Physiological changes during torpor favor association with Endozoicomonas endosymbionts in the urochordate Botrylloides leachii

Abstract

Environmental perturbations evoke down-regulation of metabolism in some multicellular organisms, leading to dormancy, or torpor. Colonies of the urochordate Botrylloides leachii enter torpor in response to changes in seawater temperature and may survive for months as small vasculature remnants that lack feeding and reproductive organs but possess torpor-specific microbiota. Upon returning to milder conditions, the colonies rapidly restore their original morphology, cytology and functionality while harboring re-occurring microbiota, a phenomenon that has not been described in detail to date. Here we investigated the stability of B. leachii microbiome and its functionality in active and dormant colonies, using microscopy, qPCR, in situ hybridization, genomics and transcriptomics. A novel lineage of Endozoicomonas, proposed here as Candidatus Endozoicomonas endoleachii, was dominant in torpor animals (53–79% read abundance), and potentially occupied specific hemocytes found only in torpid animals. Functional analysis of the metagenome-assembled genome and genome-targeted transcriptomics revealed that Endozoicomonas can use various cellular substrates, like amino acids and sugars, potentially producing biotin and thiamine, but also expressing various features involved in autocatalytic symbiosis. Our study suggests that the microbiome can be linked to the metabolic and physiological states of the host, B. leachii, introducing a model organism for the study of symbioses during drastic physiological changes, such as torpor.

Effects of color variation and physiological state on ascidian microbiomes

“…Endozoicomonas are conspicuous chemoorganoheterotrophic Gammaproteobacteria found in a wide range of animal hosts, including coral, sponges, and fish (reviewed in Neave et al, 2016 ). In ascidians, Endozoicomonas are also prevalent (Galià‐Camps et al, 2023 ; Hyams et al, 2023 ; Schreiber et al, 2016 ) and phylogenetic analyses of 16S rRNA genes have revealed an ascidian‐specific clade (Schreiber et al, 2016 ). Members of the same ascidian‐specific Endozoicomonas clade were also detected in seawater, albeit at much lower abundances, suggesting a facultative symbiosis (Schreiber et al, 2016 ).…”

“…Further analysis revealed the presence of Endozoicomonas microcolonies on the branchial sac of most of the ascidian species investigated (Schreiber et al, 2016 ). In addition, resting colonies of B. leachii were enriched with a novel lineage of Endozoicomonas that appear to occupy specific hemocytes found exclusively in resting animals (Hyams et al, 2023 ). Here, we observed a decrease in the relative abundance of Endozoicomonas in the resting forms of P. constellatum .…”

“…López‐Legentil et al ( 2016 ) showed that although both filtering and resting colonies of the aplousobranch Pseudodistoma crucigaster maintained their core microbial symbionts, shifts in rare bacteria were detected in resting colonies, including the appearance of strictly anaerobic lineages, nitrifying bacterial guilds, and additional environmental microorganisms. Similarly, Hyams et al ( 2023 ) found that the stolidobranch Botrylloides leachii harbored a resting‐specific microbiota.…”

Deep quantitative proteomics of North American Pacific coast star tunicate (Botryllus schlosseri)

“…A recent study on another botryllid ascidian has revealed that under stress conditions that lead to a torpor state, a novel lineage of the commensal bacterium Endozoicomonas, was dominant in torpor animals, and potentially occupied hemocyte types that were only detected in torpid animals [61]. This bacterial symbiont is found in close association with diverse marine invertebrate hosts, including reef-building corals, sponges, bryozoans, sea slugs, and other tunicates [62][63][64][65][66].…”