Abstract
BackgroundAscidians can associate with abundant and diverse consortia of microbial symbionts, yet these communities remain unexamined for the majority of host ascidians and little is known about host-symbiont interactions.MethodsWe 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.ResultsResting 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.ConclusionsThese 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0134-4) contains supplementary material, which is available to authorized users.
Highlights
Ascidians can associate with abundant and diverse consortia of microbial symbionts, yet these communities remain unexamined for the majority of host ascidians and little is known about host-symbiont interactions
Because these animals are sessile as adults and cannot escape their predators, ascidians are well-known as prolific producers of defensive secondary metabolites [32,33,34], several of which function as chemical protection against
The zooids were characterized by a strong regression of the branchial sac and the accumulation of reserves in the abdomen and postabdomen (Fig. 2)
Summary
Ascidians can associate with abundant and diverse consortia of microbial symbionts, yet these communities remain unexamined for the majority of host ascidians and little is known about host-symbiont interactions. Ascidians or sea-squirts (Chordata, Ascidiacea) are a diverse group of sessile marine invertebrates characterized by their secreted gelatinous or leathery tunic [31] composed of tunicine (a cellulose-like polysaccharide). Because these animals are sessile as adults and cannot escape their predators, ascidians are well-known as prolific producers of defensive secondary metabolites [32,33,34], several of which function as chemical protection against. Few studies have aimed to pinpoint the origin of these secondary metabolites and have uncovered molecules of both bacterial symbiont [40, 41] and animal host origin [42, 43]
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