New paper published

by Xavier Turon

New paper published in Frontiers in Zoology: “Feeding cessation alters host morphology and bacterial communities in the ascidian Pseudodistoma crucigaster”, by Susanna López-Legentil, Xavier Turon and Patrick M. Erwin.

This article is the result of yet another collaboration between the projects ChallenGen and MarSymbiOmics. We have coupled electron microscopy and 16S rRNA gene tag pyrosequencing to investigate the bacterial communities associated with the colonial ascidian Pseudodistoma crucigaster, a species endemic to the Mediterranean Sea that has a life cycle with two phases: actively-filtering (active) and non-filtering (resting) forms.

Active (A) and resting (B) colonies of Pseudodistoma crucigaster. The active form has functional siphonal apertures for filtering water, while the resistance form has all apertures sealed by a glassy cuticle.

Resting colonies exhibited a reduced branchial sac (feeding apparatus) and a thickened cuticle. Electron microscope images also suggested higher abundance of colonizing microorganisms on surfaces of resting colonies. Accordingly, bacterial sequences associated with environmental sources (sediment and biofilms, >99 % similarity) were detected exclusively in resting colonies. Bacterial communities of P. crucigaster colonies (active and resting) were dominated by 3 core taxa affiliated (>94 % similarity) with previously described symbiotic Alphaproteobacteria in marine invertebrates. Shifts in rare bacteria were detected when ascidians entered the resting phase, including the appearance of strictly anaerobic lineages and nitrifying bacterial guilds.

These findings suggest that physical (thickened cuticle) and metabolic (feeding cessation) changes in host ascidians have cascading effects on associated bacteria, where modified oxygen concentrations and chemical substrates for microbial metabolism may create anaerobic microhabitats and promote colonization by environmental microorganisms.

Network of bacterial OTUs in three active and three resting samples of Pseudodistoma crucigaster, with edges coded by specificity.