Abstract
Coral bleaching, i.e., the loss of photosynthetic algal endosymbionts, caused by ocean warming is now among the main factors driving global reef decline, making the elucidation of factors that contribute to thermotolerance important. Recent studies implicate high salinity as a contributing factor in cnidarians, potentially explaining the high thermotolerance of corals from the Arabian Seas. Here we characterized bacterial community composition under heat stress at different salinities using the coral model Aiptasia. Exposure of two Aiptasia host-algal symbiont pairings (H2-SSB01 and CC7-SSA01) to ambient (25°C) and heat stress (34°C) temperatures at low (36 PSU), intermediate (39 PSU), and high (42 PSU) salinities showed that bacterial community composition at high salinity was significantly different, concomitant with reduced bleaching susceptibility in H2-SSB01, not observed in CC7-SSA01. Elucidation of bacteria that showed increased relative abundance at high salinity, irrespective of heat stress, revealed candidate taxa that could potentially contribute to the observed increased thermotolerance. We identified 4 (H2-SSB01) and 3 (CC7-SSA01) bacterial taxa belonging to the orders Alteromonadales (1 OTU), Oligoflexales (1 OTU), Rhizobiales (2 OTUs), and Rhodobacterales (2 OTUs), suggesting that only few bacterial taxa are potential contributors to an increase in thermal tolerance at high salinities. These taxa have previously been implicated in nitrogen and DMSP cycling, processes that are considered to affect thermotolerance. Our study demonstrates microbiome restructuring in symbiotic cnidarians under heat stress at different salinities. As such, it underlines how host-associated bacterial communities adapt to prevailing environmental conditions with putative consequences for the environmental stress tolerance of the emergent metaorganism.
Highlights
Corals are keystone species that, through the secretion of calcium carbonate, create the three-dimensional structures that constitute one of the most biodiverse ecosystems on Earth: coral reefs (Knowlton et al, 2010)
The 16S rRNA gene sequencing of the bacterial communities associated with H2-SSB01 and CC7-SSA01 across temperatures and salinities revealed that bacterial community compositions of both host-algal symbiont pairings were significantly different from each other (PPERMANOVA = 0.001), and further, that both were significantly different from the surrounding seawater (PPERMANOVA = 0.001; Figure 1 and Table 1)
Bacteria associated with multicellular animals at large are becoming increasingly acknowledged for providing functions to their respective hosts, together comprising so-called metaorganisms or holobionts (Bosch and McFall-Ngai, 2011; McFall-Ngai et al, 2013; Jaspers et al, 2019; Voolstra and Ziegler, 2020)
Summary
Corals are keystone species that, through the secretion of calcium carbonate, create the three-dimensional structures that constitute one of the most biodiverse ecosystems on Earth: coral reefs (Knowlton et al, 2010). Given that reef corals from the Red Sea and Persian/Arabian Gulf are exposed to the highest levels of salinity in any of the world’s oceans (41 and 50 PSU, respectively; Ngugi et al, 2012; Riegl and Purkis, 2012), recent research has investigated whether high salinity could affect thermotolerance (D’Angelo et al, 2015; Gegner et al, 2017; Ochsenkühn et al, 2017). Studies using the coral model Aiptasia showed that higher salinities can lead to reduced bleaching in the cnidarian-dinoflagellate symbiosis (Gegner et al, 2017, 2019) This salinity-conveyed thermotolerance could be further linked to an increase of the osmolyte floridoside that is produced by the associated algal endosymbiont, and putatively reduces the effects of bleaching-associated reactive oxygen species (ROS) due to its ROS-scavenging capabilities (Ochsenkühn et al, 2017; Gegner et al, 2019)
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