Homogenization of Endosymbiont Communities Hosted by Equatorial Corals during the 2016 Mass Bleaching Event.

Sudhanshi S Jain,Oren Levy,Benjamin J Wainwright,Bar Feldman,Lutfi Afiq-Rosli,Jun Wei Phua,Danwei Huang

doi:10.3390/microorganisms8091370

Sudhanshi S Jain, Oren Levy + Show 5 more

Open Access

https://doi.org/10.3390/microorganisms8091370

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Abstract

Thermal stress drives the bleaching of reef corals, during which the endosymbiotic relationship between Symbiodiniaceae microalgae and the host breaks down. The endosymbiont communities are known to shift in response to environmental disturbances, but how they respond within and between colonies during and following bleaching events remains unclear. In 2016, a major global-scale bleaching event hit countless tropical reefs. Here, we investigate the relative abundances of Cladocopium LaJeunesse & H.J.Jeong, 2018 and Durusdinium LaJeunesse, 2018 within and among Pachyseris speciosa colonies in equatorial Singapore that are known to host both these Symbiodiniaceae clades. Bleached and unbleached tissues from bleaching colonies, as well as healthy colonies, during and following the bleaching event were sampled and analyzed for comparison. The nuclear ribosomal internal transcribed spacer (ITS) regions were separately amplified and quantified using a SYBR Green-based quantitative polymerase chain reaction (qPCR) method and Illumina high-throughput sequencing. We found Cladocopium to be highly abundant relative to Durusdinium. The relative abundance of Durusdinium, known to be thermally tolerant, was highest in post-bleaching healthy colonies, while bleached and unbleached tissues from bleaching colonies as well as tissue from healthy colonies during the event had depressed proportions of Durusdinium. Given the importance of Durusdinium for thermal tolerance and stress response, it is surprising that bleached tissue showed limited change over healthy tissue during the bleaching event. Moreover, colonies were invariably dominated by Cladocopium during bleaching, but a minority of colonies were Durusdinium-dominant during non-bleaching times. The detailed characterization of Symbiodiniaceae in specific colonies during stress and recovery will provide insights into this crucial symbiosis, with implications for their responses during major bleaching events.

Highlights

Over half of all stony corals (Cnidaria: Anthozoa: Scleractinia) [1,2] live in obligate symbiosis with microscopic dinoflagellates of the family Symbiodiniaceae [3], which support the existence of shallow-water coral reefs in the world’s oceans [4,5]
The abundance of Durusdinium relative to Cladocopium LaJeunesse & H.J.Jeong, 2018 is thought to be indicative of stress tolerance, with previous studies showing Durusdinium being dominant in coral colonies that have been affected by temperature fluctuations and bleaching events [16,17]
The ratios of Durusdinium to Cladocopium internal transcribed spacer 1 (ITS1) concentrations obtained from the quantitative polymerase chain reaction (qPCR) experiments were compared among the five groups of Pachyseris speciosa samples—normal (N), normal part of bleached colonies (BN), bleached part of bleached colonies (BB), post-bleaching colonies collected in 2017 (PB2017) and post-bleaching colonies collected in 2018 (PB2018)

Summary

Introduction

Over half of all stony corals (Cnidaria: Anthozoa: Scleractinia) [1,2] live in obligate symbiosis with microscopic dinoflagellates of the family Symbiodiniaceae [3], which support the existence of shallow-water coral reefs in the world’s oceans [4,5] These microalgal endosymbionts provide for most of the coral hosts’ energetic requirements for growth and production of their calcium carbonate skeleton [6,7,8]. The coral reef surveys conducted have found P. speciosa to be one of the most common reef corals in the turbid waters here [65,66,67] This species was highly susceptible to bleaching during the major event in 2010 [65] and the minor event in 2013 [68], but was found to be less susceptible and better able to recover during the 2016 GCBE [44]. Transported to the laboratory in seawater, immediately fixed in 100% molecular grade ethanol and stored at −80 ◦C until processing

DNA Extraction

Quantitative Polymerase Chain Reaction

Illumina Library Preparation and Sequencing

Statistical Analyses

Results