The effect of allometric scaling in coral thermal microenvironments

Bacterial Communities Vary from Different Scleractinian Coral Species and between Bleached and Non-Bleached Corals

Bleaching is one of the most relevant factors implicated in the integrity of coral reef ecosystems, with the increasing frequency and intensity of damaging events representing a serious threat to reef biodiversity. Here, we analyzed changes in coral-associated bacteria from three types of non-bleached and bleached scleractinian corals (Acropora digitifera, Galaxea fascicularis, and Porites pukoensis) in Hainan Luhuitou peninsula coastal areas. The community structure of symbiotic bacteria differed significantly among the three apparently healthy corals. The bleached corals had higher bacterial alpha diversity and some specific bacteria genera, including Ruegeria, Methyloceanibacter, Filomicrobium, Halioglobus, Rubripirellula, Rhodopirellula, Silicimonas, Blastopirellula, Sva0996 marine group, Woeseia, and unclassified_c_Gammaproteobacteria, were consistently increased in bleached groups. Network analysis revealed significantly different degrees of modularity between bleached and non-bleached groups at the bacterial genus level, and a higher proportion of links was dominated by positive co-occurrences. Functional prediction analysis illustrated that coral-associated bacteria remained relatively consistent in the bleached and non-bleached groups. Structure equation modeling revealed that the bacterial community diversity and function were directly influenced by host and environment factors. These findings suggested that coral-associated bacterial responses to bleaching occur in a host-dependent manner, informing novel strategies for restoring coral and aiding adaption to bleaching stress. IMPORTANCE Accumulating evidence indicates that coral-associated bacteria play an important role in the health of holobionts. However, the variability of the symbiotic bacterial community structure among coral species with different coral health statuses remains largely unknown. Here, we investigated three apparent non-bleached (healthy) and bleached coral species (sampled in situ), involving related symbiotic bacterial profiles, including composition, alpha diversity, network relationship, and potential function. Structural equation modeling analysis was used to analyze the relationship between coral status and abiotic and biotic factors. The bacterial community structure of different groups was shown to exhibit host-specific traits. Both host and environmental impacts had primary effects on coral-associated microbial communities. Future studies are needed to identify the mechanisms that mediate divergent microbial consortia.

Multiphysics modelling of photon, mass and heat transfer in coral microenvironments

Coral reefs are constructed by calcifying coral animals that engage in a symbiosis with dinoflagellate microalgae harboured in their tissue. The symbiosis takes place in the presence of steep and dynamic gradients of light, temperature and chemical species that are affected by the structural and optical properties of the coral and their interaction with incident irradiance and water flow. Microenvironmental analyses have enabled quantification of such gradients and bulk coral tissue and skeleton optical properties, but the multi-layered nature of corals and its implications for the optical, thermal and chemical microenvironment remains to be studied in more detail. Here, we present a multiphysics modelling approach, where three-dimensional Monte Carlo simulations of the light field in a simple coral slab morphology with multiple tissue layers were used as input for modelling the heat dissipation and photosynthetic oxygen production driven by photon absorption. By coupling photon, heat and mass transfer, the model predicts light, temperature and O₂ gradients in the coral tissue and skeleton, under environmental conditions simulating, for example, tissue contraction/expansion, symbiont loss via coral bleaching or different distributions of coral host pigments. The model reveals basic structure-function mechanisms that shape the microenvironment and ecophysiology of the coral symbiosis in response to environmental change.

Emergent properties of branching morphologies modulate the sensitivity of coral calcification to high PCO2

Experiments with coral fragments (i.e. nubbins) have shown that net calcification is depressed by elevated PCO2 Evaluating the implications of this finding requires scaling of results from nubbins to colonies, yet the experiments to codify this process have not been carried out. Building from our previous research demonstrating that net calcification of Pocillopora verrucosa (2-13 cm diameter) was unaffected by PCO2  (400 and 1000 µatm) and temperature (26.5 and 29.7°C), we sought generality to this outcome by testing how colony size modulates PCO2  and temperature sensitivity in a branching acroporid. Together, these taxa represent two of the dominant lineages of branching corals on Indo-Pacific coral reefs. Two trials conducted over 2 years tested the hypothesis that the seasonal range in seawater temperature (26.5 and 29.2°C) and a future PCO2  (1062 µatm versus an ambient level of 461 µatm) affect net calcification of an ecologically relevant size range (5-20 cm diameter) of colonies of Acropora hyacinthus As for P. verrucosa, the effects of temperature and PCO2  on net calcification (mg day-1) of A. verrucosa were not statistically detectable. These results support the generality of a null outcome on net calcification of exposing intact colonies of branching corals to environmental conditions contrasting seasonal variation in temperature and predicted future variation in PCO2 While there is a need to expand beyond an experimental culture relying on coral nubbins as tractable replicates, rigorously responding to this need poses substantial ethical and logistical challenges.

The effect of small-scale morphology on thermal dynamics in coral microenvironments

The thermal microenvironments of corals is a topic of current interest given their relationship to coral bleaching. We present computational fluid dynamics (CFD) model of corals with both smooth and rugged polyp surface topographies for two species of massive corals (Leptastrea purpurea and Platygyra sinensis) in order to predict their microscale surface warming. This study explores whether variation in polyp depth (PD) may directly effect a coral overall surface area-to-volume (A/V) ratio and consequently its surface warming. Validation of our models was made against detailed laboratory measurements of coral surface warming and thermal boundary layer thickness. Our results suggested that while differences in surface warming exist between smooth surfaces and surfaces covered in micro-polyps (5 mm depth), the variation in terms of surface warming is small (~0.18-0.19^∘C) and it can be largely attributed to increasing A/V ratios. Our results demonstrated good agreement with measurements of surface temperatures on living corals and that ignoring the presence of polyps by modelling heat transfer associated with a smooth surface makes no material difference to the values obtained or the interpretation of the processes leading to surface warming.

What drives phenotypic divergence among coral clonemates of Acropora palmata?

Evolutionary rescue of populations depends on their ability to produce phenotypic variation that is heritable and adaptive. DNA mutations are the best understood mechanisms to create phenotypic variation, but other, less well-studied mechanisms exist. Marine benthic foundation species provide opportunities to study these mechanisms because many are dominated by isogenic stands produced through asexual reproduction. For example, Caribbean acroporid corals are long lived and reproduce asexually via breakage of branches. Fragmentation is often the dominant mode of local population maintenance. Thus, large genets with many ramets (colonies) are common. Here, we observed phenotypic variation in stress responses within genets following the coral bleaching events in 2014 and 2015 caused by high water temperatures. This was not due to genetic variation in their symbiotic dinoflagellates (Symbiodinium "fitti") because each genet of this coral species typically harbours a single strain of S. "fitti". Characterization of the microbiome via 16S tag sequencing correlated the abundance of only two microbiome members (Tepidiphilus, Endozoicomonas) with a bleaching response. Epigenetic changes were significantly correlated with the host's genetic background, the location of the sampled polyps within the colonies (e.g., branch vs. base of colony), and differences in the colonies' condition during the bleaching event. We conclude that long-term microenvironmental differences led to changes in the way the ramets methylated their genomes, contributing to the differential bleaching response. However, most of the variation in differential bleaching response among clonemates of Acropora palmata remains unexplained. This research provides novel data and hypotheses to help understand intragenet variability in stress phenotypes of sessile marine species.

Prediction of solar irradiance using ray-tracing techniques for coral macro- and micro-habitats

Light distribution on coral reefs is very heterogeneous at the microhabitat level and is an important determinant of coral thermal microenvironments. This study implemented a solar load model that uses a backward ray-tracing method to estimate macroscale and microscale variations of solar irradiance penetrating the ocean surface and impacting the surfaces of coral colonies. We then explored whether morphological characteristics such as tissue darkness (or pigmentation) and thickness may influence the amount of light captured and its spectral distribution by two contrasting coral colony morphologies, branching and massive. Results of global horizontal irradiance above and below the sea-surface and at the surface of coral colonies were validated using spectrometer scans, field measurements, and empirical correlations. The macroscale results of horizontal, irradiated, and shaded irradiance levels and solar altitude angles for PAR, UVA and UVB compared very well with the spectrometer-based observations (typically within < 5%). In general, a comparison between the model results and field and empirical measurements indicated that the contributions of clouds, turbidity, and tides to variations in irradiance at various depth (up to 5 m) were typically within 5-10% of each other. Moreover, the effect of colony darkness or pigmentation on light microenvironment was notably more pronounced for the massive species than branching colony. This study provided insights that species with thinner tissue have the ability to intercept more light with the difference in terms of irradiance levels between 0.1 mm and 0.8 mm tissue thickness for both massive and branching colonies were approximately 2 W m^-2, which was quite unlikely to influence the overall coral heat budgets.

Transcriptional response of the heat shock gene hsp70 aligns with differences in stress susceptibility of shallow-water corals from the Mediterranean Sea

Shallow-water corals of the Mediterranean Sea are facing a dramatic increase in water temperature due to climate change, predicted to increase the frequency of bleaching and mass mortality events. However, supposedly not all corals are affected equally, as they show differences in stress susceptibility, as suggested by physiological outputs of corals along temperature gradients and under controlled conditions in terms of reproduction, demography, growth, calcification, and photosynthetic efficiency. In this study, gene expression and induction of a 70-kDa heat shock protein (HSP70) was analyzed in five common shallow-water hard corals in the Mediterranean Sea, namely Astroides calycularis, Balanophyllia europaea, Caryophyllia inornata, Cladocora caespitosa, and Leptopsammia pruvoti. The main aim was to assess the contribution of this evolutionary conserved cytoprotective mechanism to the physiological plasticity of these species that possess different growth modes (solitary vs colonial) and trophic strategies (zooxanthellate vs azooxanthellate). Using quantitative real-time PCR, in situ hsp70 baseline levels and expression profiles after a heat-shock exposure were assessed. Levels of hsp70 and heat stress induction were higher in zooxanthellate than in azooxanthellate species, and different heat stress transcriptional profiles were observed between colonial and solitary zooxanthellate corals. On the whole, the hsp70 transcriptional response to heat stress aligns with stress susceptibility of the species and suggests a contribution of trophic strategy and morphology in shaping coral resilience to stress. Understanding these molecular processes may contribute to assess the potential effects and relative resilience of Mediterranean corals under climate change.