Sulfide assimilation by ectosymbionts of the sessile ciliate, Zoothamnium niveum
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We investigated the constraints on sulfide uptake by bacterial ectosymbionts on the marine peritrich ciliate Zoothamnium niveum by a combination of experimental and numerical methods. Protists with symbionts were collected on large blocks of mangrove-peat. The blocks were placed in a flow cell with flow adjusted to in situ velocity. The water motion around the colonies was then characterized by particle tracking velocimetry. This shows that the feather-shaped colony of Z. niveum generates a unidirectional flow of seawater through the colony with no recirculation. The source of the feeding current was the free-flowing water although the size of the colonies suggests that they live partly submerged in the diffusive boundary layer. We showed that the filtered volume allows Z. niveum to assimilate sufficient sulfide to sustain the symbiosis at a few micromoles per liter in ambient concentration. Numerical modeling shows that sulfide oxidizing bacteria on the surfaces of Z. niveum can sustain 100-times higher sulfide uptake than bacteria on flat surfaces, such as microbial mats. The study demonstrates that the filter feeding zooids of Z. niveum are preadapted to be prime habitats for sulfide oxidizing bacteria due to Z. niveum’s habitat preference and due to the feeding current. Z. niveum is capable of exploiting low concentrations of sulfide in near norm-oxic seawater. This links its otherwise dissimilar habitats and makes it functionally similar to invertebrates with thiotrophic symbionts in filtering organs.
KeywordsSulfide Diffusive Boundary Layer Bacterial Symbiont Peat Surface Feeding Current
This work was supported by the Caribbean Coral Reef Ecosystems program of the Smithsonian National Museum of Natural History (Washington, DC), the Danish Research Academy and the Max Plank Society. We thank J. Ott for introducing us to the study Z. niveum, and M. Carpenter and D. Miller for support on Carrie Bow Cay. The manuscript was greatly improved by constructive reviews. This is contribution 658, Caribbean Coral Reef Ecosystems Program, Smithsonian Institution. The experiments comply with the current laws of Belize and Germany.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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