Uncovering the role of Symbiodiniaceae assemblage composition and abundance in coral bleaching response by minimizing sampling and evolutionary biases

Timothy D Swain,Vadim Backman,Luisa A Marcelino,Simon Lax

doi:10.1186/s12866-020-01765-z

Abstract

BackgroundBiodiversity and productivity of coral-reef ecosystems depend upon reef-building corals and their associations with endosymbiotic Symbiodiniaceae, which offer diverse functional capabilities to their hosts. The number of unique symbiotic partners (richness) and relative abundances (evenness) have been hypothesized to affect host response to climate change induced thermal stress. Symbiodiniaceae assemblages with many unique phylotypes may provide greater physiological flexibility or form less stable symbioses; assemblages with low abundance phylotypes may allow corals to retain thermotolerant symbionts or represent associations with less-suitable symbionts.ResultsHere we demonstrate that true richness of Symbiodiniaceae phylotype assemblages is generally not discoverable from direct enumeration of unique phylotypes in association records and that cross host-species comparisons are biased by sampling and evolutionary patterns among species. These biases can be minimized through rarefaction of richness (rarefied-richness) and evenness (Probability of Interspecific Encounter, PIE), and analyses that account for phylogenetic patterns. These standardized metrics were calculated for individual Symbiodiniaceae assemblages composed of 377 unique ITS2 phylotypes associated with 123 coral species. Rarefied-richness minimized correlations with sampling effort, while maintaining important underlying characteristics across host bathymetry and geography. Phylogenetic comparative methods reveal significant increases in coral bleaching and mortality associated with increasing Symbiodiniaceae assemblage richness and evenness at the level of host species.ConclusionsThese results indicate that the potential flexibility afforded by assemblages characterized by many phylotypes present at similar relative abundances does not result in decreased bleaching risk and point to the need to characterize the overall functional and genetic diversity of Symbiodiniaceae assemblages to quantify their effect on host fitness under climate change.

Highlights

Biodiversity and productivity of coral-reef ecosystems depend upon reef-building corals and their associations with endosymbiotic Symbiodiniaceae, which offer diverse functional capabilities to their hosts
From this functionally diverse pool of potential partners, coral species associate with a selection of phylotypes, where individual coral colonies frequently associate with a single symbiont phylotype, and across conspecific colonies few phylotypes are typically present at high frequencies and dominate the assemblage, while others are present at low frequencies and are minor components [14,15,16]
Raw-R significantly increases with increasing sampling effort, indicating that extensively sampled coral species may artificially appear to be associated with higher numbers of unique phylotypes and raw-richness may not reflect true richness of assemblages

Summary

Introduction

Biodiversity and productivity of coral-reef ecosystems depend upon reef-building corals and their associations with endosymbiotic Symbiodiniaceae, which offer diverse functional capabilities to their hosts. Symbiodiniaceae (formerly known as Symbiodinium [4]) are genetically and physiologically diverse with > 400 phylotypes identified [5, 6] with different capabilities for photosynthetic production [7, 8], oxidative stress resistance [9], and thermal stress tolerance [10,11,12,13] From this functionally diverse pool of potential partners, coral species associate with a selection of phylotypes (heretofore ‘Symbiodiniaceae assemblage’), where individual coral colonies frequently associate with a single symbiont phylotype, and across conspecific colonies few phylotypes are typically present at high frequencies and dominate the assemblage, while others are present at low frequencies and are minor components [14,15,16]. Increasingly frequent and pervasive thermal stress under climate change has heightened the urgency to understand the consequences of coralSymbiodiniaceae association patterns in response to thermal stress

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