Bacterial Communities Vary from Different Scleractinian Coral Species and between Bleached and Non-Bleached Corals

Abstract

Bleaching is one of the most relevant factors implicated in the integrity of coral reef ecosystems, with the increasing frequency and intensity of damaging events representing a serious threat to reef biodiversity. Here, we analyzed changes in coral-associated bacteria from three types of non-bleached and bleached scleractinian corals (Acropora digitifera, Galaxea fascicularis, and Porites pukoensis) in Hainan Luhuitou peninsula coastal areas. The community structure of symbiotic bacteria differed significantly among the three apparently healthy corals. The bleached corals had higher bacterial alpha diversity and some specific bacteria genera, including Ruegeria, Methyloceanibacter, Filomicrobium, Halioglobus, Rubripirellula, Rhodopirellula, Silicimonas, Blastopirellula, Sva0996 marine group, Woeseia, and unclassified_c_Gammaproteobacteria, were consistently increased in bleached groups. Network analysis revealed significantly different degrees of modularity between bleached and non-bleached groups at the bacterial genus level, and a higher proportion of links was dominated by positive co-occurrences. Functional prediction analysis illustrated that coral-associated bacteria remained relatively consistent in the bleached and non-bleached groups. Structure equation modeling revealed that the bacterial community diversity and function were directly influenced by host and environment factors. These findings suggested that coral-associated bacterial responses to bleaching occur in a host-dependent manner, informing novel strategies for restoring coral and aiding adaption to bleaching stress. IMPORTANCE Accumulating evidence indicates that coral-associated bacteria play an important role in the health of holobionts. However, the variability of the symbiotic bacterial community structure among coral species with different coral health statuses remains largely unknown. Here, we investigated three apparent non-bleached (healthy) and bleached coral species (sampled in situ), involving related symbiotic bacterial profiles, including composition, alpha diversity, network relationship, and potential function. Structural equation modeling analysis was used to analyze the relationship between coral status and abiotic and biotic factors. The bacterial community structure of different groups was shown to exhibit host-specific traits. Both host and environmental impacts had primary effects on coral-associated microbial communities. Future studies are needed to identify the mechanisms that mediate divergent microbial consortia.

Keywords: bacterial communities; bleaching; coral health status; host-dependent profile; network relationship.

FIG 1

Compassion of alpha diversity indices based on 16S rRNA genes sequencing (A, B). Asterisks indicate significant differences (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001) based on the Student's t test. N, non-bleached corals; B, bleached corals.

FIG 2

Composition of microbial prokaryotes present in three non-bleached (N) and bleached (B) coral species and seawater. Bar plots show the taxonomic classification of OTU in each sample at the phyla (A), class (B), order (C), and genus (D) levels (other: relative abundance <2%).

FIG 3

(A to C) Venn diagram for the common and unique number of OTUs among non-bleached and bleached corals and seawater among A. digitifera (A), G. fascicularis (B), and P. pukoensis (C). Prokaryotic community structure and relative dispersion of the non-bleached and bleached coral species and seawater samples, analyzed with nonmetric multidimensional scaling (NMDS) plots using Bray-Curtis dissimilarity. (D) Three coral species (non-bleached and bleached) and seawater. (E to G) A. digitifera, G. fascicularis, and P. pukoensis in non-bleached and bleached model, respectively. N, non-bleached; B, bleached.

FIG 4

Distribution of the relative abundance of the specific bacteria between corals.

FIG 5

Co-occurrence network of coral bacterial communities at the genus level between two conditions. (A, C, E) Non-bleached. (B, D, F) Bleached. The size of the nodes is proportional to the average relative abundance of the genera. Edge thickness linking two nodes is proportional to the absolute value of the Spearman’s correlation coefficients. Node colors are used to differentiate between phyla. Green edges indicate the co-occurrence of connected nodes, pink edges denote mutual exclusivity, and the width of edges reflects the strength of interaction.

FIG 6

Structure equation modeling showing the relative influence between seawater abiotic and biotic coral associated with microbial factors on coral status. Response variables are represented as solid and predictor variables are represented as bold-dashed boxes. Red and blue arrows indicate positive and negative relationships, respectively. Proportion of variance explained (R²) is represented by orange numbers for each response variable. Significant paths imply a causal influence of different variables with arrow widths proportional to the degree of influence, as shown in the bottom right. Arrow numbers represent the standardized path strength.

FIG 7

(A) Map of the South China Sea study site. (B) Corals and seawater were sampled at one coral reef site off Luhuitou peninsula coastal, which is marked with a circle. (C) The site is located approximately 200 m northeast of Luhuitou (image of sampling site). (D) In situ monitoring of coral bleaching through fixed undersea observation equipment at the Hainan Island. (E) Representative real-time captured image of bleached corallite.