Intertidal Warming Causes Mortality and Disrupts the Microbiome of Oysters.

Intertidal ecosystems are physically stressful habitats, with resident organisms often living close to their limits. These limits include the balance between host organisms and microbial partners; a balance that may be tipped by climate change. We simulated intertidal warming in the field by establishing populations of the Sydney rock oyster, Saccostrea glomerata, on black and white concrete tiles, resulting in differing thermal conditions. Tiles were placed on the intertidal shoreline among natural oyster populations. Oysters on black tiles were up to 3°C warmer than those on white tiles during low tide. We monitored the tiles for oyster survival and took gill and haemolymph samples from oysters for microbiological analysis using qPCR, 16S, and HSP60 rRNA sequencing. We found that after six days, levels of oyster mortality were 50% greater on the black tiles. Oysters on black tiles exhibited a significant shift in their microbiome, involving increases in putative pathogenic bacteria from the Vibrio genus, including the known oyster pathogen V. harveyi and the human pathogen V. parahaemolyticus. These findings demonstrate that relatively small increases in temperature within intertidal ecosystems can cause significant shifts in the microbiome and mortality among oyster populations, with putative links to bacterial pathogens.