Heat stress destabilizes symbiotic nutrient cycling in corals

Recurrent mass bleaching events are pushing coral reefs worldwide to the brink of ecological collapse. While the symptoms and consequences of this breakdown of the coral–algal symbiosis have been extensively characterized, our understanding of the underlying causes remains incomplete. Here, we investigated the nutrient fluxes and the physiological as well as molecular responses of the widespread coral Stylophora pistillata to heat stress prior to the onset of bleaching to identify processes involved in the breakdown of the coral–algal symbiosis. We show that altered nutrient cycling during heat stress is a primary driver of the functional breakdown of the symbiosis. Heat stress increased the metabolic energy demand of the coral host, which was compensated by the catabolic degradation of amino acids. The resulting shift from net uptake to release of ammonium by the coral holobiont subsequently promoted the growth of algal symbionts and retention of photosynthates. Together, these processes form a feedback loop that will gradually lead to the decoupling of carbon translocation from the symbiont to the host. Energy limitation and altered symbiotic nutrient cycling are thus key factors in the early heat stress response, directly contributing to the breakdown of the coral–algal symbiosis. Interpreting the stability of the coral holobiont in light of its metabolic interactions provides a missing link in our understanding of the environmental drivers of bleaching and may ultimately help uncover fundamental processes underpinning the functioning of endosymbioses in general.

Intracellular competition for nitrogen controls dinoflagellate population density in corals

Julia Marie Jeanne Yvonne Bodin, Stéphane Laurent Escrig, Maoz Fine, Maria-Evangelia Giovani, Anders Meibom

The density of dinoflagellate microalgae in the tissue of symbiotic corals is an important determinant for health and productivity of the coral animal. Yet, the specific mechanism for their regulation and the consequence for coral nutrition are insufficiently understood due to past methodological limitations to resolve the fine-scale metabolic consequences of fluctuating densities. Here, we characterized the physiological and nutritional consequences of symbiont density variations on the colony and tissue level in Stylophora pistillata from the Red Sea. Alterations in symbiont photophysiology maintained coral productivity and host nutrition across a broad range of symbiont densities. However, we demonstrate that density-dependent nutrient competition between individual symbiont cells, manifested as reduced nitrogen assimilation and cell biomass, probably creates the negative feedback mechanism for symbiont population growth that ultimately defines the steady-state density. Despite fundamental changes in symbiont nitrogen assimilation, we found no density-related metabolic optimum beyond which host nutrient assimilation or tissue biomass declined, indicating that host nutrient demand is sufficiently met across the typically observed range of symbiont densities under ambient conditions.

ROYAL SOC2020

Coral photosymbiosis: Linking phylogenetic identity to single cell metabolic activity in mixed symbiont populations

Isabelle Domart-Coulon, Maoz Fine, Béatrice Gaume, Anders Meibom

Tropical reef-building corals live in symbiosis with a wide range of micro-organisms, including unicellular Symbiodinium dinoflagellates also called zooxanthellae. These photosynthetic endosymbionts live inside the coral gastroderm cells. Although corals can feed on planktonic preys, the dinoflagellates significantly contribute to the nutrition of their host by transferring a large fraction (up to 90%) of photosynthates that are produced through the fixation of dissolved inorganic carbon (DIC) and nitrogen (nitrate or ammonium). Nine major clades (A-I) of Symbiodinium have been identified by molecular genetic analyses. Each clade presents distinct physiological features and the specific association between coral species and Symbiodinium clade determines the phenotype of the holobiont. Differences in the photosynthetic response to irradiance, rates of carbon fixation, and thermal tolerance can be attributed to symbiont clade. Several Symbiodinium clades can simultaneously exist within a single coral and the host can dynamically modify the proportion between dominant and background clades to adapt to changing environmental conditions. Investigating at the cellular level the metabolic exchanges between coral and Symbiodinium is of great interest to understand e.g. the bleaching process (loss of zooxanthellae) which often leads to coral death. We are aiming at quantitatively image the differential metabolic activity between symbiont clades in the same host, in the intact symbiosis. Previous studies have used mass bulk techniques to investigate the metabolism of symbionts at the colony scale. However, such studies cannot determine the specific contribution of the individual cells, as a function of their distribution in the coral host tissue. We have developed a SIMS-ISH method combining nanoscale secondary ion mass spectrometry (NanoSIMS) and in situ hybridization (ISH) for the simultaneous in situ identification of Symbiodinium genotype, and visualization of symbiont-host metabolic exchange at the level of individual cell. We focus on two reef-building coral species Pocillopora damicornis and Stylophora pistillata for which a large amount of complementary metabolic data exists. We designed specific fluorescent DNA probes to identify clade C Symbiodinium in P. damicornis and clade A in S. pistillata, and in mixed cultures. We combined the probes with pulse-chase experiments using isotopically labeled seawater (13C-bicarbonate and 15N-nitrate) to attribute a particular metabolism to a specific clade. This combined method enables us to phylogenetically identify metabolically active cells from a NanoSIMS isotopic/elemental image. The precise correlation between TEM and the NanoSIMS isotopic maps allows us to follow the turnover and translocation of metabolites with sub-cellular precision in both the symbionts and the host. Due to the complex nature of the coral symbiosis, the ability to discriminate the phylogenetic identity and metabolic role of specific Symbiodinium populations in situ is crucial to understand the effects of environmental stress on the coral holobiont plasticity. Moreover, analyzing symbiotic associations in situ provides a unique insight into the spatio-temporal patterns of metabolic interactions in holobionts. This analytical breakthrough promises to open entirely new areas of research focused on understanding the dynamics of interactions between animals and the microbial world.

2017

NanoSIMS isotopic imaging - Deciphering the nature of coral-symbiont metabolic interactions

Isabelle Domart-Coulon, Maoz Fine, Christophe Kopp, Thomas Krüger, Anders Meibom

The ability of endosymbiotic Symbiodinium to fix inorganic nutrients and their translocation to the coral host is considered a key element for the growth of coral reefs in tropical coastal waters and has been a fundamental research topic for over 50 years. With pulse-chase experiments using stable isotopes and combined TEM and NanoSIMS ultrastructural and isotopic analyses it is now possible to visualize and quantify the fixation, translocation, and turnover of essential elements such as carbon and nitrogen with unprecedented subcellular resolution. With these techniques we have monitored the assimilation of 15N nitrate, 15N ammonium, and 13C bicarbonate in the coral Pocillopora damicornis. These studies have revealed dynamic intra-symbiont storage of assimilated nitrate and ammonium in the form of uric acid crystals, and the formation of lipid droplets and starch granules in the symbiont and their subsequent remobilization. Carbon translocation toward the coral gastrodermal lipid droplets was detected within 15 min. Moreover, glycogen granules in the coral tissue were found to be an important sink for translocated carbon. This work has now been extended to the related pocilloporid species, Stylophora pistillata, with the important addition of heterotrophy (feeding the coral with dual-labelled brine shrimps) to contrast the fate and timeframes of heterotrophic vs. autotrophic (15N nitrate/13C bicarbonate) nutrient uptake under natural conditions and environmental stress due to elevated water temperatures. The in situ spatial data provided by NanoSIMS allow us for the first time to address questions that cannot be answered by traditional bulk measurements, such as 1) the influence of local symbiont densities on individual Symbiodinium assimilation rates, 2) metabolic capabilities of different symbiont ITS2 types in hospite, and (3) the effect of changes in intracellular host and symbiosome pH on individual symbiont productivity.