All ETDs from UAB

Advisory Committee Chair

Robert W Thacker

Advisory Committee Members

Charles Amsler

Asim Bej

Daniel Gleason

Deb Gochfeld

Document Type

Dissertation

Date of Award

2012

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

Microbial communities spanning nearly all evolutionary lineages of bacteria and archaea are found within many species of marine sponges. Although diverse microbial metabolic pathways may benefit sponge hosts, minimal experimental evidence exists supporting host assimilation of symbiont-derived nutrients. By coupling manipulative shading experiments with stable isotope analyses of isolated microbial and sponge cell fractions, I provide evidence that some sponge hosts assimilate a large proportion of their C and N from microbial symbionts and that these inputs are crucial to host performance. Interestingly, however, these interactions are highly variable across host species, suggesting that overall symbiont abundance alone does not drive trends across species. Instead, using light-dark bottle incubations with inorganic compounds enriched in 13C and 15N, I show that variation in the efficiency of symbiont metabolism is correlated to holobiont photosynthesis: respiration ratio (gross primary productivity [P] : respiration [R]) and the presence of specific clades of the cyanobacterial symbiont Synechococcus spongiarum. Finally, to determine if symbioses within two closely related species have fundamentally different responses to a gradient of environmental change, I conducted a shading experiment with Aplysina cauliformis and A. fulva using 6 irradiance treatments. Growth rates in A. cauliformis were variable across treatments and highest at the 2 extremes, while growth rates in A. fulva decreased with irradiance, suggesting that a gradient of environmental conditions impacts these sponges differently. These trends are especially interesting considering that both species had minimal variation in the abundance of S. spongiarum clades A, B, and C and assimilated a predominant portion of their C and N from symbiont sources across all treatments. Instead, increased symbiont carbon assimilation under the full shade in A. cauliformis may translate to added benefit to this host, while the efficiency of symbiont carbon metabolism decreased with irradiance in A. fulva, leading to a reduction in growth rate. Together, these results suggest that dense symbiont communities vary in their overall benefit to sponge hosts. Such variation may be due to multiple, complex factors including the relative presence of beneficial vs. harmful symbionts and how the benefit conferred by specific symbiont taxa differs under changing environmental conditions.

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