Diurnal temperature fluctuations drive compartment-specific microbial dynamics in tissue and skeleton of Stylophora pistillata from marginal reefs

Abstract

Coral-associated microbiota are dynamic and influenced by host identity, distinct microhabitats within coral tissue and skeleton, and surrounding environmental conditions. While increasing sea surface temperatures (SSTs) are known to disrupt coral-Symbiodiniaceae associations, most experimental studies of coral thermal stress have relied on constant or slowly changing temperature regimes that do not capture the pronounced diurnal temperature variability experienced by corals in shallow reef environments. In this study, we investigated bacterial community dynamics in coral tissue and skeleton of Stylophora pistillata under environmentally realistic, ecologically relevant diurnal temperature fluctuations. Corals were exposed to three temperature regimes, including a constant temperature control (26 degrees C), diurnal oscillations of +/- 3 degrees C, and diurnal oscillations combined with a gradual increase in mean temperature that remained below established bleaching thresholds, with cumulative thermal exposure quantified using Degree Heating Weeks (DHW). No visible bleaching or significant changes in photosynthetic performance were observed across the four-week experiment. However, bacterial communities associated with coral tissue exhibited pronounced temporal shifts under different diurnal temperature regimes, whereas endolithic microbial communities remained comparatively stable. These results indicate that coral tissue-associated microbiomes respond sensitively to short-term thermal variability prior to overt host-level stress responses. Overall, our findings highlight the importance of incorporating high-frequency temperature variability into experimental designs and environmental assessments aimed at understanding coral-microbe interactions in thermally dynamic reef environments.