Subtle Differences in Symbiont Cell Surface Glycan Profiles Do Not Explain Species-Specific Colonization Rates in a Model Cnidarian-Algal Symbiosis

Abstract

Mutualisms between cnidarian hosts and dinoflagellate endosymbionts are foundational to coral reef ecosystems. These symbioses are often re-established every generation with high specificity, but gaps remain in our understanding of the cellular mechanisms that control symbiont recognition and uptake dynamics. Here, we tested whether differences in glycan profiles among different symbiont species account for the different rates at which they initially colonize aposymbiotic polyps of the model sea anemone Aiptasia (Exaiptasia pallida). First, we used a lectin array to characterize the glycan profiles of colonizing Symbiodinium minutum (ITS2 type B1) and noncolonizing Symbiodinium pilosum (ITS2 type A2), finding subtle differences in the binding of lectins Euonymus europaeus lectin (EEL) and Urtica dioica agglutinin lectin (UDA) that distinguish between high-mannoside and hybrid-type protein linked glycans. Next, we enzymatically cleaved glycans from the surfaces of S. minutum cultures and followed their recovery using flow cytometry, establishing a 48–72 h glycan turnover rate for this species. Finally, we exposed aposymbiotic host polyps to cultured S. minutum cells masked by EEL or UDA lectins for 48 h, then measured cell densities the following day. We found no effect of glycan masking on symbiont density, providing further support to the hypothesis that glycan-lectin interactions are more important for post-phagocytic persistence of specific symbionts than they are for initial uptake. We also identified several methodological and biological factors that may limit the utility of studying glycan masking in the Aiptasia system.

Host Genotypic Effect on Algal Symbiosis Establishment in the Coral Model, the Anemone Exaiptasia diaphana, From the Great Barrier Reef

“…B. minutum density increased further over time, reaching a maximum of 127 ± 2 cells/mm 2 at 30 dpi. These results are congruent with our identification of B. minutum as the homologous symbiont in GBR anemones and are consistent with previous works showing that the homologous symbiont is the most efficient colonizer of other E. diaphana genotypes (Belda-Baillie et al, 2002;Xiang et al, 2013;Gabay et al, 2018Gabay et al, , 2019Parkinson et al, 2018;Medranos et al, 2019;Sproles et al, 2019). Such an affinity profile is not exclusive to anemones, as it has also been reported in several corals (Weis et al, 2001;Rodriguez-Lanetty et al, 2003, 2004Wolfowicz et al, 2016;Lin et al, 2019).…”

“…To monitor symbiont uptake and colonization rate in the anemones, we extended methods from earlier works (Berner et al, 1993;Neubauer et al, 2016;Chakravarti et al, 2017;Parkinson et al, 2018) by including a calibration curve that allows symbiont coverage to be expressed as in hospite cell density. Our approach takes advantage of fluorescent microscopy and symbiont red chlorophyll auto-fluorescence.…”

“…This specificity demonstrates the existence of a recognition mechanism between symbiotic partners. In other symbioses there typically exists an intricate series of inter-partner molecular signals that allow suitable partners to be identified at the onset of symbiosis (Nyholm and McFall-Ngai, 2004), but how this works for cnidarians and Symbiodinaceae is largely unknown (Davy et al, 2012;Parkinson et al, 2018).…”

Free‐living and symbiotic lifestyles of a thermotolerant coral endosymbiont display profoundly distinct transcriptomes under both stable and heat stress conditions

“…Sorek and Levy () showed that blue light spectra increased mRNA expression of free‐living Symiodiniaceae cryptochromes CRY1 and CRY2, but were downregulated when hosted in the coral Stylophora pistillata . Though an E. pallida host GFP has not yet been identified, they exhibit patterns of green autofluorescence when excited by blue light (Parkinson et al, ), which could reduce the amount of blue light transmitted to endosymbionts. In contrast with the differential expression of these blue light receptors within these categories, there is no significant change in the expression of the red light‐detecting phytochrome transcript (Appendix ), suggesting that this host‐mediated light filtering is biased towards the attenuation of blue rather than red light spectra.…”

Transcriptomic insights into the establishment of coral-algal symbioses from the symbiont perspective

“…These results are consistent with the involvement of algal N-glycans in both recognition and engulfment stages during establishment of symbiosis (Weis et al 2008; Davy et al 2012). In the case of Exaiptasia interacting with Symbiodiniaceae, glycans appear to play critical roles in post-recognition “persistence mechanisms” (Parkinson et al, 2018). Symbiodiniaceae algae that are capable of establishing symbiosis have complex repertoires of glycan processing enzymes that differ from their free-living counterparts (Lin et al 2015 and Mohamed et al 2019, in prep), suggesting that variation in glycoprotein structure may be involved in host recognition specificity.…”