Mineralogical Gradients Associated with Alvinellids at Deep-Sea Hydrothermal Vents

Deep-Sea Research I 50 (2003) 269–280

Mineralogical gradients associated with alvinellids at deep-sea hydrothermal vents

Magali Zbindena, Nadine Le Brisb, Philippe Compere" c, Isabelle Martinezd, Francois, Guyote, Francoise, Gailla,* a Laboratoire de Biologie Marine, UMR CNRS 7622, Quai Saint Bernard, 75252 Paris cedex 05, France b Departement! Environnement Profond, IFREMER, BP 70, 29280 Plouzane,! France c Institut de Zoologie, UniversitedeLi! ege," 22 Quai Van Beneden, B-4020 Liege," Belgium d Laboratoire de Geochimie! des Isotopes Stables, IPGP, 4 place Jussieu, 75252 Paris cedex 05, France e Laboratoire de Mineralogie,! LMCP and IPGP, 4 place Jussieu, 75252 Paris cedex 05, France

Received 16 November 2001; received in revised form 30 April 2002; accepted 8 November 2002

Abstract

Alvinella pompejana and Alvinella caudata live in organic tubes on active sulphide chimney walls at deep-sea hydrothermal vents. These polychaete annelids are exposed to extreme thermal and chemical gradients and to intense mineral precipitation. This work points out that mineral particles associated with Pompeii worm (A. pompejana and A. caudata) tubes constitute useful markers for evaluating the chemical characteristics of their micro-environment. The minerals associated with these worm tubes were analysed on samples recovered from an experimental alvinellid colony, at different locations in the vent ﬂuid–seawater interface. Inhabited tubes from the most upper and lower parts of the colony were analysed by light and electron microscopies, X-ray microanalysis and X-ray diffraction. A change was observed from a Fe–Zn–S mineral assemblage to a Zn–S assemblage at the millimeter scale from the outer to the inner face of a tube. A similar gradient in proportions of minerals was observed at a decimeter scale from the lower to the upper part of the colony. The marcasite/pyrite ratio of iron disulphides also displays a steep decrease along the few millimeters adjacent to the external tube surface. The occurrence of these gradients indicates that the micro- environment within the tube differs from that outside the tube, and suggests that the tube wall acts as an efﬁcient barrier to the external environment. r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Biogeochemistry; Annelids; Tubes; Biomineralisation; Zinc–iron sulphides

1. Introduction polychaete annelids dwelling in organic tubes on smoker walls of deep-sea vents of the East Paciﬁc Alvinella spp. (i.e. A. pompejana and A. cauda- Rise (review in Desbruyeres" et al., 1998). Because ta), the so-called Pompeii worms, are thermophilic of their location on active parts of chimney walls, these organisms are exposed to steep thermo- *Corresponding author. Tel.: +33-1-44-27-30-63; fax: +33- chemical gradients and intense mineral precipita- 1-44-27-52-50. tion. Still, the precise knowledge of the conditions E-mail address: [email protected] (F. Gaill). sustained by Alvinella constitutes one of the most

0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(02)00161-9 270 M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 puzzling questions of deep-sea vent biology scopies, X-ray microanalysis and X-ray diffraction (Chevaldonne! et al., 2000; Pradillon et al., 2001; (XRD). Shillito et al., 2001). Since Alvinella spp. was shown to spend most of the time within its tube (Chevaldonne! and Jollivet, 1993), the conditions 2. Materials and methods prevailing in this micro-environment are of particular interest and, especially, potential differences 2.1. Sample collection between the characteristics of the micro-surrounding in the inner space delimited by tubes and the An alvinellid colony was collected with the outside conditions have to be examined. This can ‘‘Nautile’’ submersible during the ‘‘HOT 96’’ be approached by chemical characterisation of cruise (91N East Pacific Rise, 2600 m depth). This fluid phases in contact with the inner and outer colony was recovered from a TRAC (Titanium faces of the tube, hereafter called inner and outer Ring for Alvinellid Colonisation, Gaill et al., 1996; medium fluids, respectively. Taylor et al., 1999). The TRAC was deployed for Direct characterisation of the thermal and over 70 days at the top of the M vent smoker, chemical centimeter-scale gradients in the alvinel- initially colonised by alvinellids. This device lid surrounding is expected to provide further clues (Fig. 1) enables collection of the colony that has to address this question. Temperature measure- developed in and around it, preserving its spatial ments and analyses of the chemical conditions structure. Just before recovery, discrete tempera- were performed inside Alvinella spp. tubes with ture measurements were achieved at several points specific sampling and sensing devices (Cary et al., at the bottom and at the top of the device, with a 1998; Di Meo et al., 1999; Luther III et al., 2001). probe held by the ‘‘Nautile’’ arm. The experi- Up to now, this approach did not reveal differ- mental alvinellid colony was dissected on board in ences between the inner and outer medium fluids. order to collect tube portions from two horizontal The study of minerals associated with inhabited sections of equal height (4 cm) corresponding to Alvinella spp. tubes is an interesting alternative to the uppermost (level 1) and lower (level 2) parts of the direct approach of in situ measurement of the colony (Fig. 1). Level 1 was directly in contact thermal and chemical parameters. Previous studies with the surrounding seawater-vent fluid mixing have provided information about the tube char- zone while level 2 was located at the base of the acteristics (see review in Gaill and Hunt, 1991), but TRAC, which had been directly deployed on the the nature and mineralogy of the associated mineralised surface of the smoker. Collected tubes, mineral particles have not been systematically from which the animals were removed, were fixed investigated. Recently, Zbinden et al. (2001) have in saline formalin and kept in a 701-ethanol shown that mineral particles found within Alvi- solution. Mineral deposits that had grown on the nella spp. tubes exhibit constant composition and TRAC walls and between tubes were also recov- microstructure, suggesting that mineral particles ered from these two sections. may be good indicators of chemical conditions prevailing in the worm’s micro-environment. 2.2. Ash and C/N/S content of the tube The present study is the first to combine compositional and ultrastructural information Some of the tubes from levels 1 and 2 were oven- about mineral deposits within and at the inner dried at 801C for 48 h and weighed with a and outer surfaces of recently secreted Pompeii Sartorius precision balance (104 g). They were worm tubes, from different locations at the active further heated for 2 h at 4501C (Ehret oven) and substrate–sea water interface. Samples were col- the ash weighed. Other tubes of each level were lected selectively from experimental alvinellid used for measurements of C/N/S contents. Tubes colonies obtained with a specific device. The were dried in ambient air and ground to powder. mineral particles associated with inhabited tubes For each sample, 3 replicates (about 10 mg dry were analysed by photonic and electron micro- weight) were analysed for total carbon and M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 271

the TRAC from levels 1 and 2 were also dried, ground and analysed. XRD studies were carried out with a Philips PW 1710 diffractometer operated at 40 kV, 30 mA with a Co tube. Spectra were collected between 2Y ¼ 31 and 1001 with 0:04ð2YÞ steps and 20 s per step. A Rietveld reﬁnement program was used for interpreting the multiphase spectra.

2.4. Light and scanning electron microscopies and X-ray microanalysis

Tube samples were dehydrated in ethanol and propylene oxide series and then embedded in an epoxy resin (Serlabo). Thick polished sections were obtained with a rotary saw. The sections were progressively thinned to 80 mm by abrasion on sandpaper, then polished on velvet with a 0.3 mm alumina suspension and glued on a glass slide with an epoxy resin (Epotecny, E501). Some of these sections were stained with toluidin blue to show the organic matrix of the tube by light microscopy (with a Nikon Optiphot-pol microscope). The other sections were carbon-coated in a Balzers BAF-400 evaporator prior to energy dispersive X-ray microanalysis (EDX) and observations, achieved in a scanning electron microscope (SEM: JEOL JSM-840A) operating at 20 kV. X-ray microanalysis and elemental mappings were performed in the SEM ﬁtted with a Link Pentafet Fig. 1. (A) Schematic representation of the TRAC (showing the location of the two levels of sampling). Total height of the detector and a Link eXl-10 analyser. TRAC is 15 cm and diameter is 25 cm. (B) Photograph of the TRAC in situ at the top of the smoker. 3. Results nitrogen contents with a Nitrogen Analyser 1500 (Carlo Erba Strumentazione, Milano, Italy). Three 3.1. Macroscopic observations other replicates of the same samples (about 2–3 mg dry weight) were analysed for total (i.e. both In situ observations of the TRAC showed the organic and inorganic) carbon and sulphur con- formation of mineral deposits and the colonisation tents with a carbon–sulphur Determinator CS-125 by alvinellids from the basal part to the top of the (Leco Corporation St-Joseph, MI, USA). The TRAC during the 70 days of deployment. One minimum weight of sample required for one hundred and ﬁfty alvinellids belonging to three analysis was 500 mg. species (Paralvinella grasslei, Alvinella pompejana and Alvinella caudata) were sampled from the 2.3. X-ray diffraction (XRD) device. Since the tubes of the two species of the genus Alvinella cannot be morphologically Tubes of each level were dried in ambient air distinguished from each other, they were consid- and ground to powder. Minerals recovered inside ered together for the following observations. 272 M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280

Paralvinella grasslei, which only secretes mucus, the whole sample. As sphalerite and wurtzite (the was not considered here. Obvious color differences two polymorphs of ZnS) XRD signatures are very were noticed between tubes from levels 1 and 2, close, it was not possible to determine their relative with the former white and the latter grey/black contributions. Small amounts of pyrite and (Figs. 2A and B). In situ temperatures measured marcasite form associated phases. These two before the TRAC recovery were about 201C for polymorphs of iron sulphide (FeS2) are present level 1 and 701C for level 2. in equal proportions and represent about 5 wt% of the sample (Table 2). 3.2. Ash and C/N/S composition of the tube Level 2 minerals are also predominantly composed of zinc sulphides (Fig. 3B) but, in contrast Weighing of dry tubes and ash showed that the with level 1, they represent only 60 wt% of the proportion of ash is higher in the tubes from the sample (Table 2). The two forms of FeS2 are bottom level (level 2) than in those from the top still present in equal amounts and represent one (level 1) (Table 1). Ash content represents almost half of the tube dry weight (45.5%) in the ﬁrst case, whereas it does not exceed one quarter of Table 1 the sample dry weight (24.6%) in the second case. Alvinella spp. and Riftia pachyptila tube composition in C, N The C/N/S ratio of these tubes is quite similar for and S (both organic and inorganic), and total inorganic content both levels, the percentage of sulphur being (ashes). Results are given in percentage of tube dry weight slightly larger in tubes from level 1 (Table 1). C N S Ash Alvinella 3.3. X-ray diffraction Level 1 16 4.3 59.5 24.6 Level 2 14.3 4.6 48.5 45.5 3.3.1. Minerals from the TRAC 29a Minerals collected at both levels are predomi- Riftia 40b 9b 4.5b 3a nantly zinc and iron sulphides. In level 1 samples, the predominant phase is zinc sulphide (ZnS) a From Gaill and Hunt (1986). b (Fig. 3A), which represents more than 95 wt% of Ravaux (pers. comm.).

Fig. 2. Macroscopic morphology of the tube at level 1 (A) and at level 2 (B) with the corresponding transversal sections, respectively (C) and (D). An important mineral cover is present on the outer face of tubes from level 2 and mineral particles are also visible inside the tube matrix, between the tube layers. At level 1, mineral particles are less numerous and smaller. Scale bars: A and B ¼ 1 cm; C and D ¼ 250 mm. M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 273

Fig. 3. X-ray diffraction spectra of minerals from level 1 (A) and level 2 (B), and of tubes from level 1 (C) and level 2 (D). Only peaks from major phases in each sample have been labelled (Sp: sphalerite, Py: pyrite, Ma: marcasite). For the tubes of both levels, the major peaks correspond to elemental sulphur and have not been labelled. The simulated spectrum of elemental sulphur (under S8 form) is shown immediately below the spectrum C. 274 M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280

Table 2 Mineralogical features of the mineral particles associated with the Alvinella spp. tubes and recovered on the TRAC

Tubes Minerals from the TRAC wall (wt%)

Inner face Inside Outer face

Level 1

S8 + ZnS + + + 95%

FeS2 +5%

Level 2

S8 + 10% ZnS + + + 60%

FeS2 + 30% Pyrite 25% 50% Marcasite 75% 50%

Qualitative results of scanning electron microscopy X-ray microanalysis are indicated by ‘‘+’’ in the cases where the elements are present. Quantitative results of X-ray diffraction are given in percentage of the total dry weight of the phases (wt%).

approximately 30 wt% of the sample (Table 2). medium (Figs. 2D and 4A). These particles are The XRD results also indicate the presence of coarse and lobated and range from 10 to 400 mmin native sulphur (about 10 wt%), not observed in diameter (Fig. 4A). Their bulk consists exclusively level 1 samples. of S and Fe (Figs. 4B, C and E). They locally contain small S and Zn rich-inclusions, having a 3.3.2. Tubes stoichiometry close to ZnS (Figs. 4D and F). The major crystallised phase observed in bulk Tubes from level 1 differ from those from level 2 tube samples of both levels is native sulphur in having little to no coarse particles at the tube (Figs. 3C and D). This phase is associated at both external surface, and little to no iron sulphide levels with the two iron sulphide polymorphs: pyrite present (Figs. 2C and 5A). Only one cluster of and marcasite. Unlike in the minerals recovered particles (24 mm in diameter, 65 mm long) was inside the TRAC, these two phases are not present found on the polished sections from level 1 in equal proportion in those samples, marcasite (Fig. 5A). X-ray spectra and mappings of these being far more abundant (75%) than pyrite (25%) particles (Fig. 5) reveal the presence of three main (Table 2). The main difference between samples elements: sulphur (S), iron (Fe), and zinc (Zn). from the two levels is the higher relative contribu- These elements are heterogeneously distributed tion of iron sulphide in tubes from level 2. inside the particles. These particles appear to be composed mostly of zinc sulphide (ZnS) with 3.4. Light microscopy, scanning electron minor iron content (Figs. 5D and F), with the microscopy and X-ray microanalysis exception of small rounded nuclei where sulphur and iron are the only elements present (Figs. 5B, C The tube wall structure is similar in tubes from and E). levels 1 and 2. It consists of 3–6 layers, made visible by their intensely stained boundaries. 3.4.2. Mineral particles inside the tube proteinaceous layers 3.4.1. Mineral particles at the outer tube surface In contrast with the outer face, particles are Tubes from level 2 are characterised by the small and rare at the inner side of the tube wall, in presence of iron sulphide minerals covering the contact with the inner medium ﬂuids. Alignments outer surface, at the interface with the external of small particles are also observed within the tube M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 275

Fig. 4. Aspect and elemental composition of a mineral particle from the outer face of a tube from level 2. SEM secondary electron image (A) shows the morphological aspect of a polished section of mineral particles. X-ray mappings indicate that 2 major elements are found in these mineral particles: sulphur (B) and iron (C) are present in the whole concretion. Zinc (D) is also found in the concretion but in restricted areas. In these areas, spectrum F (see the analysed beam position in Fig. 4A) shows that sulphur and zinc are present in relatively similar proportions. Iron is also present but in lower amount. In the rest of the concretion, the ratio S/Fe is 3/2 (spectrum F, see the analysed beam position in Fig. 4A). Scale bar=20 mm. wall, along the boundaries between the layers. the tube consist mainly of S and Zn (Figs. 6B and Tube wall layers are well identiﬁed on secondary C), which give rise to the major peaks in the electron images of polished sections (Fig. 6). The spectra (Fig. 6F). These elements are accompanied interboundary layers are evidenced by their lighter by phosphorus (P) and calcium (Ca) that appear to aspect when compared to the darker matrix. X-ray be associated with the layered organic matrix as spectra and mappings (Fig. 6) show that the they are also found within layers free of mineral elemental composition of the alignments of miner- deposits (Fig. 6G). These observations are consis- al particles inside the tube wall is similar, whatever tent with the transmission electron microscopy the level considered. The mineral particles inside study of Zbinden et al. (2001). 276 M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280

Fig. 5. Aspect and elemental composition of mineral particles from the outer face of a tube from level 1. SEM secondary electron image (A) shows the morphological aspect of a polished section of mineral particles. X-ray mappings indicate that 3 major elements are found in these mineral particles: sulphur (B, Ka peak at 2.307 keV), iron (C, Ka peak at 6.400 keV, Kb at 7.059 keV) and zinc (D, Ka peak at 8.631 keV, Kb at 9.572 keV). Spectrum E (see the analysed beam position in Fig. 5A) shows the relative proportion of the elements of the core area depleted with Zn. Spectrum F (see the analysed beam position in Fig. 5A) shows the relative proportion of the elements in Zn-rich zone. Scale bar=10 mm.

4. Discussion within the Alvinella spp. tubes, as compared to those of Riftia pachyptila. The main chemical and This study brings new data about the miner- mineralogical features of the tube particles and alisation of Alvinella spp. tubes. These data also corresponding smoker minerals at the two level of conﬁrm that the abundance of minerals associated the TRAC have been reported in Table 2. These with Alvinella spp. tubes is 8–15 times larger than results emphasise the existence of steep gradients that associated with Riftia pachyptila, as pre- in abundance, size and composition of mineral viously shown (Gaill and Hunt, 1986)(Table 1). phases. Gradients are observed at the colony Another striking difference, evidenced by this (decimeter) scale moving from the lower to the study, is the remarkably high sulphur content uppermost level of the TRAC, as well as at the M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 277

Fig. 6. Aspect and elemental composition of the Alvinella spp. tube organic matrix and of the included mineral particles. SEM secondary electron image (A) shows the morphological aspect of a polished section of a tube from level 2 (the tube of level 1 shows the same morphology). X-ray mappings indicate that 2 major elements are found in the mineral layers: zinc (B) and sulphur (C). A position beam analysis (spectrum F, see Fig. 5A for the analysed position) shows the two major elements and also minor elements such as P (Ka at 2.015 keV), Ca (Ka at 3.690 keV, Kb at 4.012 keV), Fe and Ni (Ka at 7.472 keV, Kb at 8.265 keV) present in low quantity. The largest peak (Cl, Ka at 2.622 keV) is due to the embedding medium. X-ray mappings also give the composition of the organic matrix in S (C), P (D) and Ca (E). A punctual analysis in the matrix (spectrum G, see the analysed beam position in Fig. 6A) shows that P and S are present in the same proportion. Calcium is slightly higher. We also observe a very low background of Fe, Ni and Zn. The largest peak (Cl) is due to the embedding medium. Scale bar=50 mm. tube wall (millimeter) scale moving from the inner TRAC, revealed similar structural and chemical to the outer face of a tube, and in the minerals differences between the inside and outside minerals within a few millimeter adjacent to the tube (Zbinden et al., 2001). Such observations strongly surface. support the assumption that our experimental The use of a colonisation device enabled us to conditions do not create significant artifacts consider mineralogical gradients associated with regarding the nature of the minerals that pre- newly secreted tubes and to relate these gradients cipitate within and in close contact to the tubes. to different thermal and chemical conditions. The minerals observed on the TRAC walls are 4.1. Predominant marcasite at the outer tube consistent with numerous observations of miner- surface alisation in chimney walls (Fouquet et al., 1988; Hannington et al., 1995), indicating that condi- XRD results have indicated that among the tions are close to natural. Furthermore tubes reported iron disulphide minerals, marcasite is recovered with conventional methods, without more abundant than pyrite in the minerals directly 278 M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 associated with the Alvinella spp. tubes than in the Locally low pH in the immediate vicinity of the bulk of minerals collected at the same level in the outer surfaces of the tubes could provide a TRAC. This finding is similar to what was potential explanation. Such low pH could observed for Paralvinella sulfincola at North East result from a number of abiotic processes (Tivey, Pacific vent sites (Juniper et al., 1992). These 1995; Seyfried and Mottl, 1995). Biotic processes authors suggested that P. sulfincola was altering could also be involved: for example, the meta- the local geochemistry, promoting marcasite pre- bolic activity of microorganisms at the tube cipitation at the chimney surface. Our results thus outer surface may locally affect the pH. Further provide the first data confirming the occurrence of in situ measurements and laboratory experi- this mineralogical characteristic for the EPR ments are needed to distinguish among those alvinellid species. Our data also demonstrate by possible interpretations. Yet, whatever the abi- direct imaging a close connection between marca- otic or biotic cause of the assumed pH anomaly, site precipitation and the outer surfaces of the its close association with the tube interface tubes. clearly suggests that the worm activity exerts The predominance of marcasite over pyrite is some direct or indirect influence on local geo- expected to result from an acidic environment chemistry. (Murowchick and Barnes, 1986; Schoonen and

Barnes, 1991). According to these authors, and 4.2. FeS2 vs. ZnS mineral precipitation providing that the zero-valent sulphur content is high enough to permit its formation (Benning In this study, we have reported a distinct et al., 2000), marcasite is the predominant product difference in the relative amounts of zinc and iron of iron disulphide precipitation below pH 5. sulphides from lower and upper levels of the Above this value, pyrite is the dominant disulphide TRAC and from the inner and outer tube surfaces. formed. Amounts of iron sulphide decrease with increasing To a first approximation, if we assume that the distance from the TRAC base and zinc sulphide is fluids present in the vicinity of the worms form as a always predominant at the inner surfaces and mixture of seawater and a hydrothermal end- within the matrix of the tubes. Possible reasons for member similar in composition to the model an absence of iron sulphides in these areas are: (1) proposed by Janecky and Seyfried (1984), then that the fluid present is not saturated with FeS2,or the pH of the medium would be about 5.5 at 701C (2) that precipitation of iron sulphide was kineti- (level 2) and about 6.5 at 201C (level 1). The cally inhibited. Again, if we assume that the fluids presence of equal amounts of marcasite and pyrite present in the vicinity of the worms form as a however indicates that the pH was lower, close to 5 mixture of seawater and hydrothermal fluid (that or even lower (Murowchik and Barnes, 1986; is similar in pH and concentrations of Fe, Zn, and Schoonen and Barnes, 1991). First in situ pH H2S to the hypothetical vent fluid considered by measurements within Alvinella colonies (Le Bris Janecky and Seyfried, 1984), then iron sulphide et al., 2001), although obtained at another site and zinc sulphide should both be saturated at the (EPR 131N), are consistent with this assumption. temperatures measured (20–701C). Thus kinetic This lower pH could result from a number of inhibition is the most likely explanation for the processes such as some amount of conductive lack of Fe sulphide. cooling of vent fluid (Janecky and Seyfried, 1984) Schoonen and Barnes (1991) have documented or from transport and mixing of fluids from that, at temperatures below 1001C, FeS2 formation diffusion and advection trough the deposits is indeed expected to be extremely slow and that (Tivey, 1995). In any case, the much greater iron disulphides can be formed only via an abundance of marcasite relative to pyrite (75:25) amorphous FeS precursor whose stability condi- in the immediate proximity of the tube’s external tions can be expressed as surfaces in level 2 suggests that the precipitation of 2þ X marcasite is promoted by the presence of the tube. Log10ð½Fe ½H2SÞ pKFeS 2pH; M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280 279 where brackets refer to the concentration of these Alvinella spp. tubes differ from the outside ions and pKFeS is the apparent solubility product conditions and suggests that the tube wall act as of amorphous iron sulphide in the medium. If this an efficient barrier to the external environment. condition is not met, FeS cannot form. Increasing These conclusions contrast with previous ideas fluid dilution with seawater, moving from the base that thermal and chemical characteristics of the to the top of the TRAC, decreases the total iron inner medium fluid phases are not significantly and sulphide contents and increases pH. The pH different from those of the external fluids. Our increase, however, may be small relative to the results suggest that the worm’s environment in the decrease in concentrations of ferrous ion and inner medium delimited by the tubes may be hydrogen sulphide, in which case there would be a controlled by particular hydrodynamical mixing decrease in the stability of the FeS intermediate conditions resulting in a moderate contribution of phase. hydrothermal fluid with respect to seawater (i.e. A possible explanation for larger proportion of reduced temperatures, lower sulphide and metal seawater inside the tubes than at the outer surface contents) or local geochemical changes induced by could be that the worm, by its behaviour, may microbial activity. In any case, the important point create particular hydrodynamic conditions within remains that differences in proportions of minerals the tube (Desbruyeres" et al., 1998). The lack of present at the outer and inner faces of the tubes are data related to the chemical composition of fluids reported at the scale of a few millimeters, reflecting at this site prevents us from quantitatively check- differences in the chemical environments at these ing this assumption, but such dilution gradients same scales. To better understand the reasons for may thus result in a considerable decrease in the the observed differences in proportions of minerals rate of iron sulphide formation from the bottom to present at different locations within and around the top level of the TRAC and at the inner surface the tubes requires more detailed studies to be of the tubes. carried out, including laboratory precipitation Yet another possible explanation for the lack of studies in the presence of biological materials, in Fe sulphides within the tubes and at their inner situ measurements, and mineralogical compari- surfaces is that the environment may be depleted sons between alvinellid-bearing and—depleted in sulphide as a result of the metabolic activity of hydrothermal environments. the worm, its epibiotic bacteria or microorganisms of the tube wall. The large amounts of elemental sulphur that we observed in Alvinella spp. tubes could result from a sulphide-oxidizing activity of Acknowledgements the worm or of its epibiotic bacteria. Such sulphide-consuming processes may result in a The authors want to thank L. Mullineaux and substantially decrease in sulphide concentration C.R. Fisher for help in the TRAC deployment. We and, thus, in a reduction of FeS stability at a very would like to acknowledge strong support from local scale. Therefore its formation may be Philippe Ildefonse and Guillaume Morin for prevented as well as further formation of iron acquisition, interpretation and quantification of disulphides. More accurate information on sul- the XRD data. We are deeply saddened that phide flux in the Alvinella surrounding are yet Philippe Ildefonse died in October 1999, while this needed to test this hypothesis. study was beginning. We also thank M.M. Loth for technical assistance in sample preparation and A. Khripounoff, from DRO/EP (IFREMER, 5. Conclusion Brest), for instrumental facilities, and URM 7. The authors wish to thank the reviewer who The difference in proportions of minerals at provided helpful comments in rewriting this paper. different locations in tubes supports the view that This work was funded with the help of INSU, the chemical characteristics at the inner faces of CNRS and the DORSALES program. 280 M. Zbinden et al. / Deep-Sea Research I 50 (2003) 269–280

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