The Nutritional Implications of Partner Switching in the Cnidarian-Dinoflagellate Symbiosis
Authors:   , Matthews Jennifer 
Published: November 2021

<p>Reef-building corals form a symbiosis with phototrophic dinoflagellates of the genus Symbiodinium. Specificity in the host-symbiont partnership is widespread, despite the potential for flexibility in endosymbiont community composition to provide a mechanism of environmental acclimatisation and adaptation. The potential for partner switching may be linked to the nutritional flux between the two partners, with optimal nutritional exchange determining success. Further research is therefore necessary to determine how novel symbiont types (i.e. not originally detected in the host) affect the nutritional biology of the cnidarian host, and ultimately the capacity for the evolution of novel associations in the cnidarian-dinoflagellate symbiosis. The specific objectives were: 1) to design an effective method of providing aposymbiotic host organisms for experimental symbiont colonisation studies; 2) to determine how colonisation with a novel symbiont affects the gene-to-metabolite response of the host; 3) to deduce how the quantity and identity of translocated photosynthetic products from symbiont to host is affected by the symbiont type; 4) to determine whether natural differences in the Symbiodinium community composition affect the metabolite profile of the same cnidarian host.  Chapter 2 demonstrates that menthol-induced bleaching effectively and efficiently provides experimental aposymbiotic (i.e. symbiont-free) sea anemones (Aiptasia sp.) for colonisation studies. The menthol treatment produced aposymbiotic hosts within just 4 weeks (97–100% symbiont loss), and the condition was maintained long after treatment when anemones were held under a standard light:dark cycle. The ability of Aiptasia to form a stable symbiosis appeared to be unaffected by menthol exposure, as demonstrated by successful reestablishment of the symbiosis when anemones were experimentally re-colonised with the homologous Symbiodinium (i.e. originating from the same host species). Furthermore, there was no significant impact on photosynthetic or respiratory performance of re-colonised anemones.  A novel application of statistically integrated transcriptomic and metabolomic analyses was applied in Chapter 3 to explore the molecular and metabolic pathways underlying symbiosis specificity in the cnidarian-dinoflagellate symbiosis. Aposymbiotic individuals of the sea anemone Aiptasia generated by the methods optimised in Chapter 2 were colonised with either the homologous Symbiodinium type B1 or the novel Symbiodinium type D1a. RNA-seq gene expression analysis and gas chromatography-mass spectrometry-based metabolite profiling were conducted on the isolated host tissues. Analysis of the gene and metabolite expression profiles revealed that a novel symbiont confers an expression pattern intermediate between hosting a homologous symbiont, and having no symbiont. Although the formation and autotrophic potential of the novel association was similar to the homologous association, as determined by O₂ flux measurements, the novel association resulted in an increase in the catabolism of metabolic stores (glycogen, lipid and protein), presumably in order to meet the energy requirements associated with the maintenance of cellular homeostasis. Integrated pathway analysis revealed reduced energy storage, an increased catabolism of stores, metabolic signalling and cellular redox homeostasis were molecular processes involved in the hosts’ response to a novel symbiont type. This raised interesting questions as to how differences in the composition of symbiont-derived metabolites might feature in the specificity of the symbiosis.  Metabolite profiling was subsequently coupled with ¹³C-labelling in Chapter 4, to identify the specific differences in the identity and quantity of photosynthetic carbon products translocated to the Aiptasia host when in symbiosis with the novel Symbiodinium type D1a versus the homologous Symbiodinium B1. A reduction in the diversity and quantity of net translocated products was observed in the novel association, despite achieving a similar autotrophic potential to the homologous association, as determined by O₂ flux measurements. Interestingly, however, there was a continued fixation and translocation of carbon products to the host, suggesting that novel associations are, at least in part, metabolically functional in terms of photosynthate provision. Nevertheless, the decreased diversity and abundance of translocated products were associated with modifications to biosynthesis, increased catabolism of host energy stores, and oxidative stress-related signalling pathways in hosts colonised with the novel Symbiodinium type D1a.The impact of symbiont type on the host’s metabolite profile in an experimental setting raised the interesting possibility that the metabolite profiles of a cnidarian that forms natural flexible associations with different Symbiodinium types may also have dissimilar metabolite profiles, with implications for the nutritional physiology of the symbiosis. This was tested in Chapter 5.  An in situ field survey of the metabolite profiles of Montipora capitata, a dominant reef-building coral in Hawai’i that forms homologous symbioses with Symbiodinium in clades C and clade D, revealed the metabolite pools of a coral host were not affected by Symbiodinium community composition. Given prior evidence that clade D can be less nutritionally beneficial to the coral host than some other symbiont types, the reasons for the similarity in metabolite profiles remains unclear. However, possible explanations are: 1) host heterotrophic compensation for the presence of a less beneficial symbiont; 2) modifications to the abundance and identity of metabolites exchanged in the symbiosis, perhaps via host selection of its symbiont community over time to provide an optimal state; or 3) adjustment to the host’s metabolic pathway activity in response to different host-symbiont interactions, that produces similar free-metabolite profiles in the host. Nevertheless, this served to highlight that a coral host in a naturally flexible association, and under ‘normal’ environmental conditions, may maintain a steady metabolite profile irrespective of its Symbiodinium community composition. Notably, this contrasts with the findings that heterologous symbionts (i.e. those not usually associated with a particular host species) may have negative nutritional implications for the host that could ultimately restrict the success and persistence of novel host-symbiont pairings.  This study provides important evidence that optimal nutritional exchange and mechanisms of coping with oxidative stress in both partners are important determinants in the evolution of novel cnidarian-dinoflagellate symbioses. However, it also raises the possibility that such novel pairings, should they persist, may evolve over time to a more beneficial symbiotic state; this is worthy of further study. Indeed, we need to continue our efforts to understand the molecular and physiological mechanisms underpinning the adaptation of the cnidarian-dinoflagellate symbiosis to climate change, to facilitate the development of robust management strategies to safeguard the world’s coral reefs.</p>