Reef-building corals typically live close to the upper limits of their thermal tolerance and even small increases in summer water temperatures can lead to bleaching and mortality. Projections of coral reef futures based on forecasts of ocean temperatures indicate that by the end of this century, corals will experience their current thermal thresholds annually, which would lead to the widespread devastation of coral reef ecosystems. Here, we use skeletal cores of long-lived Porites corals collected from 14 reefs across the northern Great Barrier Reef, the Coral Sea, and New Caledonia to evaluate changes in their sensitivity to heat stress since 1815. High-density ‘stress bands’—indicative of past bleaching—first appear during a strong pre-industrial El Niño event in 1877 but become significantly more frequent in the late twentieth and early twenty-first centuries in accordance with rising temperatures from anthropogenic global warming. However, the proportion of cores with stress bands declines following successive bleaching events in the twenty-first century despite increasing exposure to heat stress. Our findings demonstrate an increase in the thermal tolerance of reef-building corals and offer a glimmer of hope that at least some coral species can acclimatize fast enough to keep pace with global warming.

Coral reefs are increasingly in jeopardy due to global changes affecting both reef accretion and bioerosion processes. Bioerosion processes dynamics in dead reef carbonates under various environmental conditions are relatively well understood but only over a short-term limiting projections of coral reef evolution by 2100. It is thus essential to monitor and understand bioerosion processes over the long term. Here we studied the assemblage of traces of microborers in a coral core of a massive Diploastrea sp. from Mayotte, allowing us to explore the variability of its specific composition, distribution, and abundance between 1964 and 2018. Observations of microborer traces were realized under a scanning electron microscope (SEM). The area of coral skeleton sections colonized by microborers (a proxy of their abundance) was estimated based on an innovative machine learning approach. This new method with 93% accuracy allowed analyzing rapidly more than a thousand SEM images. Our results showed an important shift in the trace assemblage composition that occurred in 1985, and a loss of 90% of microborer traces over the last five decades. Our data also showed a strong positive correlation between microborer trace abundance and the coral bulk density, this latter being particularly affected by the interannual variation of temperature and cumulative insolation. Although various combined environmental factors certainly had direct and/or indirect effects on microboring species before and after the breakpoint in 1985, we suggest that rising sea surface temperature, rainfall, and the loss of light over time were the main factors driving the observed trace assemblage change and decline in microborer abundance. In addition, the interannual variability of sea surface temperature and instantaneous maximum wind speed appeared to influence greatly the occurrence of green bands. We thus stress the importance to study more coral cores to confirm the decadal trends observed in the Diploastrea sp. from Mayotte and to better identify the main factors influencing microboring communities, as the decrease of their abundance in living massive stress tolerant corals may have important consequences on their resilience.

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<ul><li>Coral reefs are increasingly in jeopardy due to global changes which affect both reef accretion and bioerosion processes. Among those processes, microborers, and especially the chlorophyte <em>Ostreobium</em> sp., play a major role in reef carbonate dissolution. The dynamics of this process in dead reef carbonates under various environmental factors such as ocean acidification, sedimentation, and eutrophication began to be relatively well understood over a short period of time (month to year scale).  In contrast, the long-term effects of environmental factors on reef microboring communities and their erosive activity remain poorly known, limiting predictions of coral reef evolution by 2100. Massive coral colonies are great bio-carbonate archives recording environmental conditions over decades and are known to be colonized by microboring floras and especially the chlorophyte <em>Ostreobium</em> sp., forming sometimes eye visible green bands.  Massive corals offer therefore the opportunity to study the long-term effects of environmental changes on microboring communities and to understand the possible implication of green bands in coral resilience. Here we studied microboring communities along a coral core of a massive <em>Diploastrea</em> sp. collected at 15 m depth on the outer slope of the northeastern barrier reef in Mayotte in October 2018. The studied coral core length (~15 cm) allowed to determine the coral vertical extension rate reconstructed via an X-ray image analysis, its skeletal density based on new image analysis of a CT scan, and microboring community abundance based on an innovative machine learning approach over the last 50 years. The machine learning approach (with a precision of 93%) allowed analyzing very quickly hundreds of scanning electronic images taken along the coral core to quantify the surface area occupied by microboring galleries within the coral skeleton. Our results show a shift in microboring community composition at a breakpoint around the ’80s (1985 – 1986). Before the ’80s, the community was dominated by large galleries mainly distributed along the main growth axis of the coral colony (most probably made by phototrophic microborers) while after the ’80s the community was mainly dominated by two types of thinner galleries widely distributed within the coral skeleton. Surprisingly, our results also revealed a significant decrease in microboring galleries’ abundance over the last fifty years. Important abundances were not correlated to the presence of green bands nor to the coral vertical extension rate but were positively correlated to the skeleton density. Those trends will be discussed in the light of historical temperature change, temperature anomalies, precipitations, wind, insolation period, and the measured coral skeleton parameters (density, coral extension rate, and calcification rate) to highlight the possible main drivers influencing microborer abundance in massive corals.</li> </ul>