The Effect of Thermal Stress on the Bacterial Microbiome of Exaiptasia diaphana

Abstract

Coral bleaching linked to climate change has generated interest in the response of coral's bacterial microbiome to thermal stress. The sea anemone, Exaiptasia diaphana, is a popular coral model, but the response of its bacteria to thermal stress has been barely explored. To address this, we compared the bacterial communities of Great Barrier Reef (GBR) E. diaphana maintained at 26 • C or exposed to increasing temperature (26-33 • C) over two weeks. Communities were analyzed by metabarcoding of the bacterial 16S rRNA gene. Bleaching and Symbiodiniaceae health were assessed by Symbiodiniaceae cell density and dark-adapted quantum yield (F v /F m ), respectively. Significant bleaching and reductions in F v /F m occurred in the heat-treated anemones above 29 • C. Overall declines in bacterial alpha diversity in all anemones were also observed. Signs of bacterial change emerged above 31 • C. Some initial outcomes may have been influenced by relocation or starvation, but collectively, the bacterial community and taxa-level data suggested that heat was the primary driver of change above 32 • C. Six bacterial indicator species were identified as potential biomarkers for thermal stress. We conclude that the bacterial microbiome of GBR E. diaphana is generally stable until a thermal threshold is surpassed, after which significant changes occur. resistance or susceptibility during thermal stress have been observed [12][13][14]. This highlights the need for controlled, laboratory-based experiments to clarify the relationship between temperature-related bacterial community shifts and bleaching in cnidarians [15][16][17].The sea anemone, Exaiptasia diaphana, is a much-used model for coral symbiosis studies [18,19]. Its ability to propagate asexually for rapid growth of clonal populations, basic maintenance requirements and coral-like bleaching response to environmental stressors have seen it widely adopted by the research community and several clonal lines of different geographic origin and algal symbiont type have been established. However, studies using E. diaphana to explore cnidarian responses to heat stress have focused largely on aspects of the host-Symbiodiniaceae relationship [20][21][22][23][24][25], whilst its bacterial microbiome has been almost wholly neglected and data from only two studies are available.In a 2010 Master's thesis, E. diaphana of unspecified origin were exposed to temperature ramped from 26 • C to 31 • C over ten days, then held at 31 • C for four days [26]. No significant differences between the associated bacterial communities of control and treated anemones were detected across the study period. However, poor resolution of chosen molecular biology methods negatively impacted the findings. In addition, exposing the anemones to a maximum of 31 • C meant the anemones may not have been thermally stressed, and no measurements of algal cell density or photosynthetic performance were taken to assess their condition. Although the study revealed few insights, it is acknowledged as the first investigation...

Short-Term Exposure to Sterile Seawater Reduces Bacterial Community Diversity in the Sea Anemone, Exaiptasia diaphana

“…DNA was extracted (Wilson et al, 2002;Hartman et al, 2019) from the anemones and RSS/fRSS samples for bacterial community analysis by metabarcoding of the 16S rRNA genes. Extraction blanks (n = 7) were processed simultaneously to identify contaminants introduced during DNA extractions.…”

“…As an alternative approach to creating the MM, we have explored a community reduction strategy of the E. diaphana microbiome by manipulating the external environment (Clavel et al, 2017), instead of the application of antibiotics. Studies on the microbiome of E. diaphana indicate a similar phylum level diversity to corals (Blackall et al, 2015) for anemones sourced from the Great Barrier Reef (GBR) (Hartman et al, 2019(Hartman et al, , 2020Dungan et al, 2020a), Hawaii (Herrera et al, 2017), Pacific and Caribbean (Brown et al, 2017), Atlantic (Röthig et al, 2016), and Red Sea (Ahmed et al, 2019). We hypothesize that selective forces acting on E. diaphana and its microbiome, when placed in sterile seawater, will reduce the complexity of the microbiome with the loss of some (e.g., transient) members.…”

“…We hypothesize that selective forces acting on E. diaphana and its microbiome, when placed in sterile seawater, will reduce the complexity of the microbiome with the loss of some (e.g., transient) members. This hypothesis is driven by existing evidence that environment has a significant influence on the cnidarian microbiome (Zhang et al, 2015;Webster and Reusch, 2017;Hartman, 2020), and that autoclavesterilized seawater is correlated with a reduction in alpha diversity for GBR-sourced E. diaphana (Hartman et al, 2019). If their microbiomes were successfully reduced, we sought to identify bacterial taxa that persisted in anemones reared in sterile seawater as these taxa may be critical to holobiont health.…”

Development of a free radical scavenging probiotic to mitigate coral bleaching

“…Microbiome engineering through the addition of probiotics has been postulated as a key strategy to manipulate host phenotypes and ecosystem functioning for coral reefs [23][24][25][26][27][28]. The differences in the bacterial species composition of healthy and thermally stressed corals [29][30][31][32][33][34] and the coral model Exaiptasia diaphana [35][36][37] suggest a role for microbiome engineering in cnidarian health. A disruption to the bacterial community of Pocillopora damicornis with antibiotic treatment diminished the resilience of the holobiont during thermal stress, whereas intact microbial communities conferred resilience to thermal stress and increased the rate of recovery after bleaching events to the coral holobiont [38].…”

A Shift Pattern of Bacterial Communities Across the Life Stages of the Citrus Red Mite, Panonychus citri

“…Many of these microbiota play important roles in interactions with their hosts for improved physiology, life history traits, environmental adaptability, reproduction, and are even essential for the host's survival (Engel and Moran, 2013;Kwong and Moran, 2016). In addition to the beneficial effect on host nutritional digestion of varied diets, especially the diets with poor or unbalanced nutrition or recalcitrant components (Hongoh et al, 2008;Huang et al, 2010), microbiota may (1) increase arthropod host fitness through protecting the host from parasites and pathogens or improving the host's tolerance to heat stress (Dillon and Dillon, 2004;Hartman et al, 2020), (2) influence the host's lifespan (Storelli et al, 2011;Noman et al, 2020), (3) improve the insect's social communication (Dillon et al, 2002), and (4) govern mating and reproductive systems (Sharon et al, 2010). In some cases, microbiota facilitated the pest host's survival under traditional pest control by enhancing pesticide resistance (Kikuchi et al, 2012;Zhou and Yao, 2020).…”