Quality or quantity: is nutrient transfer driven more by symbiont identity and productivity than by symbiont abundance? |
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Authors: | Christopher J Freeman Robert W Thacker David M Baker Marilyn L Fogel |
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Institution: | 1.Department of Biology, University of Alabama at Birmingham, Birmingham AL, USA;2.Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA;3.Smithsonian Institution, National Museum of Natural History, Washington, DC, USA |
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Abstract: | By forming symbiotic interactions with microbes, many animals and plants gain access to the products of novel metabolic pathways. We investigated the transfer of symbiont-derived carbon and nitrogen to the sponges Aplysina cauliformis, Aplysina fulva, Chondrilla caribensis, Neopetrosia subtriangularis and Xestospongia bocatorensis, all of which host abundant microbial populations, and Niphates erecta, which hosts a sparse symbiont community. We incubated sponges in light and dark bottles containing seawater spiked with 13C- and 15N-enriched inorganic compounds and then measured 13C and 15N enrichment in the microbial (nutrient assimilation) and sponge (nutrient transfer) fractions. Surprisingly, although most sponges hosting abundant microbial communities were more enriched in 13C than N. erecta, only N. subtriangularis was more enriched in 15N than N. erecta. Although photosymbiont abundance varied substantially across species, 13C and 15N enrichment was not significantly correlated with photosymbiont abundance. Enrichment was significantly correlated with the ratio of gross productivity to respiration (P:R), which varied across host species and symbiont phylotype. Because irradiance impacts P:R ratios, we also incubated A. cauliformis in 13C-enriched seawater under different irradiances to determine whether symbiont carbon fixation and transfer are dependent on irradiance. Carbon fixation and transfer to the sponge host occurred in all treatments, but was greatest at higher irradiances and was significantly correlated with P:R ratios. Taken together, these results demonstrate that nutrient transfer from microbial symbionts to host sponges is influenced more by host–symbiont identities and P:R ratios than by symbiont abundance. |
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Keywords: | cyanobacteria microbial symbionts nutrient exchange porifera stable isotopes symbiosis |
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