Diel thermal variability does little to mitigate bleaching in two endangered coral holobionts

Research has shown that tropical corals inhabiting warmer, more thermally variable reef environments often exhibit reduced bleaching. However, the influence of diel thermal variation (DTV) is rarely tested experimentally independent of changes in mean temperature, and few studies have explored responses of coral hosts together with their associated microbial consortia (Symbiodiniaceae algae and bacteria). Here, using a common garden approach with two species of endangered reef-building corals (Orbicella franksi & O. faveolata), we show that 90 days of DTV priming (26{degrees}C {+/-} 2.4{degrees}C day-1) elicited little physiological or gene expression plasticity and did not alter Symbiodiniaceae or bacterial compositions relative to corals under stable conditions (26{degrees}C). Critically, when these corals experienced a subsequent heat challenge (32{degrees}C for 15 days), both species exhibited broad physiological and gene expression responses regardless of DTV priming. A systems biology approach linking gene expression with experimental treatments and traits supports the presence of both 1) convergent gene modules associated with homeostasis in both host species and 2) divergent gene modules showcasing enrichment of catabolism in O. faveolata and classic protein folding functions in O. franksi under thermal challenge. Together, these data highlight that while congeneric coral species exhibit divergent gene expression responses to thermal challenges, DTV priming did not facilitate thermal stress resistance. These findings suggest that other environmental parameters that co-vary with DTV on a reef may play a more influential role in bleaching mitigation.