Establishment of Coral-Bacteria Symbioses Reveal Changes in the Core Bacterial Community With Host Ontogeny.

Rachele Bernasconi,Annette Koenders,Michael Stat,Andrea Paparini,Megan J Huggett,Michael Bunce

doi:10.3389/fmicb.2019.01529

Abstract

Bacterial communities are fundamental symbionts of corals. However, the process by which bacterial communities are acquired across the life history of corals, particularly in larval and early juvenile stages, is still poorly characterized. Here, transfer of bacteria of the Scleractinian coral Acropora digitifera from adults to spawned egg-sperm bundles was analyzed, as well as acquisition across early developmental stages (larvae and newly settled spat), and 6-month-old juveniles. Larvae were reared under manipulated environmental conditions to determine the source (maternal, seawater, or sediment) of bacteria likely to establish symbiotic relationships with the host using amplicon sequencing of the 16S rRNA gene. Maternal colonies directly transferred bacteria from the families Rhodobacteraceae, Cryomorphaceae, and Endozoicimonaceae to egg-sperm bundles. Furthermore, significant differences in the microbial community structure were identified across generations, yet the structure of the coral bacterial community across early life history stages was not impacted by different environmental rearing conditions. These data indicate that the uptake and structure of bacterial communities is developmentally, rather than environmentally, regulated. Both maternal coral colonies and ubiquitous bacteria found across environmental substrates represent a potential source of symbionts important in establishing the coral microbiome. Uniquely, we report the presence of variation with ontogeny of both the core and resident bacterial communities, supporting the hypothesis that microbial communities are likely to play specific roles within the distinct life history stages of the coral host.

Highlights

Corals host mutualistic relationships with a dense, dynamic and highly diverse consortium of microorganisms such as dinoflagellate unicellular algae in the family Symbiodiniaceae, bacteria, archaea, viruses and fungi
While specific Operational Taxonomic Units (OTUs) found in both adult colony and corresponding bundles were variable (Figures 2B–F and Supplementary Figure 4), taxa such as Cryomorphaceae, and Endozoicimonaceae were found in all adults and bundles (Figure 1G and Supplementary Figure 4), and Rhodobacteraceae were found in four out of five adults and associated bundles (Supplementary Figures 4B–E)
Of the 15 OTUs belonging to the Endozoicimonaceae, OTU 4397109 was reported among all adult coral colonies and 20 out of 22 bundle samples (Supplementary Figure 5)

Summary

Introduction

Corals host mutualistic relationships with a dense, dynamic and highly diverse consortium of microorganisms such as dinoflagellate unicellular algae in the family Symbiodiniaceae, bacteria, archaea, viruses and fungi. Horizontal transmission of symbionts from the environment is a commonly used mechanism and has been documented in invertebrates such as Hydra (Franzenburg et al, 2013), tubeworms (Kikuchi et al, 2007), the broad-headed bug Riptortus clavatus (Nussbaumer et al, 2006) and in the bobtail squid Euprymna scolopes where the luminous bacterium Vibrio fischeri is acquired (Nyholm and McFall-Ngai, 2004) Both vertical and horizontal transmission modes are likely to be influenced by host reproductive strategies (Apprill et al, 2009, 2012; Sharp et al, 2010, 2012) and several studies suggest that the acquisition of microbial symbionts is consistent with reproductive mode (brooding vs broadcast spawning) within corals (Hartmann et al, 2017). Understanding of the mechanisms for acquiring bacterial associations, and how these evolve and are maintained through development stages, is poorly understood

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Discussion

Conclusion