Abstract
The importance of Symbiodinium algal endosymbionts and a diverse suite of bacteria for coral holobiont health and functioning are widely acknowledged. Yet, we know surprisingly little about microbial community dynamics and the stability of host‐microbe associations under adverse environmental conditions. To gain insight into the stability of coral host‐microbe associations and holobiont structure, we assessed changes in the community structure of Symbiodinium and bacteria associated with the coral Pocillopora verrucosa under excess organic nutrient conditions. Pocillopora‐associated microbial communities were monitored over 14 days in two independent experiments. We assessed the effect of excess dissolved organic nitrogen (DON) and excess dissolved organic carbon (DOC). Exposure to excess nutrients rapidly affected coral health, resulting in two distinct stress phenotypes: coral bleaching under excess DOC and severe tissue sloughing (>90% tissue loss resulting in host mortality) under excess DON. These phenotypes were accompanied by structural changes in the Symbiodinium community. In contrast, the associated bacterial community remained remarkably stable and was dominated by two Endozoicomonas phylotypes, comprising on average 90% of 16S rRNA gene sequences. This dominance of Endozoicomonas even under conditions of coral bleaching and mortality suggests the bacterial community of P. verrucosa may be rather inflexible and thereby unable to respond or acclimatize to rapid changes in the environment, contrary to what was previously observed in other corals. In this light, our results suggest that coral holobionts might occupy structural landscapes ranging from a highly flexible to a rather inflexible composition with consequences for their ability to respond to environmental change.
Highlights
Anthropogenic environmental change has caused global degradation and loss of coral reef cover at an unprecedented scale over the last decades (Hughes et al, 2017)
Part of the data presented for the excess dissolved organic carbon (DOC) experiment were published previously in a companion paper (Pogoreutz, Rädecker, Cárdenas, Gärdes, Wild, et al, 2017); this applies to the bacterial 16S rRNA gene amplicon sequencing data, Symbiodinium population density data, and seawater DOC concentrations for the excess DOC experiment presented in this study
This is complemented by new data on Symbiodinium chlorophyll a content and internal transcribed spacer 2 (ITS2) sequencing for the excess DOC and dissolved organic nitrogen (DON) experiments, as well as bacterial 16S rRNA gene amplicon sequencing data for the DON experiment
Summary
Anthropogenic environmental change has caused global degradation and loss of coral reef cover at an unprecedented scale over the last decades (Hughes et al, 2017). We sought to investigate the flexibility of microbial association of the common Indo-Pacific coral Pocillopora verrucosa, previously suggested to display a limited acclimatization potential (Pogoreutz, Rädecker, Cárdenas, Gärdes, Voolstra, et al, 2017; Pogoreutz, Rädecker, Cárdenas, Gärdes, Wild, et al, 2017; Sawall et al, 2015; Ziegler, Roder, Büchel, & Voolstra, 2014) We approached this by exposing the P. verrucosa holobiont to different excess dissolved organic nutrient treatments. We integrated microbial community data from the here-conducted experiment assessing the effects of excess dissolved organic nitrogen (DON; 40- to 50-fold enrichment compared to ambient conditions) and complemented these data with a previously published companion experiment exposing P. verrucosa holobionts to excess labile dissolved organic carbon (DOC; >10-fold enrichment; Pogoreutz, Rädecker, Cárdenas, Gärdes, Wild, et al, 2017) In these two independent 14- day experimental treatments, corals exhibited bleaching and mortality reflected by progressed tissue sloughing, respectively. We assessed such responses in algal and bacterial symbionts by assessing community composition dynamics via ITS2 and 16S rRNA gene-typing, respectively, alongside algal symbiont density and chlorophyll a content measurements
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