Shallow Reef Corals Research Articles

Plasticity of shallow reef corals across a depth gradient

Global warming harms coral reefs. Mesophotic coral reef ecosystems (MCEs) have been suggested to serve as refugia for shallow reefs. Information on the adaptation potential of shallow corals at MCEs is a prerequisite for understanding the refuge potential of MCEs. In this study, we investigated the photoacclimation potential of four shallow coral species transplanted at different depths over 1 year. The results showed that the corals—Pocillopora damicornis, Porites cylindrica, and Turbinaria reniformis—survived and acclimated to a wide range of light regimes at the depths of 5, 20, and 40 m. However, Acropora tenuis survived only at 5 and 20 m depth and showed significant morphological alteration at 20 m depth. Our results indicate that shallow corals have substantial plasticity with respect to depth changes. Changes in photosynthetic performance and phenotypic plasticity within these coral species may act as a buffer for depth-related changes and as modulators of evolutionary responses.

Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima

The physiology of ectotherms living in marine environments is strongly influenced by their local thermal experience. Scleractinian corals living near their thermal optimums are increasingly vulnerable to bleaching and mortality as oceanic heat waves increase globally. Mesophotic coral ecosystems (MCEs) below 30 m depths are characteristically cooler than adjacent shallow water reefs, which according to theory should result in differential metabolic responses to temperature between depths. How local temperatures influence physiological responses in mesophotic corals is poorly understood. We compared thermal sensitivities of four coral species between a shallow (5–10 m) and upper-mesophotic (30−35 m) reef in Bermuda. Thermal performance curves (TPC) were measured in laboratory mesocosms for four common coral species (Diploria labyrinthiformis, Orbicella franksi, Montastraea cavernosa and Porites astreoides) across a wide range of temperatures (19−36). Our results indicate that the maximum rate of gross photosynthetic (GP) performance (GP-Pmax) and the mean overall photosynthetic rates (GP-lnc) varied significantly among species. In contrast, thermal sensitivity (Pmax, Topt, lnc, E, Eh, or Th) did not vary between depths for conspecifics except for deactivation energy (GP-Eh) in D. labyrinthiformis. Additionally, gross respiration (R) did not differ among species or between depths for any thermal metric. Similar metabolic responses between depths suggest that local adaptation and/or acclimatization to different thermal conditions is likely not occurring. Instead, upper-mesophotic corals in Bermuda do not have lower bleaching thresholds than shallow water conspecifics, but similar thermal sensitivities supporting the potential for MCEs to function as a thermal refuge.

Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima

Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima

Energy Sources of the Depth-Generalist Mixotrophic Coral Stylophora pistillata.

Energy sources of corals, ultimately sunlight and plankton availability, change dramatically from shallow to mesophotic (30–150 m) reefs. Depth-generalist corals, those that occupy both of these two distinct ecosystems, are adapted to cope with such extremely diverse conditions. In this study, we investigated the trophic strategy of the depth-generalist hermatypic coral Stylophora pistillata and the ability of mesophotic colonies to adapt to shallow reefs. We compared symbiont genera composition, photosynthetic traits and the holobiont trophic position and carbon sources, calculated from amino acids compound-specific stable isotope analysis (AA-CSIA), of shallow, mesophotic and translocated corals. This species harbors different Symbiodiniaceae genera at the two depths: Cladocopium goreaui (dominant in mesophotic colonies) and Symbiodinium microadriaticum (dominant in shallow colonies) with a limited change after transplantation. This allowed us to determine which traits stem from hosting different symbiont species compositions across the depth gradient. Calculation of holobiont trophic position based on amino acid δ15N revealed that heterotrophy represents the same portion of the total energy budget in both depths, in contrast to the dogma that predation is higher in corals growing in low light conditions. Photosynthesis is the major carbon source to corals growing at both depths, but the photosynthetic rate is higher in the shallow reef corals, implicating both higher energy consumption and higher predation rate in the shallow habitat. In the corals transplanted from deep to shallow reef, we observed extensive photo-acclimation by the Symbiodiniaceae cells, including substantial cellular morphological modifications, increased cellular chlorophyll a, lower antennae to photosystems ratios and carbon signature similar to the local shallow colonies. In contrast, non-photochemical quenching remains low and does not increase to cope with the high light regime of the shallow reef. Furthermore, host acclimation is much slower in these deep-to-shallow transplanted corals as evident from the lower trophic position and tissue density compared to the shallow-water corals, even after long-term transplantation (18 months). Our results suggest that while mesophotic reefs could serve as a potential refuge for shallow corals, the transition is complex, as even after a year and a half the acclimation is only partial.

Effects of UV radiation on the growth, photosynthetic and photoprotective components, and reproduction of the Caribbean shallow-water coral Porites furcata

Shallow reef corals can frequently be subjected to high doses of ultraviolet radiation [280–400 nm (UVR)] and have developed mechanisms to cope with this. Nevertheless, slight changes in this stressor may impact their physiology and ultimately their survival. Here, we present results on the effects of artificially enhanced UVR on the growth, reproduction, production of photosynthetic pigments and photoprotective compounds of the Caribbean shallow-water branching coral Porites furcata. Corals were randomly located in one of the three different treatments: normal photosynthetically active radiation (PAR) + UVR; normal PAR+ enhanced UVR; normal PAR+ depleted UVR. Growth rates were measured using the Alizarin red staining method, photosynthetic pigments as well as mycosporine-like amino acids (MAAs) were quantified through high-performance liquid chromatography, and fecundity was estimated after histological analyses. Growth and photosynthetic pigment concentration were negatively correlated with increased UVR, compared to controls exposed to normal UVR. A significant increase in MAAs was also found in colonies under enhanced UVR. Based on their respective concentrations, the primary mycosporine-glycine (λmax = 310 nm) and shinorine (λmax = 333 nm) are the main contributors to UVR absorption in this species, while the levels of the secondary MAA palythine (λmax = 320 nm) tripled toward the end of the 128 days of the experimental period. While several physical factors may influence reef coral physiology, the results suggest that slight increases in UVR can debilitate the skeletal constitution and severely reduce the fecundity of corals living in shallow waters.

Symbiodinium diversity in mesophotic coral communities on the Great Barrier Reef: a first assessment

Despite a growing interest in mesophotic coral ecosystems (MCEs), information on the photosynthetic endosymbionts (genus Symbiodinium) associated with scleractinian corals inhabiting deep reef ecosystems is sparse. Here, the deep-water Symbiodinium diversity is assessed from 10 different coral genera at a depth range of 45 to 70 m on the Great Barrier Reef (GBR), Australia. Symbiodinium identity was established using denaturing gradient gel elec- trophoresis (DGGE) fingerprinting of the internal transcribed spacer region 2 (ITS2) of the riboso- mal DNA. Except for the novel Symbiodinium type C131 (found in Porites), all Symbiodinium types have previously been identified in shallow reef corals across the Pacific. Specimens of Seri- atopora, Montipora, and Porites harboured similar symbionts as reported in shallow water (e.g. C3n, C3n-hh, C15, and C17), thus adhering to patterns of host-specificity across a wide depth range. However, several other Symbiodinium types were found to transcend previously estab- lished patterns of host-specificity at mesophotic depths. For example, 'host-specialist' types C3i and C3k (previously only reported in Acropora spp.) were found here to associate with a range of different genera (Leptoseris, Pachyseris, Fungia, and Echinophyllia). Although limited in sample size, this preliminary survey indicates that mesophotic habitats on the GBR may not represent an isolated community in terms of Symbiodinium diversity, which has significant relevance to their potential to act as refugia. Moreover, the present study identifies the need to examine symbiont diversity across broad environmental ranges (including MCEs) in order to gain an accurate understanding of symbiosis specificity and distribution range of specific coral-Symbiodinium associations.

Mortality of shallow reef corals in the western Arabian Gulf following aerial exposure in winter

Aerial exposure of patch reef corals occurred in Tarut Bay, western Arabian Gulf, (Saudi Arabia) between December 1991 and May 1992, and coincided with extreme low spring tides (below the predicted lowest astronomical tide-LAT). Colonies of Acropora and Stylophora occurring at the highest levels on the tops of patch reef platforms were most affected by the low tides. Corals fully exposed to air suffered total mortality, whereas those not fully exposed suffered tissue damage to their upper parts. Exposure occurred during winter months when air and water temperatures are at their lowest in the gulf. Coupling of extremely low spring tides with wind-induced negative surges (below LAT) are not regular events but are not infrequent. Cold temperatures and exposure may act in concert to produce disproportionate mortalities of reef flat corals in the shallow coastal areas of eastern Saudi Arabia. It is highly unlikely that the Gulf War oil spill played any role in the observed damage to reef corals in the Gulf in 1992.