The Meta-Organism Response of the Environmental Generalist Pocillopora damicornis Exposed to Differential Accumulation of Heat Stress

Abstract

Coral bleaching events in the marine environment are now occurring globally, and the frequency and severity of these events are increasing. Critically, these events can cause the symbiosis between Symbiodiniaceae and their coral hosts to break down, but how the microbial community within the coral responds to bleaching is still equivocal. We investigated the impact of thermal stress exposure on the meta-organism responses of the generalist scleractinian coral species Pocillopora damicornis. Using mesocosms to recreate warming scenarios previously observed at Heron Island, we show that P. damicornis symbiont densities and photophysiological parameters declined at a similar rate under thermal stress regardless of the length of pre-bleaching thermal stress, defined here as temperatures above the monthly maximum mean (MMM) for Heron Island but below the local bleaching threshold (MMM + 2°C). However, we find that the P. damicornis microbiome remains stable over time regardless of the degree of thermal stress and the accumulation of pre-bleaching thermal stress. Our study therefore suggests that while P. damicornis is physiologically impacted by bleaching temperatures, the microbial community identified through 16S rRNA sequencing remains unchanged at the ASV level throughout bleaching. Understanding the capacity of a generalist species to withstand bleaching events is imperative to characterizing what coral species will exist on coral reefs following disturbances, as it has been suggested that the success of environmental generalist species may simplify community structure and lead to changes in biodiversity following environmental disturbance.

Assessing the coral microbiome at the scale of tissue-specific habitats within the coral meta-organism

“…Consistently, tissue samples excluding skeletal material show lower read counts than whole fragments or mucus samples (Table 1, Meistertzheim et al, 2016;Zou et al, 2022). Specifically, for P. damicornis, read counts per sample as high as 96,134 counts have been recorded in the literature for whole fragments (Bergman et al, 2021), compared to 9,414 -12,091 for tissue samples excluding coral skeleton (Li et al, 2021;Zou et al, 2022). Pollock et al (2018) also found an average of 13,629 reads per sample in the P. damicornis mucus layer and 10,928 reads per sample in the P. damicornis skeleton, compared to only 7,317 reads per sample in the P. damicornis tissue, highlighting how sample size and read count per sample may bias the overall read count.…”

“…The communities observed in Acropora and Pocillopora samples were likely also influenced by both preservation method and anthropogenic factors. While Alphaproteobacteria dominated the microbiome of both Acropora and Pocillopora in the present study, Bergman et al (2021) previously found Gammaproteobacteria to dominate the microbiome of whole Pocillopora fragments. The samples in the present study were collected directly from the reef, whereas Bergman et al (2021) collected corals and conducted an ex situ tank experiment using multiple warming trajectories, which likely influenced the differences from the microbial communities observed herein due to environmental effects.…”

“…While Alphaproteobacteria dominated the microbiome of both Acropora and Pocillopora in the present study, Bergman et al (2021) previously found Gammaproteobacteria to dominate the microbiome of whole Pocillopora fragments. The samples in the present study were collected directly from the reef, whereas Bergman et al (2021) collected corals and conducted an ex situ tank experiment using multiple warming trajectories, which likely influenced the differences from the microbial communities observed herein due to environmental effects. Additionally, both Acropora and Pocillopora samples were almost entirely lacking Endozoicomonas, a common bacterial associate in corals (Pogoreutz et al, 2018;Epstein et al, 2019;Glasl et al, 2019;Voolstra and Ziegler, 2020) that has been observed to be highly abundant in both Acropora and Pocillopora species (Ziegler et al, 2019).…”

“…Additionally, the present study investigated the microbiome of corals using fixed tissue, whereas many studies investigating the Pocillopora or Acropora microbiome analyze whole fragments snap-frozen in liquid nitrogen (e.g. Epstein et al, 2019, Ziegler et al, 2019, Bergman et al, 2021. While Hernandez-Agreda et al (2018) found composition of microbial communities to be comparable between fixed and snap-frozen coral samples, some low abundance and low occurrence phylotypes were found to be variable between preservation methods.…”

Core microbiome of corals Pocillopora damicornis and Pocillopora verrucosa in the northeastern tropical Pacific

“…OTUs present in at least 80% of the samples of the same substrate were considered members of the core microbiome, following the criterion proposed by Hernandez-Agreda et al (2016) andBergman et al (2021). As a result, the core microbiome OTUs of each substrate were present in at least 10 of 12 samples.…”