Unveiling microbiome changes in Mediterranean octocorals during the 2022 marine heatwaves: quantifying key bacterial symbionts and potential pathogens

Abstract

Background Climate change has accelerated the occurrence and severity of heatwaves in the Mediterranean Sea and poses a significant threat to the octocoral species that form the foundation of marine animal forests (MAFs). As coral health intricately relies on the symbiotic relationships established between corals and microbial communities, our goal was to gain a deeper understanding of the role of bacteria in the observed tissue loss of key octocoral species following the unprecedented heatwaves in 2022. Results Using amplicon sequencing and taxon-specific qPCR analyses, we unexpectedly found that the absolute abundance of the major bacterial symbionts, Spirochaetaceae (C. rubrum) and Endozoicomonas (P. clavata), remained, in most cases, unchanged between colonies with 0% and 90% tissue loss. These results suggest that the impairment of coral health was not due to the loss of the main bacterial symbionts. However, we observed a significant increase in the total abundance of bacterial opportunists, including putative pathogens such as Vibrio, which was not evident when only their relative abundance was considered. In addition, there was no clear relation between bacterial symbiont loss and the intensity of thermal stress, suggesting that factors other than temperature may have influenced the differential response of octocoral microbiomes at different sampling sites. Conclusions Our results indicate that tissue loss in octocorals is not directly caused by the decline of the main bacterial symbionts but by the proliferation of opportunistic and pathogenic bacteria. Our findings thus underscore the significance of considering both relative and absolute quantification approaches when evaluating the impact of stressors on coral microbiome as the relative quantification does not accurately depict the actual changes in the microbiome. Consequently, this research enhances our comprehension of the intricate interplay between host organisms, their microbiomes, and environmental stressors, while offering valuable insights into the ecological implications of heatwaves on marine animal forests.

The Changing Biogeography of the Ligurian Sea: Seawater Warming and Further Records of Southern Species

“…Proving that a species no longer exists in a given area is difficult, especially in the marine environment [176]. Frequent and intense heatwaves associated with the periods of rapid warming have caused mass mortalities of many Ligurian Sea autochthonous species [26,177,178], some of which, however, survive in deeper waters [28,179,180]. Primary productivity alteration [181], together with other dysfunctions [182] and disturbances [183], have heavily impacted the Ligurian Sea biota [28].…”

Coral microbiomes are structured by environmental gradients in deep waters

“…Measuring absolute microbial abundances would aid the interpretation of differences in richness and evenness between species. For example, Endozoicomonas dropped in relative abundance in a Mediterranean coral in response to a heatwave, but this was shown to be due to the growth of opportunistic bacteria while the absolute abundance of Endozoicomonas remained the same [ 84 ].…”