Mar Biol (2009) 156:669–677 DOI 10.1007/s00227-008-1117-6 ORIGINAL PAPER SulWde assimilation by ectosymbionts of the sessile ciliate, Zoothamnium niveum Hans Røy · Kay Vopel · Markus Huettel · Bo Barker Jørgensen Received: 17 October 2008 / Accepted: 15 December 2008 / Published online: 3 February 2009 © The Author(s) 2009. This article is published with open access at Springerlink.com Abstract We investigated the constraints on sulWde suggests that they live partly submerged in the diVusive uptake by bacterial ectosymbionts on the marine peritrich boundary layer. We showed that the Wltered volume allows ciliate Zoothamnium niveum by a combination of experi- Z. niveum to assimilate suYcient sulWde to sustain the sym- mental and numerical methods. Protists with symbionts biosis at a few micromoles per liter in ambient concentra- were collected on large blocks of mangrove-peat. The tion. Numerical modeling shows that sulWde oxidizing blocks were placed in a Xow cell with Xow adjusted to in bacteria on the surfaces of Z. niveum can sustain 100-times situ velocity. The water motion around the colonies was higher sulWde uptake than bacteria on Xat surfaces, such as then characterized by particle tracking velocimetry. This microbial mats. The study demonstrates that the Wlter feed- shows that the feather-shaped colony of Z. niveum gener- ing zooids of Z. niveum are preadapted to be prime habitats ates a unidirectional Xow of seawater through the colony for sulWde oxidizing bacteria due to Z. niveum’s habitat with no recirculation. The source of the feeding current was preference and due to the feeding current. Z. niveum is the free-Xowing water although the size of the colonies capable of exploiting low concentrations of sulWde in near norm-oxic seawater. This links its otherwise dissimilar hab- itats and makes it functionally similar to invertebrates with Communicated by M. Kühl. thiotrophic symbionts in Wltering organs. Electronic supplementary material The online version of this article (doi:10.1007/s00227-008-1117-6) contains supplementary material, which is available to authorized users. Introduction H. Røy · B. B. Jørgensen The peritrich ciliate Zoothamnium niveum (Ehrenberg Max Planck Institute for Marine Microbiology, 1838) forms attached contractile colonies that are visible to Celsiusstr. 1, 28359 Bremen, Germany the naked eye. The colony shape resembles a feather with Present Address: an average length between 3 and 5 mm. The side-branches H. Røy (&) of the “feather” carry several hundred microzooids Department of Biological Sciences, (zooid = single ciliate) in a regular pattern (see Bauer- Center for Geomicrobiology, University of Aarhus, Ny Munkegade 1535, 8000 Aarhus C, Denmark Nebelsick et al. 1996a, b for a detailed description). The e-mail: [email protected] entire surface of the zooids, stalk and branches are covered by a single layer of sulWde-oxidizing bacteria belonging to K. Vopel the Gamma-proteobacteria (Rinke et al. 2007). The peri- School of Applied Sciences, W Auckland University of Technology, Mail No C43, trich ciliates are lter-feeders, and the microzooids of Private Bag 92006, Auckland 1142, New Zealand Z. niveum possess a fully developed oral ciliature and cytopharynx, similar to Zoothamnium species that are not M. Huettel covered by sulWde oxidizing bacteria (Bauer-Nebelsick et al. Department of Oceanography, Florida State University, 117 N Woodward Ave., 1996b). Rapid contractions of zooids, branches and the dis- OSB 517, Tallahassee, FL 32306-4320, USA tal parts of the stalk are characteristic of all Zoothamnium 123 670 Mar Biol (2009) 156:669–677 ectosymbionts (Ott et al. 1998; Vopel et al. 2001). Ott et al. (1998) proposed that Z. niveum supplies the bacterial sym- bionts with sulWde through periodic contraction into the anoxic and sulWdic diVusive boundary layer (DBL). Studies of the physical and chemical microenvironment around the colonies (Vopel et al. 2001, 2002, 2005) could not conWrm this mechanism, but suggested that the feeding currents intercept sulWde and brings it to the symbiotic bacteria. In this study, we link the sulWde transport quantitatively from the turbulent water column, via the feeding current, to the bacteria on the surface of Z. niveum. We analyze the Xow Weld around individual colonies at high resolution. These data identify the source, pathway and volume of sea- water passing through the colonies. We use literature data on the oxygen consumption of Z. niveum to estimate the sulWde requirements of the consortia based on the stoichi- ometry of lithothropic growth by sulWde oxidation. The Wltered seawater volume together with the sulWde require- ment provides a rough estimate of the constraints on sulWde uptake by the ecto-symbionts on Z. niveum. We reWned the constraints further using numerical modeling. Fig. 1 White spot around a 1 cm wide conduit into the peat wall. Several Zoothamnium niveum colonies can be seen around the hole. Material and methods Recorded in situ with a Nikon coolpix 960 in underwater housing Field site and sampling species. For Z. niveum, the contraction takes only about 4 ms, and the average velocity reaches 520 mm s¡1 (Vopel Experimental work was conducted on the island of Carrie et al. 2002). During contractions and the following slow Bow Cay, Belize, in April 2002, at the Weld station operated expansion the cilia cease. by the Caribbean Coral Reef Ecosystem program of the The type organisms for a recent re-description of Z. niv- National Museum of Natural History (Washington, DC, eum (Bauer-Nebelsick et al. 1996b) were collected on man- USA). Fieldwork was done in the channel that separates the grove islands of the Belize barrier reef. Several ecological two islands of Twin Cays, in an area locally known as Bat- investigations have been performed on this population. Wsh Point. Detailed description of Twin Cays and the tidal Aggregations of Z. niveum and non-symbiotic sulfur-bacte- channels can be found in Rützler and Macintyre (1982) and ria form characteristic “white spots” (Fig. 1) on undercut Ott et al. (1998). peat-banks under Red Mangrove (Rhizophora mangle) In situ observations and ciliate collection were per- stands. Typical spots are 5–25 mm in diameter and contain formed while SCUBA diving along undercut peat banks at 9–43 Zoothamnium colonies (Ott et al. 1998). The spots BatWsh Point. Current speed and direction were mea- often occur around bark-lined holes in the peat. The holes sured by timing 10 cm displacements of natural particles form when a mangrove root dies and the core of the root in the seawater with a stopwatch. Typical currents were decomposes. The seawater in these bark conduits is anoxic 5–20 mm s¡1 measured 1–3 cm from the peat surface. and contains up to 1 mmol L¡1 sulWde. SulWdic water in the Then the spots with ciliate colonies were cut out together conduits mixes with the surrounding seawater, primarily with a 20 £ 20 £ 20 cm3 block of surrounding peat and driven by wave-generated oscillating boundary Xow. This placed in a closed container while still under water. makes the holes function like miniature sulWde vents (Vopel et al. 2005). Laboratory Xow cell Though the published evidence is mostly suggestive, the association between Z. niveum and its ectosymbionts has After transport to the laboratory, the Wbrous peat blocks been assumed to be obligatory. A trophic character of the were trimmed to Wt a glass Xow-cell that accommodated a symbiosis is suggested from observation of bacteria mor- 45 mm thick 90 £ 150-mm peat block. Care was taken to photypes in the food vacuoles (Bauer-Nebelsick et al. align the surface of the peat Xush with the inXow and to 1996b), and by poor growth of the ciliates without place the blocks with the same orientation to the current as 123 Mar Biol (2009) 156:669–677 671 they were found in situ. Peat and ciliates were never Multiphysics (Stockholm), which can perform reaction– exposed to air. diVusion–advection calculations in an arbitrary geometry Water in the Xow cell was re-circulated through an aer- by dividing it into triangular Wnite elements. The Xow ated container holding 7 L of seawater. At the entrance of around individual zooids was calculated from the “Incom- the Xow cell, seawater was passed through 50 mm of coarse pressible Navier–Stokes” application mode from the “Earth Wlter-foam (blue EHEIM) to dissipate inXow turbulence. A Science Module”. Solute dynamics was calculated in the similar foam block was placed downstream of the peat to “Convection and DiVusion” application mode based on prevent the Xow from converging toward the outXow. The the results of the Navier–Stokes solution. Geometry of the free-Xow section between the foam blocks was 150 mm and zooids was based on Bauer-Nebelsick et al. (1996a, b) and the water depth during measurements 50 mm. Free Xow that of T. zoothamnicoli on Rinke et al. (2007). The density velocity was adjusted to 1 cm s¡1 reproducing a setting of the calculation mesh was adjusted to give adequate cal- within the velocity range measured in situ (see below). The culation speed and tested not to inXuence the model results. entire system was allowed to reach equilibrium at 29°C and Time steps were controlled dynamically to assure numerical ¡1 190 mol O2 L before measurements. These values were stability. The applied model of Z. niveum describes a single within the range observed in the habitat. All laboratory radial-symmetric zooid with surrounding seawater. The observations were performed within 18 h from the time of purpose of the model was to describe the Xux to the bacteria collection. on the ciliates surface. Three scenarios were modeled: (1) one micro-zooid with functional cilliary apparatus; (2) a Particle-tracking velocimetry hypothetical scenario with a single isolated micro-zooid without feeding current, and (3) a single bacterial symbiont The Xow cell was mounted on a metal frame together with a suspended in stagnant water.