Fine-Scale Skeletal Banding Can Distinguish Symbiotic from Asymbiotic Species among Modern and Fossil Scleractinian Corals

Understanding the evolution of scleractinian corals on geological timescales is key to predict how modern reef ecosystems will react to changing environmental conditions in the future. Important to such efforts has been the development of several skeleton-based criteria to distinguish between the two major ecological groups of scleractinians: zooxanthellates, which live in symbiosis with dinoflagellate algae, and azooxanthellates, which lack endosymbiotic dinoflagellates. Existing criteria are based on overall skeletal morphology and bio/geo-chemical indicators-none of them being particularly robust. Here we explore another skeletal feature, namely fine-scale growth banding, which differs between these two groups of corals. Using various ultra-structural imaging techniques (e.g., TEM, SEM, and NanoSIMS) we have characterized skeletal growth increments, composed of doublets of optically light and dark bands, in a broad selection of extant symbiotic and asymbiotic corals. Skeletons of zooxanthellate corals are characterized by regular growth banding, whereas in skeletons of azooxanthellate corals the growth banding is irregular. Importantly, the regularity of growth bands can be easily quantified with a coefficient of variation obtained by measuring bandwidths on SEM images of polished and etched skeletal surfaces of septa and/or walls. We find that this coefficient of variation (lower values indicate higher regularity) ranges from similar to 40 to similar to 90% in azooxanthellate corals and from similar to 5 to similar to 15% in symbiotic species. With more than 90% (28 out of 31) of the studied corals conforming to this microstructural criterion, it represents an easy and robust method to discriminate between zooxanthellate and azooxanthellate corals. This microstructural criterion has been applied to the exceptionally preserved skeleton of the Triassic (Norian, ca. 215 Ma) scleractinian Volzeia sp., which contains the first example of regular, fine-scale banding of thickening deposits in a fossil coral of this age. The regularity of its growth banding strongly suggests that the coral was symbiotic with zooxanthellates.

Physiological and isotopic responses of scleractinian corals to ocean acidification

Maoz Fine, Anders Meibom

Uptake of anthropogenic CO2 by the oceans is altering seawater chemistry with potentially serious consequences for coral reef ecosystems due to the reduction of seawater pH and aragonite saturation state (Omega(arag)) The objectives of this long-term study were to investigate the viability of two ecologically important reef-building coral species, massive Ponies sp. and Stylophora pistillata, exposed to high pCO(2) (or low pH) conditions and to observe possible changes in physiologically related parameters as well as skeletal isotopic composition. Fragments of Ponies sp. and S. pistillata were kept for 6-14 months under controlled aquarium conditions characterized by normal and elevated pCO(2) conditions, corresponding to pH(T) values of 8.09, 7.49, and 7.19, respectively. In contrast with shorter, and therefore more transient experiments, the long experimental time-scale achieved in this study ensures complete equilibration and steady state with the experimental environment and guarantees that the data provide insights into viable and stably growing corals. During the experiments, all coral fragments survived and added new skeleton, even at seawater Omega(arag) < 1, implying that the coral skeleton is formed by mechanisms under strong biological control. Measurements of boron (B), carbon (C), and oxygen (O) isotopic composition of skeleton, C isotopic composition of coral tissue and symbiont zooxanthellae, along with physiological data (such as skeletal growth, tissue biomass, zooxanthellae cell density, and chlorophyll concentration) allow for a direct comparison with corals living under normal conditions and sampled simultaneously. Skeletal growth and zooxanthellae density were found to decrease, whereas coral tissue biomass (measured as protein concentration) and zooxanthellae chlorophyll concentrations increased under high pCO(2) (low pH) conditions. Both species showed similar trends of delta B-11 depletion and delta O-18 enrichment under reduced pH, whereas the delta C-13 results imply species-specific metabolic response to high pCO(2) conditions. The skeletal delta B-11 values plot above seawater delta B-11 vs. pH borate fractionation curves calculated using either the theoretically derived alpha(B) value of 1.0194 (Kakihana et al. (1977) Bull. Chem. Soc. Jpn. 50, 158) or the empirical alpha(B) value of 1.0272 (Klochko et al. (2006) EPSL 248, 261). However, the effective alpha(B) must be greater than 1.0200 in order to yield calculated coral skeletal delta B-11 values for pH conditions where Omega(arag) >= 1. The delta B-11 vs. pH offset from the seawater delta B-11 vs. pH fractionation curves suggests a change in the ratio of skeletal material laid down during dark and light calcification and/or an internal pH regulation, presumably controlled by ion-transport enzymes. Finally, seawater pH significantly influences skeletal delta C-13 and delta O-18. This must be taken into consideration when reconstructing paleo-environmental conditions from coral skeletons. (C) 2010 Elsevier Ltd. All rights reserved.

2010

Symbiont diversity and coral bleaching: An antioxidant view on thermal stress

Thomas Krüger

The functioning of coral reef systems, as biodiversity hotspots, is largely dependent on the symbiotic association between dinoflagellate symbionts (Symbiodinium spp.) and scleractinian coral hosts. The breakdown of this symbiosis (coral bleaching), as a result of global warming and other stressors, therefore has profound implications for the tropical marine environment. Corals associate with a variety of Symbiodinium genotypes, and it is this mosaic nature that contributes to the variable stress thresholds of corals. Research over the past 25 years has established that the generation and scavenging of reactive oxygen species (ROS) in both partners, under light and thermal stress, is a fundamental element of the bleaching response. However, while the existence of more thermally susceptible and tolerant symbiont types has been recognized, the differences in the antioxidant systems that may accompany these properties have received less attention. The purpose of this thesis was to explore the role of the antioxidant network in explaining the different thermal susceptibilities of various symbiont types and how the activity of key antioxidants in both partners under thermal stress relates to bleaching patterns in different corals. Thus, the specific objectives were to: (1) assess the antioxidant network response in different Symbiodinium types; (2) investigate the activity and structural diversity of key enzymatic antioxidants in different Symbiodinium types; (3) examine the regulation of these antioxidants at the transcriptomic and proteomic levels; and (4) contrast the symbiont’s and host’s antioxidant responses under bleaching conditions. Symbiodinium types in culture were found to differ significantly with regards to the concentration and activity of specific antioxidants, exhibiting magnitude scale differences in some of them. However, the response of the main removal pathway, involving superoxide dismutase (SOD) and ascorbate peroxidase (APX), under lethal thermal stress was fairly similar. Instead, the typespecific differences were found to lie in more downstream systems, and particularly in those associated with the maintenance of the glutathione redox state. A declining glutathione redox state was the common feature of the three thermally susceptible Symbiodinium types: B1, C1, and E. Indeed, in comparison to the most sensitive type (B1), the tolerant type F1 exhibited stronger antioxidant up-regulation and the successful preservation of the highly reduced glutathione pool. Comparing antioxidant gene orthologues from members of different Symbiodinium clades (A-E) revealed a higher degree of sequence variation at the amino acid level for peroxidases, which reflected the genetic radiation of the genus. In contrast, primary defences in the form of SOD isoforms were highly conserved. Sequence variations between Symbiodinium types involved residues that constitute binding sites of substrates and co-factors, and therefore likely affect the catalytic properties of these enzymes. While expression of antioxidant genes was successfully measured in Symbiodinium B1, it was not possible to assess the link between transcriptomic expression and proteomic activity due to high variability in expression between replicates, and little response in their enzymatic activity over three days. In contrast to previous findings, up-regulation of antioxidant defences was not evident in Symbiodinium cells inside the host (i.e. in hospite). In fact, oxidative stress in the thermally sensitive corals Acropora millepora and Pocillopora damicornis was only apparent from increased host catalase activity, which interestingly preceded photosynthetic dysfunction of their symbionts. Baseline antioxidant activities of thermally tolerant and susceptible host species showed no differences, though the scavenging activities of the hosts were considerably higher than those of the symbionts. Baseline activities for the symbionts were different, however, with Symbiodinium C15 from the thermally tolerant coral Montipora digitata exhibiting the lowest activities for SOD and catalase peroxidase. This thesis provides significant findings with respect to the variability in antioxidant activity, structure, and network response in different Symbiodinium types in culture, and how these relate to thermal tolerance. What effect these differences have on the response in the intact symbiosis remains unclear, however, as the findings contradict the classic bleaching model of photoinhibition and symbiont-derived ROS. I argue, using previously published data, that heating rates might profoundly affect the way we perceive the antioxidant response of both partners to thermal stress, and that host antioxidant defences might not be as easily overwhelmed by symbiont ROS as suggested previously. This thesis reports important findings on the antioxidant system in different Symbiodinium types, but also raises new questions about the antioxidant response of the intact coral.

Victoria University of Wellington2014

Role of photosymbiosis in the Late Triassic expansion of shallow-water Scleractinia

Anders Meibom

Since their appearance in the Anisian (ca. 240 Ma) scleractinian corals rapidly diversified and expandedacross shallow marine environments, becoming the main builders of reefs. This ecological success coralsowe to their symbiotic relationship with photosynthesizing dinoflagellate algae. Although mechanismsunderlying this relationship have been broadly studied, fundamental questions regarding its origin andevolution remain open. The main obstacle in studies of symbiosis in the past is that coral skeletons do notpreserve zooxanthellae. Conventionally, zooxanthellate corals are regarded as forming highly integratedcolonies with small corallites, whereas the corals lacking zooxanthellae tend to have solitary forms or lessintegrated colonies with relatively large corallites. These features, however, are not exclusive for eitherecological group, pointing to the need for more definitive indicators. Isotopic signatures of the skeletons i.e.,carbonate 13C/12C and 18O/16O, and intracrystalline skeletal organic matter (SOM) 15N/14N have been also usedto distinguish symbiotic from asymbiotic corals. However, their application is restricted only to unalteredskeleton. This feature, although rare in the fossil record, was reported in coral skeletons as old as Triassic.Recent development of two new methods gives opportunity for investigation of symbiosis in fossilscleractinians: microscale growth banding and “persulfate-denitrifier” high precision analysis of nitrogenisotopic composition of intracrystalline SOM. These methods were applied to a set of exceptionallypreserved skeletons of Early Norian (ca. 212 Ma) scleractinian corals from Antalya (Turkey). Selectedskeletons represented both, solitary or poorly integrated (phaceloid) and highly integrated (cerioid,meandroid and thamnasterioid) growth forms. Microscale growth bands of Triassic corals were compared with those of living scleractinians. Theexamined fossil specimens, including forms traditionally considered as asymbiotic, exhibited a highlyregular and continuous banding pattern typical for modern zooxanthellates, contrary to modern asymbioticcorals with irregular, often discontinuous bands. Nitrogen isotopic signatures of intracrystalline SOM inTriassic scleractinians fall below the range measured for modern asymbiotic corals, but overlaps data formmodern zooxanthellates, indicating that each of examined Triassic corals harbored photosymbionts. If theseEarly Norian corals were symbiotic, their δ15N suggests that local source values were similar to those ofmodern oligotrophic (sub)tropical North Atlantic. In fact, similarity between δ15N from Triassic corals andmodern Bermuda specimens suggests that they lived in similarly nutrient-poor waters. Further analyses showthat Triassic specimens with pristine biogenic aragonite exhibit oxygen and carbon isotopic signaturescompatible with those of modern symbiotic corals. The presented combination of microstructural and geochemical indicators, provides a new, powerfultoolkit for assessing symbiosis in well-preserved fossil corals. All criteria support our interpretation thatEarly Norian corals from Antalya lived in symbiosis with dinoflagellates, including small solitary andphaceloid forms, which based on morphology alone would be classified as asymbiotic. Since coral generaexamined herein were widespread in the Late Triassic reefs of NW Tethys, we propose that symbiosis wasthe dominant if not the exclusive lifestyle in shallow-water corals from the Tethyan realm. Theseoligotrophic environments, provided strong impetus for establishing of symbiosis with photosynthesizingalgae. Since the Triassic, the symbiotic relationship of corals and zooxanthellates facilitated the formation ofthe greatest oceanic bioconstructions - coral reefs.

2019