Symbiotic interactions between bacteria and protist hosts are widespread, diverse in function and frequently profoundly shape the evolution and metabolism of one or multiple partners. Whereas endosymbioses can represent an irreversible degree of cellular integration, episymbioses, where symbionts are attached to the surface of the host cell, capture a state in which metabolically coupled partners remain physically independent. As such, episymbioses offer unique opportunities to investigate how autonomous organisms establish, maintain, and evolve increasingly intimate cellular partnerships.

 

Episymbioses in euglenids

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My PhD research explored episymbiotic interactions between deltaproteobacteria and uncultivated euglenid (Euglenozoa) hosts using culture-independent approaches. I combined electron microscopy, genomics and transcriptomics to generate phylogenetic trees for both the hosts and their bacterial symbionts, and examine the structural basis of their partnerships. Molecular data generated from individual cells was used to reconstruct metabolic pathways, identifying potential processes and metabolite exchanges between host and symbiont that drive these symbioses. This work led to the discovery of previously unrecognized symbiotic interactions in euglenids similar to those seen in much more commonly studied larger organisms, and has resulted in the description of four new species and the first documented occurrence of anaerobic adaptation in euglenids.

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Episymbioses in symbiontid euglenozoans

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Symbiontids form the fourth major lineage of euglenozoans and are a poorly understood group of putatively anaerobic marine protists that inhabit low oxygen marine environments (e.g., oxygen minimum zones and oxygen depleted sediments) and are characterized by a conspicuous coat of epibiotic bacteria that completely cover the eukaryotic host cell surface. Despite being phylogenetically distinct from all other euglenozoans and incredibly diverse in hypoxic water columns based on environmental DNA sequencing surveys, only three species have been described to date. Curiously, the cellular diversity of symbiontids, their phylogenetic relationships with one another, the identity of their symbionts and the processes that drive this unique symbiosis remain unknown. However, the general cellular architecture of symbiontids suggests an intimate association between the bacteria and their hosts, driven by the transfer of metabolic byproducts among partners. 

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I am interested in integrating fieldwork, electron microscopy, molecular biology, and bioinformatics to expand the known diversity of symbiontids, and to build a comparative framework for understanding symbiontid biology. This includes defining host and symbiont metabolism using the cultivation-independent techniques and using these data to inform symbiontid cultivation attempts of symbiontids. Cultivated symbiontids can be used to experimentally test hypotheses for the processes that drive these partnerships as well as how these symbioses shape the evolution and physiology of both partners. Characterizing the cellular identity of symbiontid hosts and their symbionts will transform each lineage from an environmental sequence on a phylogenetic tree into a living symbiotic system and can inform how episymbiosis can serve as an adaptive strategy for persistence in hypoxic marine environments, habitats that continue to expand as climate changes.

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