HOLOSPONGIA · Sponge diseases under climate change: a multi-omics approach to the holobiont’s molecular functions

The Mediterranean Sea is a climate-change hotspot, which is often considered a miniature model of the world ocean. Therefore, it may allow foreseeing some of the expected global effects of climate change. Sponges hold numerous key roles in benthic ecosystems functioning. In the Mediterranean Sea, they create complex habitats serving as spawning and nursery grounds for fish and invertebrates, and their filter-feeding activity affects entire food webs and bentho-pelagic couplings. These keystone organisms are frequently suffering disease outbreaks and mass mortalities that are mostly triggered by marine heatwaves. Sponges are well-known holobionts that host complex symbiotic microbial communities.While some members of these communities offer adaptive advantages, others may contribute to disease outbreaks when environmental conditions change.This interdisciplinary research aims to discover the complex mechanism of thermal-stress-related necrosis and mortality of the sponge holobiont by studying the effect of heatwaves on the Mediterranean bath sponge Spongia officinalis through a holistic approach that includes morphological, transcriptomic, metagenomic, and metabolomic responses of the holobiont. To study causative as well as correlative disease events, the experimental setting of this project will combine in situ sampling of healthy and diseased individuals, as well as exposure of individuals kept in aquaria to simulated marine heatwaves. These will provide the first insight, not only to the change in the microbial community composition but alsoto the functionality of its members during thermal stress. It is imperative to understand the mechanism of disease prevalence and mortality and to likely suggest ways to mitigate this phenomenon. The expected increasing frequency of marine heatwaves can cause irreversible damage to entire ecosystems that rely on sessile invertebrates such as sponges and thus to numerous ecosystem services provided to human societies.