The volatilome of the cnidarian-dinoflagellate symbiosis

With a rapidly changing environment, understanding the endosymbiotic relationship between cnidarians and dinoflagellates is crucial to elucidate the ways in which corals may respond to future conditions. At the basis of this symbiosis is the exchange of metabolites and signalling molecules between partners, all of which contribute to the establishment, maintenance, and ultimately dissociation of this important relationship. A subset of metabolites, biogenic volatile organic compounds (BVOCs) are low molecular weight, weakly lipophilic molecules that diffuse quickly through water and air, making them potential candidates for signalling molecules in inter-species interactions. This thesis sought to characterise patterns of BVOC generation in the cnidarian-dinoflagellate symbiosis across symbiotic states and thermal dysfunction in the model cnidarian Aiptasia (Exaiptasia diaphana).

In Chapter 2, I characterised the suite of BVOCs (collectively, the ‘volatilome’) emitted by the Aiptasia model system in symbiosis, and by each partner in isolation. Relative to symbiotic anemones, the volatilome of cultured symbionts (Breviolum minutum) was more distinct than it was to aposymbiotic (symbiont-free) anemones, suggesting that symbiosis alters the physiological state of the dinoflagellate more dramatically than that of Aiptasia. For example, cultured B. minutum produced dimethyl sulphide (DMS) in highest abundance, while anemones produced halogenated methanes like bromochloromethane, bromodichloromethane, tribromomethane and trichloromethane, regardless of symbiotic state. Alternatively, the relative lack of BVOC alteration in the cnidarian host, regardless of symbiotic state, may suggest a high degree of metabolic integration between the symbiotic partners.

In Chapter 3, I examined the role of symbiont identity on the microbiome and volatilome of Aiptasia. Microbiome analysis revealed distinct populations of bacteria in each symbiotic state, with bacteria in the family Vibrionaceae being the most abundant in aposymbiotic anemones. As prominent members of bacterial pathogens, the higher proportion of bacteria in this family could indicate disease susceptibility in the aposymbiotic state. Relative to the volatilomes of aposymbiotic anemones and those symbiotic with native B. minutum, symbiosis with the non-native dinoflagellate Durusdinium trenchii emitted a volatilome indistinct from either aposymbiosis or symbiosis with the native B. minutum. This suggests that the presence of a symbiont that is known to form a sub-optimal and potentially stressful symbiosis with Aiptasia impacts the metabolome. Indeed, anemones with native symbionts produced the BVOC isoprene in highest abundance, while aposymbiotic anemones and those containing the non-native symbiont produced a higher abundance of the aldehydes octanal, nonanal and dodecanal, suggesting a potentially future role for these molecules as biomarkers.

In Chapter 4, I investigated the impact of thermal stress on the microbiome and volatilome of Aiptasia in symbiosis with its native symbiont B. minutum, and in the aposymbiotic state. Aposymbiotic and symbiotic anemones were exposed to control (25 °C), sub-bleaching (30 °C) and bleaching (33.5 °C) temperatures. In both aposymbiotic and symbiotic anemones, I observed a restructuring of the microbiome between 25 °C and 33.5 °C, with anemones at 30 °C exhibiting an intermediate state. This is consistent with previous experiments showing that cnidarian microbiota can shift in response to changing environmental conditions. Anemones at 30 °C produced the highest number of significantly different BVOCs, including acetone and naphthalene. In contrast, symbiotic anemones at the highest temperature (33.5 °C) produced a distinct volatilome relative to the lower temperature treatments, a shift largely driven by higher quantities of dimethyl sulphide, eucalyptol and 1-iodododecane. Overall, aposymbiotic anemones exhibited a decline in BVOC richness at progressively higher temperatures, perhaps revealing the onset of metabolic collapse; this decrease was not observed for symbiotic anemones at higher temperature, suggesting a stabilising effect of the dinoflagellate endosymbionts.

In Chapter 5, I describe a method with which to assess chemotactic responses in Symbiodiniaceae, and defined tryptone as a positive control in B. minutum, Cladocopium spp., and D. trenchii. I assessed the chemotactic response of B. minutum and Cladocopium spp. to a pervasive marine metabolite dimethylsulphoniopropionate (DMSP), and volatiles bromodichloromethane (BrCl2CH) and diiodomethane (I2CH2). Despite their production by aposymbiotic anemones in Chapter 2, neither BrCl2CH nor I2CH2 elicited a chemotactic response in B. minutum or Cladocopium sp. The precursor to BVOC dimethyl sulphide (DMS), multifunctional and widespread DMSP has functions in osmoregulation, antioxidant defence and acts as a chemoattractant for multiple marine organisms. I found that, while B. minutum was repelled by DMSP, Cladocopium spp. did not respond chemotactically to this molecule. These differing responses by distinct species of Symbiodiniaceae may reflect differing chemical cues used by Symbiodiniaceae to locate and establish a symbiosis with new cnidarian hosts, and adds to the literature describing the functional diversity of these endosymbionts.

Collectively, my thesis elucidates the synthesis and release of BVOCs by the cnidarian-dinoflagellate symbiosis, both in response to symbiotic state and thermal stress. This foundational study provides a platform from which to explore the functional roles of identified BVOCs and bacterial associates. Additionally, the non-invasive technology of volatilomics applied here may serve to identify biomarkers for ecosystem health in natural habitats. Ultimately, this work contributes to our understanding of the ways in which cnidarian-dinoflagellate symbiosis is altered in response to stress at a time when coral reefs are threatened with extinction.