Abstract
Most tropical reef corals live at temperatures near 27°C and pH values near 8. Conditions outside of these can stress corals and lead to bleaching, disease, and death. However, some corals can survive in marginal or extreme habitats outside of these ranges. To date there is a paucity of knowledge about the role that associated microbes may play in the acclimation of corals to such extreme habitats. Here, we explore differences in the compositional and functional profile of the microbiomes of the scleractinian coral Porites panamensis living both on and off potentially stressful shallow-water hydrothermal vents. The environment near the vents is extreme, with temperatures exceeding 80°C and pH values below 6. Coral microbiomes under stress often exhibit increased diversity, increased abundance of pathogenic bacteria, and functional profiles that shift toward pathways associated with pathogenic taxa. Samples from along a transect that crossed an arc of hydrothermal vents were sequenced for the 16S rRNA gene (V4 region). On-vent coral microbiomes were distinct from those of off-vent corals, but did not have increased alpha or beta diversity. On-vent samples had a higher relative abundance of the beneficial endosymbiont, Endozoicomonas. On- and off-vent microbiomes did not differ in overall abundance of the endolithic green alga Ostreobium, however, a single ASV, close to O. quekettii was more abundant in on-vent corals. Ostreobium can provide many of the same benefits to corals as zooxanthellae and their symbiosis is better maintained under thermal stress. Surprisingly, on-vent coral microbiomes had fewer microbial taxa that are known to be pathogenic or associated with stress than did off-vent corals. The predicted functional profiles of on-vent microbiomes revealed enrichment of pathways related to aerobic respiration, fermentation and amino acid biosynthesis, but not of virulence-related pathways. Our results suggest that P. panamensis microbiomes have acclimated to the extreme environment of the hydrothermal vent habitat rather than showing signs of stress. These results exemplify the need to focus efforts on examining the mechanisms of resilience, including symbioses with microbiota, in corals living in extreme environments in an effort to design better management strategies for reef-building corals under thermal and pH stress.
Highlights
Reef-building corals are declining globally in response to ocean acidification and thermal stresses associated with rapidly changing seas (Gardner et al, 2003; Carpenter et al, 2008; Toth et al, 2019)
Post-Quality control (QC), P. panamensis samples retained 3,644,484 microbial reads that clustered into 11,422 unique Amplicon Sequence Variants (ASVs) and 1,167,759 Ostreobium reads (Supplementary Table 1) in 47 ASVs
On-vent coral microbiomes were not enriched for typical stressrelated pathways such as antimicrobial production, bacterial motility, chemotaxis and invasion (Thurber et al, 2009; Zaneveld et al, 2016) nor did we find any increases in pathogenic taxa like Vibrio spp. (Ben-Haim et al, 2003; Thurber et al, 2009)
Summary
Reef-building corals are declining globally in response to ocean acidification and thermal stresses associated with rapidly changing seas (Gardner et al, 2003; Carpenter et al, 2008; Toth et al, 2019). Corals from more challenging and variable habitats generally have higher phenotypic plasticity and can withstand more stress than others (Oliver and Palumbi, 2011; Barshis et al, 2013; Putnam et al, 2016). It is not always clear how much of these adaptive responses are owed to the coral host and how much falls to the many coral-associated symbionts (Bourne et al, 2016; van Oppen et al, 2018)
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