Marine heatwaves alter shell microbiomes and denitrification capacity: An oyster family-specific response

Marine heatwaves (MHWs) are increasing in frequency and intensity, significantly affecting biodiversity and ecosystem processes. However, the impact of MHWs on microbiome structure and function, and whether such changes are influenced by intraspecific differences among hosts, remains unclear. This study investigated the effect of a simulated MHW on external shell microbiomes (alpha diversity, community and taxonomic compositions, and denitrification capacity) of four oyster families (1, 11, 14 and 15), with different levels of disease resistance and growth rate. Oysters were exposed to 29 °C (MHW) and 24 °C (control) temperatures for 6 days, followed by a 21-day recovery period. The MHW shifted bacterial community composition and decreased alpha diversity in Family 14, together with reductions in the relative abundance of Pseudomonadota and Planctomycetota, and in denitrification genes (nirK and nosZ). In Family 15, MHW exposure decreased the abundance of Rhizobiaceae and the denitrification gene nirS, which recovered within three weeks. Reduction in denitrification genes suggests a potential decline in nitrogen removal capacity, which could negatively affect coastal systems, as bioavailable nitrogen may accumulate and lead to eutrophication. Conversely, in Families 1 and 11 the shell microbiomes remained stable under heat stress. Although mechanisms were not directly assessed, these faster-growing families may possess physiological traits (e.g. efficient metabolism and filtration) that support the resilience of nitrogen-cycling microbes during thermal stress. This study highlights the importance of intraspecific differences among hosts in shaping microbiome responses to MHWs, suggesting that selecting certain oyster families could help maintain key microbial-driven processes like denitrification under climate stress.