| Sponges (phylum Porifera) evolved in Precambrian times, and research suggests that many sponge species began associations with large, diverse microbial communities early in their evolutionary history. This long history of close interactions suggests that the relationships are beneficial to both partners, but there is little conclusive evidence to support this hypothesis. By comparing sponge species with large, diverse microbial communities, termed high microbial abundance (HMA) sponges, to sponge species nearly devoid of associated bacteria, termed low microbial abundance (LMA) sponges, my research sought to test the hypotheses of positive microbial influences on sponges and that a symbiosis exists between the partners.; HMA sponges were found to have significantly higher tissue densities, significantly lower in situ pumping rates, and significantly different in situ fluxes of various nitrogen species, suggesting that bacteria influenced the morphology, physiology, and nitrogen budget of their host sponges. Measurements of stable carbon and nitrogen isotope ratios of bulk sponge tissue for a variety of sponges from the Florida Keys, North Carolina, and Papua New Guinea revealed that although carbon metabolism is most influenced by habitat, nitrogen metabolism varies with species, with most HMA sponges showing low nitrogen isotopic ratios generally indicating microbial metabolic processes. Transmission electron microscopy and denaturing gradient gel electrophoresis of two HMA sponges and an LMA sponge allowed me to characterize a portion of the microbial communities of the HMA sponges and to confirm the lack of extensive and diverse microbial communities among LMA sponges. Because anaerobic microbes were found in the HMA sponges, I used submersible oxygen microsensors and tetrazolium salts to detect in situ zones of hypoxia within sponge tissue, and found hypoxia in HMA sponges but not in LMA sponges. Finally, I measured the stable carbon and nitrogen isotope ratios of spongin fibers, which makes up the sponge skeleton, and sponge larvae or embryos, and found evidence of a transfer of nitrogen between the sponges and their associated bacteria. My research supports the hypothesis that HMA sponge evolution and ecology is impacted by their associated microbial community and that this community directly benefits the host sponge, resulting in a symbiotic relationship. |
