initially present in a concentration of 0.192 lb per barrel of mud, completely disappeared in 9 hr at 22 to 23 C. A significant decrease in paraformaldehyde concentration was detected 0.5 hr after preparation of the mud. It is suggested that the presence of relatively high concentrations of ammonia and chloride in the mud may facilitate the disappearance of paraformaldehyde. The failure of 0.1% (w/w) paraformaldehyde to inhibit the strong starch-fermenting microorganisms isolated from fermenting drilling mud, and the rapid disappearance of paraformaldehyde from the mud, explains the fermentation of starch which occurred in this mud, despite the addition of paraformaldehyde.     

Influence of the drilling mud formulation process on the bacterial communities in thermogenic natural gas wells of the Barnett Shale

The Barnett Shale in north central Texas contains natural gas generated by high temperatures (120 to 150°C) during the Mississippian Period (300 to 350 million years ago). In spite of the thermogenic origin of this gas, biogenic sulfide production and microbiologically induced corrosion have been observed at several natural gas wells in this formation. It was hypothesized that microorganisms in drilling muds were responsible for these deleterious effects. Here we collected drilling water and drilling mud samples from seven wells in the Barnett Shale during the drilling process. Using quantitative real-time PCR and microbial enumerations, we show that the addition of mud components to drilling water increased total bacterial numbers, as well as the numbers of culturable aerobic heterotrophs, acid producers, and sulfate reducers. The addition of sterile drilling muds to microcosms that contained drilling water stimulated sulfide production. Pyrosequencing-based phylogenetic surveys of the microbial communities in drilling waters and drilling muds showed a marked transition from typical freshwater communities to less diverse communities dominated by Firmicutes and Gammaproteobacteria. The community shifts observed reflected changes in temperature, pH, oxygen availability, and concentrations of sulfate, sulfonate, and carbon additives associated with the mud formulation process. Finally, several of the phylotypes observed in drilling muds belonged to lineages that were thought to be indigenous to marine and terrestrial fossil fuel formations. Our results suggest a possible alternative exogenous origin of such phylotypes via enrichment and introduction to oil and natural gas reservoirs during the drilling process.     

The rôle of zooxanthellae in the hermatypic coral Plesiastrea urvillei (Milne Edwards and Haime) From cold waters

The presence of zooxanthellae in tissues of the cold-temperate water coral Plesiastrea urvillei (Milne Edwards and Haime) has been confirmed histologically. Numbers of zooxanthellae per unit surface area and increases in submerged wet weight as a measure of calcification have been followed for 150 days under four different conditions: light-fed, light-starved, dark-fed, and dark-starved. No significant difference was found in density of zooxanthellae or calcification rates between light-fed and light-starved, and between dark-fed and dark-starved. After Day 48 the calcification rate in the dark dropped, however, by a factor of ≈4 to a constant lower rate and was correlated with a decrease in density of zooxanthellae. Zooxanthellae thus enhance calcification about 4 times during photosynthesis. Measurements of oxygen consumption and production indicated that even at the low light intensities experienced on a cloudy winter day by the coral in its natural environment more energy was fixed during photosynthesis than was required by the host. The retention of zooxanthellae and continued calcification in the dark for upwards of 48 days is considered to be an adaptation to the lower light levels experienced by P. urvillei compared with tropical corals.     

Long‐term salinity tolerance is accompanied by major restructuring of the coral bacterial microbiome

Scleractinian corals are assumed to be stenohaline osmoconformers, although they are frequently subjected to variations in seawater salinity due to precipitation, freshwater run‐off and other processes. Observed responses to altered salinity levels include differences in photosynthetic performance, respiration and increased bleaching and mortality of the coral host and its algal symbiont, but a study looking at bacterial community changes is lacking. Here, we exposed the coral Fungia granulosa to strongly increased salinity levels in short‐ and long‐term experiments to disentangle temporal and compartment effects of the coral holobiont (i.e. coral host, symbiotic algae and associated bacteria). Our results show a significant reduction in calcification and photosynthesis, but a stable microbiome after short‐term exposure to high‐salinity levels. By comparison, long‐term exposure yielded unchanged photosynthesis levels and visually healthy coral colonies indicating long‐term acclimation to high‐salinity levels that were accompanied by a major coral microbiome restructuring. Importantly, a bacterium in the family Rhodobacteraceae was succeeded by Pseudomonas veronii as the numerically most abundant taxon. Further, taxonomy‐based functional profiling indicates a shift in the bacterial community towards increased osmolyte production, sulphur oxidation and nitrogen fixation. Our study highlights that bacterial community composition in corals can change within days to weeks under altered environmental conditions, where shifts in the microbiome may enable adjustment of the coral to a more advantageous holobiont composition.     

Sedimentation stress in a scleractinian coral exposed to terrestrial and marine sediments with contrasting physical, organic and geochemical properties

Terrestrial runoff increases siltation and nutrient availability on coastal coral reefs worldwide. However the factors determining stress in corals when exposed to short-term sedimentation, including the interactions between sediments and nutrients, are little understood. We exposed corals to ten different sediment types at environmentally relevant concentrations (33 to 160 mg DW cm⁻²) and exposure times (12 to 60 h) in laboratory and field experiments. The sediments originated from 2 estuaries, 2 nearshore and one offshore locations and also included ground-up aragonite. For two of these sediments, three grain size fractions were used (silt < 63 μm, fine sand: 63-250 μm, medium sand: 250-500 μm). Sediments were characterised by 19 parameters grouped into “physical”, “organic and nutrient-related” and “geochemical” parameters. Changes in the photosynthetic yield of the coral Montipora peltiformis was measured by pulse-amplitude modulated chlorophyll fluorometry (PAM) as proxy for photophysiological stress from exposure, and to determine rates of recovery. Different sediments exerted greatly contrasting levels of stress in the corals. Our results show that grain size and organic and nutrient-related sediment properties are key factors determining sedimentation stress in corals after short-term exposure. Photophysiological stress was measurable after 36 h of exposure to most of the silt-sized sediments, and coral recovery was incomplete after 48 to 96 h recovery time. The four sandy sediment types caused no measurable stress at the same concentration for the same exposure time. Stress levels were strongly related to the values of organic and nutrient-related parameters in the sediment, weakly related to the physical parameters and unrelated to the geochemical parameters measured. M. peltiformis removed the sandy grain size classes more easily than the silt, and nutrient-poor sediments were removed more easily than nutrient-rich sediments. Anoxia developed on the sediment surfaces of the nutrient-rich silts, which had become slimy and smelled of hydrogen sulphide, suggesting increased bacterial activity. Our finding that silt-sized and nutrient-rich sediments can stress corals after short exposure, while sandy sediments or nutrient-poor silts affect corals to a lesser extent, will help refining predictions of sedimentation threats to coral reefs at given environmental conditions.     

Evaluation of deep subsurface sampling procedures using serendipitous microbial contaminants as tracer organisms

Surface microbiological investigations are critically dependent on the procedures used to collect samples for study. It can be difficult to distinguish between indigenous organisms and those encountered as contaminants during the drilling process. We found that coliform bacteria contaminated drilling mud slurries. These bacteria proved useful as tracer organisms in evaluating the degree of microbial contamination accidentally encountered while drilling for subterranean samples. While these organisms were found in high numbers in both the circulating muds and in the mud reservoir, few subsurface samples harbored conforms. Subsurface slurries did not inhibit the growth of a known coliform inoculum. These results indicate that the methods used to collect and field‐process cores from Atlantic coastal plain sediments were sufficient to prevent a large degree of bacterial contamination in most samples. The microflora in drilling fluids did not quantitatively or qualitatively account for the number and diversity of bacteria in subsurface samples. We conclude that a large and viable bacterial community is present in deep regions of the terrestrial subsurface.     

Sponge erosion under acidification and warming scenarios: differential impacts on living and dead coral

Ocean acidification will disproportionately impact the growth of calcifying organisms in coral reef ecosystems. Simultaneously, sponge bioerosion rates have been shown to increase as seawater pH decreases. We conducted a 20‐week experiment that included a 4‐week acclimation period with a high number of replicate tanks and a fully orthogonal design with two levels of temperature (ambient and +1 °C), three levels of pH (8.1, 7.8, and 7.6), and two levels of boring sponge (Cliona varians, present and absent) to account for differences in sponge attachment and carbonate change for both living and dead coral substrate (Porites furcata). Net coral calcification, net dissolution/bioerosion, coral and sponge survival, sponge attachment, and sponge symbiont health were evaluated. Additionally, we used the empirical data from the experiment to develop a stochastic simulation of carbonate change for small coral clusters (i.e., simulated reefs). Our findings suggest differential impacts of temperature, pH and sponge presence for living and dead corals. Net coral calcification (mg CaCO₃ cm^?2 day^?1) was significantly reduced in treatments with increased temperature (+1 °C) and when sponges were present; acidification had no significant effect on coral calcification. Net dissolution of dead coral was primarily driven by pH, regardless of sponge presence or seawater temperature. A reevaluation of the current paradigm of coral carbonate change under future acidification and warming scenarios should include ecologically relevant timescales, species interactions, and community organization to more accurately predict ecosystem‐level response to future conditions.     

BICARBONATE STIMULATION OF CALCIFICATION AND PHOTOSYNTHESIS IN TWO HERMATYPIC CORALS1

A wide range of bicarbonate concentrations was used to monitor the kinetics of bicarbonate (HCO₃^?) use in both photosynthesis and calcification in two reef‐building corals, Porites porites and Acropora sp. Experiments carried out close to the P. porites collection site in Barbados showed that additions of NaHCO₃ to synthetic seawater proportionally increased the calcification rate of this coral until the concentration exceeded three times that of seawater (6 mM). Photosynthetic rates were also stimulated by HCO₃^? addition, but these became saturated at a lower concentration (4 mM). Similar experiments on aquarium‐acclimated colonies of Indo‐Pacific Acropora sp. showed that calcification and photosynthesis in this coral were enhanced to an even greater extent than P. porites, with calcification continuing to increase above 8 mM HCO₃^?, and photosynthesis saturating at 6 mM. Calcification rates of Acropora sp. were also monitored in the dark, and, although these were lower than in the light for a given HCO₃^? concentration, they still increased dramatically with HCO₃^? addition, showing that calcification in this coral is light stimulated but not light dependent.     

Spatial and seasonal reef calcification in corals and calcareous crusts in the central Red Sea

The existence of coral reef ecosystems critically relies on the reef carbonate framework produced by scleractinian corals and calcareous crusts (i.e., crustose coralline algae). While the Red Sea harbors one of the longest connected reef systems in the world, detailed calcification data are only available from the northernmost part. To fill this knowledge gap, we measured in situ calcification rates of primary and secondary reef builders in the central Red Sea. We collected data on the major habitat-forming coral genera Porites, Acropora, and Pocillopora and also on calcareous crusts (CC) in a spatio-seasonal framework. The scope of the study comprised sheltered and exposed sites of three reefs along a cross-shelf gradient and over four seasons of the year. Calcification of all coral genera was consistent across the shelf and highest in spring. In addition, Pocillopora showed increased calcification at exposed reef sites. In contrast, CC calcification increased from nearshore, sheltered to offshore, exposed reef sites, but also varied over seasons. Comparing our data to other reef locations, calcification in the Red Sea was in the range of data collected from reefs in the Caribbean and Indo-Pacific; however, Acropora calcification estimates were at the lower end of worldwide rates. Our study shows that the increasing coral cover from nearshore to offshore environments aligned with CC calcification but not coral calcification, highlighting the potentially important role of CC in structuring reef cover and habitats. While coral calcification maxima have been typically observed during summer in many reef locations worldwide, calcification maxima during spring in the central Red Sea indicate that summer temperatures exceed the optima of reef calcifiers in this region. This study provides a foundation for comparative efforts and sets a baseline to quantify impact of future environmental change in the central Red Sea.

     

Coral calcification rates by the buoyant weight technique: Effects of alizarin staining

The buoyant weight method has been used in a laboratory experiment over an 18-day period to assess effects of alizarin staining on the calcification rate of the hermatypic coral Diploria strigosa (Dana). Exposure of corals to a concentration of 10 mg/l alizarin for 24 h in a flow-through system caused a significant depression in calcification for a period of up to 6 days. It is suggested that such initial calcification depressions after staining could affect absolute growth measurements and cause synergistic effects with experimental manipulation of environmental conditions unless an appropriate recovery period is allowed.     

Testing clove oil as an anaesthetic for long-distance transport of live fish: the case of the Lake Victoria cichlid Haplochromis obliquidens

Clove oil can be used as an anaesthetic in the handling of marine and freshwater fish. Few studies report on its use for periods up to 48 h, for example, under long‐distance transport conditions. This study tested the effect of different clove oil concentrations for 1–48 h on recovery and survival of the cichlid Haplochromis obliquidens, an ornamental fish species endemic to Lake Victoria. Haplochromis obliquidens were anaesthetized for 1 h using 5–25 μl L^?1 clove oil. There was no correlation between clove oil concentration and post‐anaesthesia recovery time (P = 0.15). On average, fish recovered within 9.5 ± 2 min, and no fish died within 24 h after recovery. Results from exposure of fish to 18–20 μl L^?1 clove oil for up to 48 h suggested a narrow margin of safety as this concentration range induced mortality. At 18 μl L^?1 recovery times ranged from 3 to 43 min between 24 and 36 h exposure, while fish exposed longer than 36 h recovered within 1–10 min, or within 1–2 min after 44–48 h. At the end of a 48‐h transport experiment total ammonia levels were higher in transport water containing anaesthetized fish than for non‐anaesthetized fish (1.65 ± 0.19 and 0.54 ± 0.08 mg L^?1 NH + NH₃, respectively). The combined use of clove oil and the selective ammonium ion exchanger zeolite was considered feasible as ammonia levels could be reduced by up to 82% compared to control bags without zeolite.     

Scaling up calcification,respiration, and photosynthesis rates of six prominent coral taxa

Coral reefs provide a range of important services to humanity, which are underpinned by community‐level ecological processes such as coral calcification. Estimating these processes relies on our knowledge of individual physiological rates and species‐specific abundances in the field. For colonial animals such as reef‐building corals, abundance is frequently expressed as the relative surface cover of coral colonies, a metric that does not account for demographic parameters such as coral size. This may be problematic because many physiological rates are directly related to organism size, and failure to account for linear scaling patterns may skew estimates of ecosystem functioning. In the present study, we characterize the scaling of three physiological rates — calcification, respiration, and photosynthesis — considering the colony size for six prominent, reef‐building coral taxa in Mo''orea, French Polynesia. After a seven‐day acclimation period in the laboratory, we quantified coral physiological rates for three hours during daylight (i.e., calcification and gross photosynthesis) and one hour during night light conditions (i.e., dark respiration). Our results indicate that area‐specific calcification rates are higher for smaller colonies across all taxa. However, photosynthesis and respiration rates remain constant over the colony‐size gradient. Furthermore, we revealed a correlation between the demographic dynamics of coral genera and the ratio between net primary production and calcification rates. Therefore, intraspecific scaling of reef‐building coral physiology not only improves our understanding of community‐level coral reef functioning but it may also explain species‐specific responses to disturbances.     

Calcification rate and the effect of temperature in a zooxanthellate and an azooxanthellate scleractinian reef coral

Calcification rates, normalized to skeletal mass, in the zooxanthellate Galaxea fascicularis and the azooxanthellate Dendrophyllia sp. were similar over the whole temperature range of 18–29 °C. Calcification was measured by Ca⁴⁵ incorporation in corals that were naturally acclimated to the prevailing seawater temperature. In both species maximum calcification rate occurred at about 25 °C and calcification rates can be fitted to a Gaussian distribution with respect to temperature. The similarity in temperature dependence of the zooxanthellate and azooxanthellate coral suggests that temperature affects some fundamental process of calcification that is independent of light effects. It is shown that two different populations of Galaxea fascicularis have distinctly different ratios of tissue protein to skeletal mass per polyp. This indicates that tissue protein may not be suitable for normalizing calcification rates in individual coral polyps, both within and between species. Intra- and interspecific comparisons of calcification rates may be better made on the basis of skeletal mass when polyps are similar in size and shape.Communicated by Topic Editor C. Barnes     

Patterns of resistance and resilience of the stress-tolerant coral Siderastrea radians (Pallas) to sub-optimal salinity and sediment burial

The coastal lagoons of south Florida, U.S., experience fluctuating levels of sedimentation and salinity and contain only a subset of the coral species found at the adjacent reefs of the Florida Reef Tract. The dominant species within these habitats is Siderastrea radians, which can reach densities of up to 68 colonies m^- 2 and is commonly exposed to salinity extremes (< 10 psu to > 37 psu) and chronic sediment burial. In this study, we document the patterns of resistance and resilience of S. radians to sub-optimal salinity levels and sediment burial in a series of short-term, long-term, acute, chronic, single-stressor, and sequential-stressor experiments.S. radians displayed remarkable patterns of resistance and resilience and mortality was documented only under prolonged (≥ 48 h) continuous exposure to salinity extremes (15 and 45 psu) and chronic sediment burial. A reduction in photosynthetic rates was documented for all salinity exposures and the decrease in photosynthesis was linearly related to exposure time. Negative impacts on photosynthetic rates were more severe under low salinity (15 psu) than under high salinity (45 psu). Corals exposed to intermediate, low-salinity levels (25 psu), exhibited initial declines in photosynthesis that were followed by temporary increases that may represent transient acclimatization patterns. The impacts of sediment burial were influenced by the duration of the burial period and ranged from a temporary reduction in photosynthesis to significant reductions in growth and tissue mortality. The maintenance of P/R ratios > 1 and the rapid (< 24 h) recovery of photosynthetic rates after burial periods of 2-24 h indicates that S. radians is able to resist short-term burial periods with minor physiological consequences. However, as burial periods increase and colonies become covered at multiple chronic intervals, sediment burial resulted in extended photosynthetic recovery periods, reduced growth, and mortality. Under normal conditions (i.e., no salinity stress), S. radians was very effective at clearing sediments, and > 50% of the colonies' surface area was cleared within 1 h. However, clearing rates were influenced by physiological status, and prior exposure to sub-optimal salinity significantly reduced the clearing rates of stressed colonies.The response of S. radians to disturbance documented in this study characterizes this species as highly stress-tolerant and provides an explanation for its present high abundance in both reef and marginal environments. Moreover, the key life-history attributes of S. radians, such as brooding reproductive strategy, small colony size, high stress-tolerance, and high recruitment rates, suggest the potential for this species to replace reef-building taxa under increased disturbance scenarios in Florida and elsewhere in the region.     

Resilience and acclimation to bleaching stressors in the scleractinian coral Porites cylindrica

‘Resilience’, the capacity of the coral symbiosis with dinoflagellate algal symbionts (‘zooxanthellae’) to recover after bleaching, is a little-studied but crucial aspect of coral responses to bleaching stressors. This study investigated the response of the zooxanthella population in the coral Porites cylindrica after bleaching either naturally on a shallow subtidal reef or experimentally in response to elevated temperature and darkness. Coral resilience was influenced by the nature and duration of the stressor. Corals strongly bleached by natural stressors were less resilient than those that had been partially bleached; and a similar recovery profile was obtained for corals experimentally bleached by exposure to elevated temperature, in which recovery was slower for corals thermally-stressed 96 h than for 72 h. The opposite trend was evident for corals exposed to darkness, indicating that the bleaching trigger had a strong impact on coral resilience. When P. cylindrica recently recovered from bleaching was subjected to a repetition of bleaching stressors, it did not display acclimation, i.e. experience-mediated acquisition of resistance to bleaching stressors. The zooxanthella populations in all corals tested throughout the experiments were typed by PCR-RFLP as clade C, indicating that coral responses were not accompanied by any substantial change in zooxanthella composition at the cladal level.     

Determinants of surfactant function in acute lung injury and early recovery

Relationships between lung function and surfactant function and composition were examined during the evolution of acute lung injury in guinea pigs. Lung mechanics and gas exchange were assessed 12, 24, or 48 h after exposure to nebulized lipopolysaccharide (LPS). Bronchoalveolar lavage (BAL) fluid was processed for phospholipid and protein contents and surfactant protein (SP) A and SP-B levels; surfactant function was measured by pulsating bubble surfactometry. Lung elastance, tissue resistance, and arterial-alveolar gradient were moderately elevated by 12 h after LPS exposure and continued to increase over the first 24 h but began to recover between 24 and 48 h. Similarly, the absolute amount of 30,000 g pelleted SP-A and SP-B, the phospholipid content of BAL fluid, and surfactant function declined over the first 24 h after exposure, with recovery between 24 and 48 h. BAL fluid total protein content increased steadily over the first 48 h after LPS nebulization. In this model of acute lung injury, the intra-alveolar repletion of surfactant components in early recovery led to improved surfactant function despite the presence of potentially inhibitory plasma proteins.