Nutrient starvation impairs the trophic plasticity of reef-building corals under ocean warming

WOS:000466375600010International audienceGlobal warming of the world's oceans is driving reef-building corals towards their upper thermal limit, inducing bleaching, nutrient starvation and mortality. In addition, corals are predicted to experience large fluctuations in seawater nutrient concentrations, following water column stratification or eutrophication problems, which can further alter their nutritional capacities and ultimately their resilience to global change. We investigated the effect of thermal stress and dissolved inorganic nutrient (DINUT) availability on the auto- and heterotrophic nutritional capacities of corals. In particular, we assessed the effect of nitrogen enrichment or DINUT depletion (both in nitrogen and in phosphorus) on the assimilation of heterotrophic nutrients and on the heat-stress tolerance of the reef-building coral Stylophora pistillata. Here, we show that DINUT depletion enhanced coral bleaching under thermal stress and more importantly, significantly impaired rates of heterotrophic nutrient assimilation, inducing coral starvation. In contrast, corals grown under nitrogen enrichment maintained high rates of heterotrophic nutrient assimilation and avoided bleaching, although nutrient uptake rates were lowered. We therefore observed a positive coupling between auto- and heterotrophy within the coral-dinoflagellate symbiosis, indicating that heterotrophic processes require a minimum of autotrophically acquired nutrients to be functional. These findings show that the trophic plasticity of corals directly depends on the availability of dissolved inorganic nutrients in seawater. The lack of a shift towards greater heterotrophy under DINUT depletion may lead to substantial modifications of the role that feeding plays in the response of reef-building corals to climate change

Symbiotic stony and soft corals: Is their host‐algae relationship really mutualistic at lower mesophotic reefs?

Mesophotic coral ecosystems (30–150 m depth) present a high oceanic biodiversity, but remain one of the most understudied reef habitats, especially below 60 m depth. Here, we have assessed the rates of photosynthesis and dissolved inorganic carbon (DIC) and nitrogen (DIN) assimilation by Symbiodiniaceae associated with four soft coral species of the genus Sinularia and two stony coral species of the genus Leptoseris collected respectively at 65 and 80–90 m depth in the Gulf of Eilat. Our study demonstrates that both Leptoseris and Sinularia species have limited autotrophic capacities at mid-lower mesophotic depths. DIC and DIN assimilation rates were overall ~ 10 times lower compared to shallow corals from 10 m depth in the same reef. While Leptoseris symbionts transferred at least 50% of the acquired nitrogen to their host after 8-h incubation, most of the nitrogen was retained in the symbionts of Sinularia. In addition, the host tissue of Sinularia species presented a very high structural carbon to nitrogen ratio (C : N) compared to Leptoseris or to the shallow coral species, suggesting nitrogen limitation in these mesophotic soft corals. The limited capacity of soft coral symbionts to acquire DIN and transfer it to the coral animal, as well as the high C : N ratios, might explain the scarcity of symbiotic soft corals at mid-lower mesophotic depths compared to their prevalence in the shallower reef. Overall, this study highlights the significance of DIN for the distribution of the Cnidarian- Symbiodiniaceae association at mesophotic depth

Seasonal Stability in the Microbiomes of Temperate Gorgonians and the Red Coral Corallium rubrum Across the Mediterranean Sea

Populations of key benthic habitat-forming octocoral species have declined significantly in the Mediterranean Sea due to mass mortality events caused by microbial disease outbreaks linked to high summer seawater temperatures. Recently, we showed that the microbial communities of these octocorals are relatively structured; however, our knowledge on the seasonal dynamics of these microbiomes is still limited. To investigate their seasonal stability, we collected four soft gorgonian species (Eunicella singularis, Eunicella cavolini, Eunicella verrucosa and Leptogorgia sarmentosa) and the precious red coral (Corallium rubrum) from two coastal locations with different terrestrial impact levels in the Mediterranean Sea, and used next-generation amplicon sequencing of the 16S rRNA gene. The microbiomes of all soft gorgonian species were dominated by the same 'core microbiome' bacteria belonging to the Endozoicomonas and the Cellvibrionales clade BD1-7, whereas the red coral microbiome was primarily composed of 'core' Spirochaetes, Oceanospirillales ME2 and Parcubacteria. The associations with these bacterial taxa were relatively consistent over time at each location for each octocoral species. However, differences in microbiome composition and seasonal dynamics were observed between locations and could primarily be attributed to locally variant bacteria. Overall, our data provide further evidence of the intricate symbiotic relationships that exist between Mediterranean octocorals and their associated microbes, which are ancient and highly conserved over both space and time, and suggest regulation of the microbiome composition by the host, depending on local conditions.publishe