All ETDs from UAB

Advisory Committee Chair

Robert Thacker

Advisory Committee Members

Asim Bej

Julie Olson

Daniel Warner

Scott Santos

Document Type

Dissertation

Date of Award

2017

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

Intimate partnerships between marine invertebrates and microbial symbionts have garnered renewed interest, as advances in next generation sequencing have revealed substantial microbial diversity in tropical sponges and ascidians. These datasets have generated a suite of new questions concerning the evolutionary and ecological mechanisms that structure these microbial partnerships. The research presented herein addresses such questions by integrating in situ ecological manipulations with modern molecular techniques to characterize the interactions between the host invertebrate and microbiome and its potential role in structuring marine communities. Implementing in situ manipulative shading experiments alongside replicated bacterial clone libraries, I documented significant variation in host benefit across sponge species harboring different clades of the sponge-specific cyanobacteria Synechococcus spongiarum. Sponges hosting specialist S. spongiarum clades were unable to subsist under low light, while species retaining multiple symbiont clades persisted. These results not only highlight the variable benefit between specialist and generalist symbiont clades, but also document potential consequences of obligate mutualisms, particularly in areas vulnerable to changing environmental conditions. Nutritional contributions from S. spongiarum were predicted to serve as a source of bottom-up regulation for Caribbean sponge populations, due to its association with one-third of the dominant sponge species. Field experiments testing the relative roles of predation and photosymbiont-derived nutrition on sponge communities provided evidence that these factors interact, implying that sponges community structure is driven by more than one trophic mechanism. Importantly, I show that cyanobacterial symbionts may act as an alternative source of bottom-up regulation, with facultative mutualisms serving a compensatory role when predator density is high. Shading experiments coupled with nutrient enriched conditions examined the variable responses among invertebrate species hosting unique microbial populations. I documented strong host resilience and symbiont stability under elevated stress for all sponge-microbe partnerships. Alternatively, ascidians and their associated microbiome were susceptible to increased environmental stress, suggesting that some symbioses demonstrate intolerance to specific environmental variables. Collectively, these results suggest that dense symbiont communities vary in their overall benefit to sponge hosts, which may have broader implications on sponge community dynamics. However, further investigations are necessary to enhance our understanding of host–symbiont dynamics and clarify the role of symbiont stability and host resilience in a broader ecological context.

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