Supporting data for "Coral-Algal Symbiosis Through Stress and Recovery"
Coral-Algal Symbiosis Through Stress and Recovery
It’s thought that almost every organism on the planet partakes in mutualistic associations, though few so prominently as reef-building coral. An evolutionarily successful relationship, endosymbiotic algae (family Symbiodiniaceae) support coral metabolism by provisioning photosynthetic nutrients, in return for shelter and access to nitrogenous waste products. Central to this success is the idea that the costs of contributing to a partners fitness must be outweighed by the benefits gained, which, under normal circumstances, is the case for the coral-algal relationship. Coral hosts may even actively protect their own investment by rationing the nitrogen access and expelling excess algal cells, maintaining a sustainable population of Symbiodiniaceae. Unfortunately, adverse environmental conditions disrupt this delicate balance – promoting symbiont nutrient hoarding or the breakdown of photosynthesis – and cause corals to bleach, in a phenomenon that threatens coral reef ecosystems worldwide.
While a large body of literature has focused on what happens up to and during bleaching in an attempt to prevent these devastating effects, researchers have more recently shifted their focus to post-bleaching recovery. For example, a driving question is whether introducing experimentally evolved Symbiodiniaceae into bleached adult corals may improve their resistance and resilience to further stress. To that end, it’s imperative to understand a) are recovering corals capable of forming novel, sustainable associations and b) do they function like a healthy mutualism?
In this thesis, I explore the influence of natural and introduced Symbiodiniaceae diversity on the stress tolerance and subsequent recovery of reef-building coral. Firstly, I leverage natural symbiont diversity of Acropora millepora from distinct habitats across the Great Barrier Reef to investigate how environment and symbiont identify interact to shape tolerance. I demonstrate that more tolerant Symbiodiniaceae can form successful and comparable mutualism to generalist species, and that symbiont identity alone may not predict coral resistance. I further show how stable isotope tracers, including novel incorporation of 2H, can be used detect sub-lethal indicators of heat stress, particularly early damage to photosynthetic apparatus. I then shift the lens to coral recovery, presenting gene expression profiles of bleached Galaxea fascicularis following exposure to Cladocopium goreaui, Durusdinium trenchii or a mixture of both. Despite initial uptake, regional specificity may limit their capacity for establishing novel Symbiodiniaceae communities. This reiterates challenges for large-scale application of symbiont manipulation as a way to bolster coral resilience. Finally, I combine bulk and compound-specific stable isotope analysis to compare nutrient exchange in a healthy G. fascicularis mutualism and those recovering under variable environmental conditions. I explore host vs symbiont-centric recovery strategies and determine that while the quantity of shared nutrients is consistent between recovering and healthy holobionts, the quality is not. Reduced translocation of essential amino acids may enhance the vulnerability of corals throughout recovery, highlighting the value of comprehensive methods when quantifying the function of coral-algal symbiosis. Building foundational knowledge of how corals and their Symbiodiniaceae in this way will not only aid in defining natural association, but also improve the design and success of potential interventions.
