Greater functional diversity and redundancy of coral endolithic microbiomes align with lower coral bleaching susceptibility.

The skeleton of reef-building coral harbors diverse microbial communities that could compensate for metabolic deficiencies caused by the loss of algal endosymbionts, i.e., coral bleaching. However, it is unknown to what extent endolith taxonomic diversity and functional potential might contribute to thermal resilience. Here we exposed Goniastrea edwardsi and Porites lutea, two common reef-building corals from the central Red Sea to a 17-day long heat stress. Using hyperspectral imaging, marker gene/metagenomic sequencing, and NanoSIMS, we characterized their endolithic microbiomes together with 15N and 13C assimilation of two skeletal compartments: the endolithic band directly below the coral tissue and the deep skeleton. The bleaching-resistant G. edwardsi was associated with endolithic microbiomes of greater functional diversity and redundancy that exhibited lower N and C assimilation than endoliths in the bleaching-sensitive P. lutea. We propose that the lower endolithic primary productivity in G. edwardsi can be attributed to the dominance of chemolithotrophs. Lower primary production within the skeleton may prevent unbalanced nutrient fluxes to coral tissues under heat stress, thereby preserving nutrient-limiting conditions characteristic of a stable coral-algal symbiosis. Our findings link coral endolithic microbiome structure and function to bleaching susceptibility, providing new avenues for understanding and eventually mitigating reef loss

Projecting coral responses to intensifying marine heatwaves under ocean acidification

Over this century, coral reefs will run the gauntlet of climate change, as marine heatwaves (MHWs) become more intense and frequent, and ocean acidification (OA) progresses. However, we still lack a quantitative assessment of how, and to what degree, OA will moderate the responses of corals to MHWs as they intensify throughout this century. Here, we first projected future MHW intensities for tropical regions under three future greenhouse gas emissions scenario (representative concentration pathways, RCP2.6, RCP4.5 and RCP8.5) for the near-term (2021-2040), mid-century (2041-2060) and late-century (2081-2100). We then combined these MHW intensity projections with a global data set of 1,788 experiments to assess coral attribute performance and survival under the three emissions scenarios for the near-term, mid-century and late-century in the presence and absence of OA. Although warming and OA had predominately additive impacts on the coral responses, the contribution of OA in affecting most coral attributes was minor relative to the dominant role of intensifying MHWs. However, the addition of OA led to greater decreases in photosynthesis and survival under intermediate and unrestricted emissions scenario for the mid- and late-century than if intensifying MHWs were considered as the only driver. These results show that role of OA in modulating coral responses to intensifying MHWs depended on the focal coral attribute and extremity of the scenario examined. Specifically, intensifying MHWs and OA will cause increasing instances of coral bleaching and substantial declines in coral productivity, calcification and survival within the next two decades under the low and intermediate emissions scenario. These projections suggest that corals must rapidly adapt or acclimatize to projected ocean conditions to persist, which is far more likely under a low emissions scenario and with increasing efforts to manage reefs to enhance resilience

Analysis of carbon and nitrogen assimilation in the Aiptasia-Symbiodinium symbioses

NanoSIMS analysis data for region of interests (ROIs) of Aiptaisa-Symbiodinium combinations. Host = host tissue; Sym = Symbiodinium cell; Lip = Lipid body. d13C and d15N enrichment reported in ‰ relative to unlabelled controls. Please refer to linked manuscript for details

Analysis of carbon and nitrogen assimilation in the Aiptasia-Symbiodnium symbioses

NanoSIMS analysis data for region of interests (ROIs) of Aiptaisa-Symbiodinium combinations. Host = host tissue; Sym = Symbiodinium cell; Lip = Lipid body. d13C and d15N enrichment reported in ‰ relative to unlabelled controls. Please refer to linked manuscript for details

Down to the bone: the role of overlooked endolithic microbiomes in reef coral health

© 2019, The Author(s). Reef-building corals harbour an astonishing diversity of microorganisms, including endosymbiotic microalgae, bacteria, archaea, and fungi. The metabolic interactions within this symbiotic consortium are fundamental to the ecological success of corals and the unique productivity of coral reef ecosystems. Over the last two decades, scientific efforts have been primarily channelled into dissecting the symbioses occurring in coral tissues. Although easily accessible, this compartment is only 2–3 mm thick, whereas the underlying calcium carbonate skeleton occupies the vast internal volume of corals. Far from being devoid of life, the skeleton harbours a wide array of algae, endolithic fungi, heterotrophic bacteria, and other boring eukaryotes, often forming distinct bands visible to the bare eye. Some of the critical functions of these endolithic microorganisms in coral health, such as nutrient cycling and metabolite transfer, which could enable the survival of corals during thermal stress, have long been demonstrated. In addition, some of these microorganisms can dissolve calcium carbonate, weakening the coral skeleton and therefore may play a major role in reef erosion. Yet, experimental data are wanting due to methodological limitations. Recent technological and conceptual advances now allow us to tease apart the complex physical, ecological, and chemical interactions at the heart of coral endolithic microbial communities. These new capabilities have resulted in an excellent body of research and provide an exciting outlook to further address the functional microbial ecology of the “overlooked” coral skeleton

Ocean acidification rapidly reduces dinitrogen fixation associated with the hermatypic coral Seriatopora hystrix

Since productivity and growth of coral-associated dinoflagellate algae is nitrogen (N)-limited, dinitrogen (N2) fixation by coral-associated microbes is likely crucial for maintaining the coral-dinoflagellate symbiosis. It is thus essential to understand the effects future climate change will have on N2 fixation by the coral holobiont. This laboratory study is the first to investigate short-term effects of ocean acidification on N2 fixation activity associated with the tropical, hermatypic coral Seriatopora hystrix using the acetylene reduction assay in combination with calcification measurements. Findings reveal that simulated ocean acidification ( pCO2 1080 µatm) caused a rapid and significant decrease (53%) in N2 fixation rates associated with S. hystrix compared to the present day scenario ( pCO2 486 µatm). In addition, N2 fixation associated with the coral holobiont showed a positive exponential relationship with its calcification rates. This suggests that even small declines in calcification rates of hermatypic corals under high CO2 conditions may result in decreased N2 fixation activity, since these 2 processes may compete for energy in the coral holobiont. Ultimately, an intensified N limitation in combination with a decline in skeletal growth may trigger a negative feedback loop on coral productivity exacerbating the negative long-term effects of ocean acidification

Corrigendum: Using aiptasia as a model to study metabolic interactions in cnidarian-Symbiodinium symbioses [Front. Physiol, 9, (2018) (214)] doi: 10.3389/fphys.2018.00214

© 2018 Rädecker, Raina, Pernice, Perna, Guagliardo, Kilburn, Aranda and Voolstra. During submission of the final version of the manuscript for publication, a previous version of Figure 3 was accidentally uploaded. The labeling of this previous Figure version does not match the annotation in the figure legend. The correct version of Figure 3 and its legend appear below. The authors sincerely apologize for the error. This error does not change the scientific conclusions of the research article