Chemical and Thermal Description of the Environment of the Genesis Hydrothermal Vent Community (13°N, EPR)

Cah. Biol. Mar. (1998) 39 : 159-167 Chemical and thermal description of the environment of the Genesis hydrothermal vent community (13°N, EPR) Pierre-Marie SARRADIN1, Jean-Claude CAPRAIS1, Patrick BRIAND1, Françoise GAILL2, Bruce SHILLITO2 et Daniel DESBRUYÈRES1 1 DRO/EP/LEA IFREMER centre de BREST - BP 70 - F-29280 PLOUZANE (France), 2 INSU / CNRS / UPMC Roscoff / Paris (France) Fax: (33) 2 98 22 47 57; e-mail: pmsarrad@ifremer.fr Abstract: The objective of this study is to describe the chemical and physical environment surrounding the vent organisms at the Genesis site (EPR, 2640 m). The main chimney is colonized by Riftia pachyptila, fishes Zoarcidae and crabs Bythograeidae. The top of the smoker is covered with tubes of polychaetes Alvinellidae, the frontier zone by limpet gastropods. Temperature measurements and water sampling were made on an axis along the chimney. The environment was characterized using relationships between chemical concentrations and temperature to provide a chemo-thermal model of the site. Discrete temperature ranges were l-1.6°C in sea water, 1.6- 10°C among Riftia plumes (up to 25°C at the tube base), 7-91°C close to the alvinellid tubes, fluid emission was 262-289°C. This study describes the habitat - 2- -1 -1 of Alvinellidae emphasizing its chemical (£S [H2S + HS + S ] 3-300 µmol l , CO2 3.5-6 mmol l , pH 5.7-7.5) and thermal specificities compared to the Riftia ones (IS 4-12 µmol l-1 ; CO2 2-3.5 mmol 1-1 ; pH 5.8-7.7). The size of Riftia allowed us to define its environment at the organism (temperature gradient along the tube 0.5-1°C cm-1) and population scales (temperature difference between organisms from the same clump: 10-20°C). Such spatial heterogeneity may play a significant role in the animal physiology and calls for further investigations. Résumé : Description des conditions chimiques et thermiques de l'environnement des communautés biologiques du site hydrothermal Genesis (13°N, ride Est-Pacifique). Le but de ce travail est de décrire l'environnement physico-chimique des organismes hydrothermaux sur le site Genesis (EPR, 2 640 m). La cheminée principale est colonisée par Riftia pachyptila, des poissons Zoarcidae et des crabes Bythograeidae. Le sommet du fumeur est couvert de polychètes Alvinellidae, la zone frontière par des gastéropodes patelliformes. Un modèle reliant la température et la concentration en 2S et CO2 est proposé. Les valeurs discrètes de températures s'échelonnent depuis 1-1,6 °C dans l'eau de mer, 1,6-10 °C autour de l'organe branchial de Riftia et jusqu'à 25 °C à la base -1 des tubes. Elles atteignent 7-91°C à proximité des alvinellidés et 262-289 °C pour le fluide pur. Un environnement spécifique pour chaque communauté d'organismes est présenté : autour des Alvinellidae XS 3-300 µmol l , CO2 3,5-6 -1 -1 -1 -1 mmol l , pH 5,7-7,5 ; autour de Riftia pachyptila £S 4-12 µmol l ; CO2 2-3,5 mmol l ; pH 5,8-7,7. La taille de R. pachyptila a permis de préciser cet environnement à l'échelle de l'organisme (gradient de 0,5-1 °C cm le long du tube) et de la communauté (différence de 10-20 °C au sein d'un même bouquet d'individus). Les travaux ultérieurs devront porter sur le rôle physiologique de cette hétérogénéité spatiale. Keywords : hydrothermal ecosystem, hydrogen sulphide, carbon dioxide, temperature. Introduction Reçu le 4 décembre 1997; accepté après révision le 10 juillet 1998. In the absence of light, the food-web of deep-sea Received 4 December 1997; accepted in revised form 10 July 1998. hydrothermal communities is based mainly on the 160 HYDROTHERMAL ENVIRONMENT AT THE GENESIS SITE chemolithoautotrophic bacterial primary production. A based on evacuated bottles was used for the first time to large part of the dominant primary consumers is associated collect water samples in the vicinity of the animals. The with autotrophic bacteria in closed symbiotic systems 150 ml Whitey Teflon coated bottle is equipped with two relying on a periodical access to reduced and oxygenated valves, the first one (Swagelock, 1/4 turn) to open the bottle compounds (Childress et al., 1992; Fisher, 1990; with the main arm of the submersible, allowing the sample Tunnicliffe, 1991). The sessile primary consumers are living to enter through a stainless steel tubing (o.d. 1/8,1 = 20 cm), in the turbulent mixing zone where the environmental the second one (Swagelock, needle valve) to distribute the conditions are constantly switching from hydrothermal to sample on board. The system is manipulated by the two sea water dominated conditions. As Johnson et al., (1986, arms of the submersible. The filling is obtained using the 1988a) demonstrated using in situ chemical measurements, difference between the inner and outer pressure. the mixing gradient within interfacial region is steep and Water sampling and temperature measurements were environmental conditions vary at the centimetre scale. made on several points along an axis (Fig. 2) including the Temporal studies using thermal probes demonstrate also main smoker, the animal communities and the surrounding (Johnson et al., 1988b) that, at the scale of micro-habitats, cold sea water. Table 1 presents the chemical treatment and temperature is extremely variable on time scales from few analytical methods used during the water samples analysis. seconds (turbulence) to days (harmonic of the tidal signal) pH was measured on board using an electrode for sulphidic (Chevaldonné et al., 1991; Chevaldonné et al., 1992). The medium, at 25 °C. Dissolved gases (CO2 and CH4) were definition of physical and chemical environment of analysed using a new method described in Sarradin et al. organisms in fluctuating micro-environment is thus deeply (1996) and based on an headspace injection, separation by affected by the scale of measuring and sampling and by gas chromatography with column selection and detection on procedures and strategies. In situ chemical analysis is a 3 detectors device. Hydrogen sulphide was determined by probably the most accurate tool when considering the methylene blue method (Fonselius, 1983), Mg with an microbiotope definition, but it must be associated with ionic selective electrode after complexation with EDTA. temporal series due to its sensitivity to micro- The sampling points performed on the site are summarized hydrodynamics conditions. Because of the variability of this in Fig. 2. turbulent environment, discrete sampling of hydrothermal Discrete temperature measurements were obtained with a water is by far less resolving but provide a wider range of probe held by the Nautile arm. These values were corrected possible analysis. Nevertheless it gives averaged values during the cruise using a comparison with a standard which can be used for comparison between micro-habitats reversal thermometer. The long term measurements (30 only. days) were obtained from 3 HOBO probes (C. Fisher, pers. The main objective of this study is to compare the gross com.) placed on 3 different clumps of Riftia pachyptila physical and chemical conditions surrounding two of the (Jones, 1981) (Fig. 2). most prominent populations of worms (Riftia and Alvinella) The description of the associated communities (Fig. 2) living within a typical vent field area (Genesis) at was obtained from the video records of the Nautile (mobile 13°N/EPR. These two species were described by Fustec et camera) as previously described (Fustec et al., 1987). al. (1987) to be dominant organisms among two different megafaunal assemblages based on in situ observation, Results faunal analysis and discrete temperature measurements. Our aim is to characterize and to compare the two micro-habitats I. Ecological setting in term of ecological settings as well as thermal and chemical characteristics. A schematic 3D view of the site is proposed in Fig. 2. The black smoker of the site, identified by the PP HOT2 marker, set down by the submersible during the cruise, was Material and Methods venting a clear hot fluid (262 to 269°C). It is 9 m high and is built on the 5 m high upright wall oriented NW-SW All the sampling and experiments were done during the delimiting the western side of the Genesis hydrothermal HOT 96 French-American cruise (9° and 13°N, East Pacific vent field. The upper part of the chimney (2638 m depth at Rise, February and March 1996) on the N/O Nadir hosting its highest point) is located above the plateau like a 4 m high the submersible Nautile. During this cruise, 10 dives out of needle. 31 were partially or totally devoted to the study of the The top 2 m of the chimney (3.5 m2) were totally covered Genesis site (12°48.63N, 103°56.41W, depth 2639-2647 m). with alvinellid worms including Alvinella pompejana 750 ml titanium syringes (Von Damm, 1983) were used (Desbruyères & Laubier, 1980), A. caudata (Desbruyères & for the sampling of hot vent fluid. A new device (Fig. 1) Laubier, 1986) and Paralvinella grasslei (Desbruyères & P.-M. SARRADIN, J-C. CAPRAIS, P. BRIAND, F. GAILL, B. SHILLITO, D. DESBRUYÈRES valve opening using the main Nautile arm prehension plate stainless steel sample inlet sample collection valve Figure 1. Schematic view of the water sampling device. Figure 1. Schéma du dispositif de prélèvement. Figure 2. 3D map of the Genesis chimney, ecological setting and location of the sampling points. Figure 2. Carte 3D de la cheminée principale de Genesis, description écologique et points de prélèvement. Table 1. Sample treatment and analytical methods. Tableau 1. Traitement des échantillons et méthodes analytiques utilisées. Parameter Preservation method Analytical method potentiometry electrode for sulphidic medium 0.1 pH Head space-GC-HWD FID (Sarradin et al, 1996) 10% colorimetry (Fonselius, 1983) 10% Mg HNO3 suprapur, 20 µl in 20 ml sample, ambient T ion selective electrode, EDTA complex 3% s* relative standard deviation (standard deviation for pH) 162 HYDROTHERMAL ENVIRONMENT AT THE GENESIS SITE Laubier, 1982) (700 ind m-2).

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