Symbiont community stability through severe coral bleaching in a thermally extreme lagoon

E G Smith,G O Vaughan,D Mcparland,J A Burt,R N Ketchum

doi:10.1038/s41598-017-01569-8

Abstract

Coral reefs are threatened by climate change as coral-algal symbioses are currently living close to their upper thermal limits. The resilience of the algal partner plays a key role in determining the thermal tolerance of the coral holobiont and therefore, understanding the acclimatory limits of present day coral-algal symbioses is fundamental to forecasting corals’ responses to climate change. This study characterised the symbiont community in a highly variable and thermally extreme (Max = 37.5 °C, Min = 16.8 °C) lagoon located in the southern Persian/Arabian Gulf using next generation sequencing of ITS2 amplicons. Despite experiencing extreme temperatures, severe bleaching and many factors that would be expected to promote the presence of, or transition to clade D dominance, the symbiont communities of the lagoon remain dominated by the C3 variant, Symbiodinium thermophilum. The stability of this symbiosis across multiple genera with different means of symbiont transmission highlights the importance of Symbiodinium thermophilum for corals living at the acclimatory limits of modern day corals. Corals in this extreme environment did not undergo adaptive bleaching, suggesting they are living at the edge of their acclimatory potential and that this valuable source of thermally tolerant genotypes may be lost in the near future under climate change.

Highlights

Marine ecosystems worldwide are threatened by climate change[1]
Two lagoon sites were surveyed in this study, each located on the southern coastline of the Persian/Arabian Gulf (PAG), in the Emirate of Umm al Quwain (UAQ)
Corals in the UAQ lagoon are living at the thermal limits of present day corals and provide an ideal opportunity to investigate the symbiont communities on extreme reefs

Summary

Introduction

Marine ecosystems worldwide are threatened by climate change[1]. Coral reefs are vulnerable due to the thermal sensitivity of the symbiotic relationship between the coral host and its algal partner of the genus Symbiodinium[2]. In order to understand how coral symbioses will respond to climate change, there has been an increasing focus on the coral symbiont communities in present day extreme reefs, in highly variable tidal environments[20,21,22] These lagoons and tide pools can experience thermal maxima that are >2.5 °C greater than neighbouring environments[23]. Corals in lagoonal systems in the southern PAG are exposed to considerable chronic and acute thermal stress, with long-term summer temperatures on reefs 1.5 °C greater than the adjacent open water locations, and with diurnal temperature ranges exceeding 10 °C33–35 It is unclear whether the highly variable thermal regime impacts the stability of the symbiont community, during bleaching events which have altered communities elsewhere[12, 13, 17]

Methods

Results

Conclusion