Ability Of Corals Research Articles

Crystal orientation mapping and microindentation reveal anisotropy in Porites skeletons

Structures made by scleractinian corals support diverse ocean ecosystems. Despite the importance of coral skeletons and their predicted vulnerability to climate change, few studies have examined the mechanical and crystallographic properties of coral skeletons at the micro- and nano-scales. Here, we investigated the interplay of crystallographic and microarchitectural organization with mechanical anisotropy within Porites skeletons by measuring Young’s modulus and hardness along surfaces transverse and longitudinal to the primary coral growth direction. We observed micro-scale anisotropy, where the transverse surface had greater Young’s modulus and hardness by ∼ 6 GPa and 0.2 GPa, respectively. Electron backscatter diffraction (EBSD) revealed that this surface also had a higher percentage of crystals oriented with the a-axis between ± 30–60∘, relative to the longitudinal surface, and a broader grain size distribution. Within a region containing a sharp microscale gradient in Young’s modulus, nanoscale indentation mapping, energy dispersive spectroscopy (EDS), EBSD, and Raman crystallography were performed. A correlative trend showed higher Young’s modulus and hardness in regions with individual crystal bases (c-axis) facing upward, and in crystal fibers relative to centers of calcification. These relationships highlight the difference in mechanical properties between scales (i.e. crystals, crystal bundles, grains). Observations of crystal orientation and mechanical properties suggest that anisotropy is driven by microscale organization and crystal packing rather than intrinsic crystal anisotropy. In comparison with previous observations of nanoscale isotropy in corals, our results illustrate the role of hierarchical architecture in coral skeletons and the influence of biotic and abiotic factors on mechanical properties at different scales. Statement of significanceCoral biomineralization and the ability of corals’ skeletal structure to withstand biotic and abiotic forces underpins the success of reef ecosystems. At the microscale, we show increased skeletal stiffness and hardness perpendicular to the coral growth direction. By comparing nano- and micro-scale indentation results, we also reveal an effect of hierarchical architecture on the mechanical properties of coral skeletons and hypothesize that crystal packing and orientation result in microscale anisotropy. In contrast to previous findings, we demonstrate that mechanical and crystallographic properties of coral skeletons can vary between surface planes, within surface planes, and at different analytical scales. These results improve our understanding of biomineralization and the effects of scale and direction on how biomineral structures respond to environmental stimuli.

Seabird-Derived Nutrients Supply Modulates the Trophic Strategies of Mixotrophic Corals

The ability of corals to modulate their nutrition strategy in response to variable nutrient supply remains poorly understood, limiting our understanding of energy flow in coral reef ecosystems and thus our comprehension of their resilience to global changes. We used a naturally occurring nutrient gradient along the reef flat of two seabird-inhabited islets in the SW Pacific to characterize spatiotemporal fluctuations in coastal nutrient availability, and how it modulates the trophic response of the mixotrophic coral Pocillopora damicornis. The clear gradients in dissolved [NOx] and δ15N values of macroalgae and both P. damicornis tissues and symbionts observed along the reef flat during the dry and the rainy season revealed that seabird-derived-N is supplied year-round to the reef flat. Yet, nitrogen isotope values of macroalgae show that the seabirds’ effect on coral reefs varies with sites and seasons. Metrics derived from the SIBER framework revealed that coral nutrition seasonally favored autotrophy when exposed to higher seabird guano concentrations and at inshore stations, while heterotrophy dominated in corals less exposed to seabird-derived nutrient supply. P. Damicornis is therefore able to cope with large changes in nitrogen supply induced by seabird island communities by switching between autotrophy and heterotrophy. These results shed light on the flexibility of resource sharing within the coral-algae symbiosis and highlight the importance of seabird populations to the functioning of coral reef ecosystems.

Inter-partner specificity limits the acquisition of thermotolerant symbionts in a model cnidarian-dinoflagellate symbiosis.

The ability of corals and other cnidarians to survive climate change depends partly on the composition of their endosymbiont communities. The dinoflagellate family Symbiodiniaceae is genetically and physiologically diverse, and one proposed mechanism for cnidarians to acclimate to rising temperatures is to acquire more thermally tolerant symbionts. However, cnidarian-dinoflagellate associations vary in their degree of specificity, which may limit their capacity to alter symbiont communities. Here, we inoculated symbiont-free polyps of the sea anemone Exaiptasia pallida (commonly referred to as 'Aiptasia'), a model system for the cnidarian-dinoflagellate symbiosis, with simultaneous or sequential mixtures of thermally tolerant and thermally sensitive species of Symbiodiniaceae. We then monitored symbiont success (relative proportional abundance) at normal and elevated temperatures across two to four weeks. All anemones showed signs of bleaching at high temperature. During simultaneous inoculations, the native, thermally sensitive Breviolum minutum colonized polyps most successfully regardless of temperature when paired against the non-native but more thermally tolerant Symbiodinium microadriaticum or Durusdinium trenchii. Furthermore, anemones initially colonized with B. minutum and subsequently exposed to S. microadriaticum failed to acquire the new symbiont. These results highlight how partner specificity may place strong limitations on the ability of certain cnidarians to acquire more thermally tolerant symbionts, and hence their adaptive potential under climate change.

Can benthic algae mediate larval behavior and settlement of the coral Acropora muricata?

The resilience of coral reefs relies significantly on the ability of corals to recover successfully in algal-dominated environments. Larval settlement is a critical but highly vulnerable stage in the early life history of corals. In this study, we analyzed how the presence of two upright fleshy algae, Sargassum mcclurei (SM) and Padina australis (PA), and one crustose coralline algae, Mesophyllum simulans (MS), affects the settlement of Acropora muricata larvae. Coral larvae were exposed to seawater flowing over these algae at two concentrations. Larval settlement and mortality were assessed daily through four variables related to their behavior: swimming, substratum testing, metamorphosis, and stresses. Temperature, dissolved oxygen, pH, algal growth, and photosynthetic efficiency were monitored throughout the experiment. Results showed that A. muricata larvae can settle successfully in the absence of external stimuli (63 ± 6 % of the larvae settled in control treatments). While algae such as MS may stimulate substrate testing and settlement of larvae in the first day after competency, they ultimately had a lower settlement rate than controls. Fleshy algae such as PA, and in a lesser measure SM, induced more metamorphosis than controls and seemed to eventually stimulate settlement. A diverse combination of signals and/or modifications of microenvironments by algae and their associated microbial communities may explain the pattern observed in coral settlement. Overall, this study contributes significantly to the knowledge of the interaction between coral and algae, which is critical for the resilience of the reefs.

Physiological responses to oxidative stress associated with pH variations in host tissue and zooxanthellae of hermatypic coral Pocillopora capitata

To study the adaptive mechanisms of host and symbiont in a decrease in pH, coral branches of Pocillopora capitata from La Boquita reef in the Bay of Santiago, México were assigned to experimental units (n = 38 per treatment) and three pH treatments were evaluated: (a) pH 8.00–8.40 (control; Treatment C1), (b) pH 7.85–7.95 (Treatment C2) and (c) pH 7.60–7.70 (Treatment C3). Corals were collected at the beginning of the experiment (T0) and at different times: T1 (5 h), T2 (12 h), T3 (48 h) and T4 (168 h). We examined malondialdehyde (MDA), antioxidant enzyme activities (SOD and CAT), antioxidant capacity and chlorophyll a (Chl a) content. The results showed a different response to the treatment between zooxanthellae and Cnidaria tissues. Initially in zooxanthellae, we observed a SOD response for the C3 treatment; however this response did not prevent lipid degradation. In Cnidaria, the C3 resulted in a greater SOD activity from the middle to the end of the experiment. The results improve our understanding of the effects of ocean acidification, the ability of coral to respond to environmental changes and the use of biochemical indicators related to the physiological condition for the recognition of coral communities affected by a decreases in pH.