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Abstract
The bacterial microbiome is an essential component of many corals, although knowledge of the microbiomes in scleractinian corals far exceeds that for octocorals. This study characterized the bacterial communities present in shallow water Caribbean gorgonian octocorals over time and space, in addition to determining the bacterial assemblages in gorgonians exposed to environmental perturbations. We found that seven shallow water Caribbean gorgonian species maintained distinct microbiomes and predominantly harbored two bacterial genera, Mycoplasma and Endozoicomonas. Representatives of these taxa accounted for over 70% of the sequences recovered, made up the three most common operational taxonomic units (OTUs), and were present in most of the gorgonian species. Gorgonian species sampled in different seasons and/or in different years, exhibited significant shifts in the abundances of these bacterial OTUs, though there were few changes to overall bacterial diversity, or to the specific OTUs present. These shifts had minimal impact on the relative abundance of inferred functional proteins within the gorgonian corals. Sequences identified as Escherichia were ubiquitous in gorgonian colonies sampled from a lagoon but not in colonies sampled from a back reef. Exposure to increased temperature and/or ultraviolet radiation (UVR) or nutrient enrichment led to few significant changes in the gorgonian coral microbiomes. While there were some shifts in the abundance of the prevalent bacteria, more commonly observed was “microbial switching” between different OTUs identified within the same bacterial genus. The relative stability of gorgonian coral bacterial microbiome may potentially explain some of the resistance and resilience of Caribbean gorgonian corals against changing environmental conditions.
Highlights
Scleractinian corals and octocorals contain organisms such as photosynthetic dinoflagellates [family Symbiodiniaceae; LaJeunesse et al (2018)], corallicolids (Kwong et al, 2019), bacteria (Rohwer et al, 2002), Archaea (Kellogg, 2004), and fungi (Kendrick et al, 1982) that together constitute the holobiont
In the Caribbean, the microbiome of five gorgonian species was previously characterized, and our study of seven Caribbean gorgonian species concurred with the finding that each gorgonian species hosted a distinct bacterial community (Sunagawa et al, 2010; Correa et al, 2013; Tracy et al, 2015; McCauley et al, 2016; Robertson et al, 2016; Shirur et al, 2016)
At the gorgonian coral genus level, some microbiome similarities existed for the genus Pseudoplexaura, with P. porosa and P. flagellosa, sampled at ambient environmental conditions from the lagoon in the same month and year, hosting very similar bacterial assemblages
Summary
Scleractinian (hard) corals and octocorals (soft corals, sea pens, gorgonians) contain organisms such as photosynthetic dinoflagellates [family Symbiodiniaceae; LaJeunesse et al (2018)], corallicolids (Kwong et al, 2019), bacteria (Rohwer et al, 2002), Archaea (Kellogg, 2004), and fungi (Kendrick et al, 1982) that together constitute the holobiont. Exposure to elevated temperatures can induce a shift from beneficial bacterial communities to opportunistic populations (Bourne et al, 2008; Garren et al, 2009; Littman et al, 2010; Kelly et al, 2014) which include a greater abundance of commensal or parasitic heterotrophic bacterial species (Thurber et al, 2009; Littman et al, 2011) These changes to the coral bacterial community can lead to increased susceptibility of the coral to diseases and infections (Thurber et al, 2009; Vezzulli et al, 2010; Shirur et al, 2016), lesions and tissue loss (Cárdenas et al, 2011; Meyer et al, 2014) and to bleaching (loss of Symbiodiniaceae) or even colony mortality (Ziegler et al, 2017; Morrow et al, 2018). Our second goal was to determine the effects of elevated seawater temperatures, and/or ultraviolet radiation (UVR), or nutrient enrichment, on the bacterial assemblages of Caribbean gorgonian corals
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