Niche specialization of reef-building corals in the mesophotic zone: metabolic trade-offs between divergent Symbiodinium types.

The photobiology of two reef corals and the distribution of associated symbiont types were investigated over a depth gradient of 0-60 m at Scott Reef, Western Australia. Pachyseris speciosa hosted mainly the same Symbiodinium C type similar to C3 irrespective of sampling depth. By contrast, Seriatopora hystrix hosted predominantly Symbiodinium type D1a or D1a-like at shallow depths while those in deeper water were dominated by a Symbiodinium C type closely related to C1. The photosynthesis/respiration (P/R) ratio increased consistently with depth at the two sampling times (November 2008 and April 2009) for P. speciosa and in November 2008 only for S. hystrix, suggesting a reduction in metabolic energy expended for every unit of energy obtained from photosynthesis. However, in April 2009, shallow colonies of S. hystrix exhibited decreased P/R ratios down to depths of approximately 23 m, below which the ratio increased towards the maximum depth sampled. This pattern was mirrored by changes in tissue biomass determined as total protein content. The depth of change in the direction of the P/R ratio correlated with a shift from Symbiodinium D to C-dominated colonies. We conclude that while photobiological flexibility is vital for persistence in contrasting light regimes, a shift in Symbiodinium type may also confer a functional advantage albeit at a metabolic cost with increased depth.

The role of deep reefs in shallow reef recovery: an assessment of vertical connectivity in a brooding coral from west and east Australia.

Approximately one quarter of zooxanthellate coral species have a depth distribution from shallow waters (<30 m) down to mesophotic depths of 30-60 m. The deeper populations of such species are less likely to be affected by certain environmental perturbations, including high temperature/high irradiance causing coral bleaching. This has led to the hypothesis that deep populations may serve as refuges and a source of recruits for shallow reef habitats. The extent of vertical connectivity of reef coral species, however, is largely unquantified. Using 10 coral host microsatellite loci and sequences of the host mtDNA putative control region, as well as ribosomal DNA (rDNA) ITS2 sequences of the coral's algal endosymbionts (Symbiodinium), we examine population structure, connectivity and symbiont specificity in the brooding coral Seriatopora hystrix across a depth profile in both northwest (Scott Reef) and northeast Australia (Yonge Reef). Strong genetic structuring over depth was observed in both regions based on the microsatellite loci; however, Yonge Reef exhibited an additional partitioning of mtDNA lineages (associated with specific symbiont ITS2 types), whereas Scott Reef was dominated by a single mtDNA lineage (with no apparent host-symbiont specificity). Evidence for recruitment of larvae of deep water origin into shallow habitats was found at Scott Reef, suggesting that recovery of shallow water habitats may be aided by migration from deep water refuges. Conversely, no migration from the genetically divergent deep slope populations into the shallow habitats was evident at Yonge Reef, making recovery of shallow habitats from deeper waters at this location highly unlikely.

Combining the use of gradients and reference areas to study bioaccumulation in wild oysters in the Hunter River estuary, New South Wales, Australia.

Two approaches for measuring the effects of human activities on aquatic biota are gradient studies and comparison of impacted areas to external references. Wild oysters were sampled at 12 sites adjacent to, upstream and downstream of a steelworks in the Hunter River estuary and in two reference estuaries. Regression analyses for the Hunter indicated reduced concentrations with distance from the point source for some metals and PAHs. Data compared from the reference estuaries to the two sites nearest and the two furthest from the point source in the Hunter indicated elevated concentrations of contaminants both near the point source and on an estuary-wide basis. The gradient approach was useful in identifying the industrial effluent as a point source for bioavailable chemicals. Combining this with the use of reference estuaries provided a broad geographic context in which to interpret results from the Hunter and identified estuary-wide effects.

Pigmentation of massive corals as a simple bioindicator for marine water quality.

Photo-acclimatisation by the algal endosymbionts of scleractinian corals to changes in environmental conditions may influence their density and/or the concentration of photosynthetic pigments, and hence coral brightness, on short time-scales. To examine coral pigmentation as a bioindicator of water quality, the brightness of massive corals was quantified using colour charts, concentrations of the pigment chlorophyll a and reflectance spectrometry in the field and with manipulative experiments. Along a water quality gradient, massive Porites became progressively lighter as nutrients decreased and irradiance increased. A laboratory experiment showed that Porites nubbins darkened within 25 days following exposure to reduced water quality. The results of a transplantation experiment of Porites nubbins in a manipulation incorporating multiple depths and zones of water quality confirmed colony brightness as a simple tool to monitor changes in marine water quality, provided effects due to other influences on pigmentation, e.g. seawater temperatures, are taken into consideration.

Growth of Western Australian corals in the anthropocene.

Anthropogenic increases of atmospheric carbon dioxide lead to warmer sea surface temperatures and altered ocean chemistry. Experimental evidence suggests that coral calcification decreases as aragonite saturation drops but increases as temperatures rise toward thresholds optimal for coral growth. In situ studies have documented alarming recent declines in calcification rates on several tropical coral reef ecosystems. We show there is no widespread pattern of consistent decline in calcification rates of massive Porites during the 20th century on reefs spanning an 11° latitudinal range in the southeast Indian Ocean off Western Australia. Increasing calcification rates on the high-latitude reefs contrast with the downward trajectory reported for corals on Australia's Great Barrier Reef and provide additional evidence that recent changes in coral calcification are responses to temperature rather than ocean acidification.

A bioindicator system for water quality on inshore coral reefs of the Great Barrier Reef.

Responses of bioindicator candidates for water quality were quantified in two studies on inshore coral reefs of the Great Barrier Reef (GBR). In Study 1, 33 of the 38 investigated candidate indicators (including coral physiology, benthos composition, coral recruitment, macrobioeroder densities and FORAM index) showed significant relationships with a composite index of 13 water quality variables. These relationships were confirmed in Study 2 along four other water quality gradients (turbidity and chlorophyll). Changes in water quality led to multi-faceted shifts from phototrophic to heterotrophic benthic communities, and from diverse coral dominated communities to low-diversity communities dominated by macroalgae. Turbidity was the best predictor of biota; hence turbidity measurements remain essential to directly monitor water quality on the GBR, potentially complemented by our final calibrated 12 bioindicators. In combination, this bioindicator system may be used to assess changes in water quality, especially where direct water quality data are unavailable.

Chronic exposure of corals to fine sediments: lethal and sub-lethal impacts.

Understanding the sedimentation and turbidity thresholds for corals is critical in assessing the potential impacts of dredging projects in tropical marine systems. In this study, we exposed two species of coral sampled from offshore locations to six levels of total suspended solids (TSS) for 16 weeks in the laboratory, including a 4 week recovery period. Dose-response relationships were developed to quantify the lethal and sub-lethal thresholds of sedimentation and turbidity for the corals. The sediment treatments affected the horizontal foliaceous species (Montipora aequituberculata) more than the upright branching species (Acropora millepora). The lowest sediment treatments that caused full colony mortality were 30 mg l(-1) TSS (25 mg cm(-2) day(-1)) for M. aequituberculata and 100 mg l(-1) TSS (83 mg cm(-2) day(-1)) for A. millepora after 12 weeks. Coral mortality generally took longer than 4 weeks and was closely related to sediment accumulation on the surface of the corals. While measurements of damage to photosystem II in the symbionts and reductions in lipid content and growth indicated sub-lethal responses in surviving corals, the most reliable predictor of coral mortality in this experiment was long-term sediment accumulation on coral tissue.

Development and validation of computational fluid dynamics models for prediction of heat transfer and thermal microenvironments of corals.

We present Computational Fluid Dynamics (CFD) models of the coupled dynamics of water flow, heat transfer and irradiance in and around corals to predict temperatures experienced by corals. These models were validated against controlled laboratory experiments, under constant and transient irradiance, for hemispherical and branching corals. Our CFD models agree very well with experimental studies. A linear relationship between irradiance and coral surface warming was evident in both the simulation and experimental result agreeing with heat transfer theory. However, CFD models for the steady state simulation produced a better fit to the linear relationship than the experimental data, likely due to experimental error in the empirical measurements. The consistency of our modelling results with experimental observations demonstrates the applicability of CFD simulations, such as the models developed here, to coral bleaching studies. A study of the influence of coral skeletal porosity and skeletal bulk density on surface warming was also undertaken, demonstrating boundary layer behaviour, and interstitial flow magnitude and temperature profiles in coral cross sections. Our models compliment recent studies showing systematic changes in these parameters in some coral colonies and have utility in the prediction of coral bleaching.

Symbiodinium genotypic and environmental controls on lipids in reef building corals.

BACKGROUND: Lipids in reef building corals can be divided into two classes; non-polar storage lipids, e.g. wax esters and triglycerides, and polar structural lipids, e.g. phospholipids and cholesterol. Differences among algal endosymbiont types are known to have important influences on processes including growth and the photobiology of scleractinian corals yet very little is known about the role of symbiont types on lipid energy reserves. METHODOLOGY/PRINCIPAL FINDINGS: The ratio of storage lipid and structural lipid fractions of Scott Reef corals were determined by thin layer chromatography. The lipid fraction ratio varied with depth and depended on symbiont type harboured by two corals (Seriatopora hystrix and Pachyseris speciosa). S. hystrix colonies associated with Symbiodinium C1 or C1/C# at deep depths (>23 m) had lower lipid fraction ratios (i.e. approximately equal parts of storage and structural lipids) than those with Symbiodinium D1 in shallow depths (<23 m), which had higher lipid fraction ratios (i.e. approximately double amounts of storage relative to structural lipid). Further, there was a non-linear relationship between the lipid fraction ratio and depth for S. hystrix with a modal peak at ∼23 m coinciding with the same depth as the shift from clade D to C types. In contrast, the proportional relationship between the lipid fraction ratio and depth for P. speciosa, which exhibited high specificity for Symbiodinium C3 like across the depth gradient, was indicative of greater amounts of storage lipids contained in the deep colonies. CONCLUSIONS/SIGNIFICANCE: This study has demonstrated that Symbiodinium exert significant controls over the quality of coral energy reserves over a large-scale depth gradient. We conclude that the competitive advantages and metabolic costs that arise from flexible associations with divergent symbiont types are offset by energetic trade-offs for the coral host.

Environmental factors controlling the distribution of symbiodinium harboured by the coral Acropora millepora on the Great Barrier Reef.

BACKGROUND: The Symbiodinium community associated with scleractinian corals is widely considered to be shaped by seawater temperature, as the coral's upper temperature tolerance is largely contingent on the Symbiodinium types harboured. Few studies have challenged this paradigm as knowledge of other environmental drivers on the distribution of Symbiodinium is limited. Here, we examine the influence of a range of environmental variables on the distribution of Symbiodinium associated with Acropora millepora collected from 47 coral reefs spanning 1,400 km on the Great Barrier Reef (GBR), Australia. METHODOLOGY/PRINCIPAL FINDINGS: The environmental data included Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data at 1 km spatial resolution from which a number of sea surface temperature (SST) and water quality metrics were derived. In addition, the carbonate and mud composition of sediments were incorporated into the analysis along with in situ water quality samples for a subset of locations. Analyses were conducted at three spatio-temporal scales [GBR (regional-scale), Whitsunday Islands (local-scale) and Keppel Islands/Trunk Reef (temporal)] to examine the effects of scale on the distribution patterns. While SST metrics were important drivers of the distribution of Symbiodinium types at regional and temporal scales, our results demonstrate that spatial variability in water quality correlates significantly with Symbiodinium distribution at local scales. Background levels of Symbiodinium types were greatest at turbid inshore locations of the Whitsunday Islands where SST predictors were not as important. This was not the case at regional scales where combinations of mud and carbonate sediment content coupled with SST anomalies and mean summer SST explained 51.3% of the variation in dominant Symbiodinium communities. CONCLUSIONS/SIGNIFICANCE: Reef corals may respond to global-scale stressors such as climate change through changes in their resident symbiont communities, however, management of local-scale stressors such as altered water quality is also necessary for maintenance of coral-Symbiodinium associations.

The quaternary structure of pyruvate kinase type 1 from Escherichia coli at low nanomolar concentrations.

Pyruvate kinase (PK) is the key control point of glycolysis-the biochemical pathway central to energy metabolism and the production of precursors used in biosynthesis. PK type 1 from Escherichia coli (Ec-PK1) is activated by both fructose-1,6-bisphosphate (FBP) and its substrate, phosphoenol pyruvate (PEP). To date, it has not been possible to determine whether the enzyme is tetrameric at the low concentrations (i.e. low nM range) used to study the steady-state kinetics, or assess whether its allosteric effectors alter the oligomeric state of the enzyme at these concentrations. Employing the new technique of analytical ultracentrifugation with fluorescence detection we have, for the first time, shown that the K(D)(4-2) for Ec-PK1 is in the subnanomolar range, well below the concentrations used in kinetic studies. In addition, we show that, unlike some other PK isoenzymes, the modulation of oligomeric state by the allosteric effectors FBP and PEP does not occur at a concentration of 10 nM or above.