Shallow-water Hydrothermal Vent Research Articles

The Early Valanginian ammonite, brachiopod and crustacean fauna of the Mecsek Mts. and its relationships with the embryonic shallow water hydrothermal vent at Zengővárkony (Mecsek Mts., South Hungary)

Early Cretaceous development of the Mecsek Mts. (South Hungary) is characterized by continental rifting that produced mixed volcano-sedimentary rocks in the Tihonian/Berriasian and built up a volcanic edifice in the earliest Cretaceous of ankaramite–alkaline basalts. During the Valanginian, marl sedimentation occurred in the basinal part containing rich pelagic fauna dominated by ammonites. Beside phylloceratids, lytoceratids and haploceratids, Kilianella roubaudiana, K. lucensis, Sarasinella eucyrta, Thurmanniceras pertransiens, Th. ( Clavithurmannia) foraticostata, Busnardoites campylotoxus, Olcostephanus drumensis, O. densicostatus, and Sabbaiceras stefanescui are indicating the Early and Late Valanginian Thurmanniceras pertransiens, Busnardoites campylotoxus and Saynoceras verrucosum zones. The Early Valanginian is also supported by Calpionellites darderi indicating the calpionellid “E” zone. The marl sequence contains a turbidite bed with rich fauna of shallow marine origin from the volcanic slope of lagoonal, reef, and tidal environs, however it contains metasomatized limestone olistoliths of different origin. Source of these olistoliths is the near iron-ore deposit, which is considered an embryonic shallow water hydrothermal vent. It includes a remarkable fauna of rich crustacean coprolite ichnospecies of Favreina dispentochetarius, F. hexaochetarius, F. octoochetarius, Palaxius decaochetarius, P. tetraochetarius, P. triochetarius. In the neighbourhood of the embryonic vent, a remarkable megafauna occurs, which is an almost monospecific brachiopod fauna dominated by Lacunosella hoheneggeri. Other brachiopods as Nucleata veronica, Moutonithyris moutoniana, Karadagithyris sp., and Zittelina pinguicula also occur as stray individuals. Together with benthic megafauna, nektonic elements as Nautiloidea gen. et sp. ind, phylloceratid- and lytoceratid ammonites and echinoid spines of Balanocidaris rysacantha occur.

Temporal Trends and Matrix-Dependent Behaviors of Trace Elements Closed to a Geothermal Hot-Spot Source (Aeolian Archipelago, Italy)

Degassing activity from marine shallow-water hydrothermal vents represents a natural important source of trace elements of particular ecotoxicological concern, such as Hg, Cd, Pb, and As. In 2002, on November 3rd, a submarine area of the Aeolian Archipelago (Italy) was affected by an exceptional hydrothermal activity, presumably associated to a significant trace element release. This study developed in the highest impacted area, was focused on different environmental matrices (water, sediment, primary producers, and fishes) with the aim to evaluate on a statistical basis temporal dynamics involving trace elements closed to the emission source. Results evidenced that this event produced significant changes in water column, modifying the general assessment of the chemical descriptors selected (Na, K, Mg, Ca, Cl, Br, SO2-4, HCO-3). Furthermore, the degassing activity represents a significant input of Hg, Cd, Pb, and As for all of the sampled matrices, being it responsible of the higher levels observed in the study area compared to controls. Concentrations measured in sediments and biota (Cystoseira compressa, Serranus cabrilla) at temporal intervals of ten months distanced, supported the occurrence of different time scale dynamics which are both element and matrix-dependent. In particular, levels of Cd and As measured in liver tissues of S. cabrilla evidenced a significant trend to increase with the time, allowing to exclude a quick recovery of the study area. Measured values in edible fishes tissues are, also, notably higher than risk limits proposed by the 2001/22/CE Directive for human consumption, so, local-based caught have to be accurately regulated to avoid the occurrence of diet over-exposure.

Changes in Benthic Macrofauna Associated with a Shallow-Water Hydrothermal Vent Gradient in Papua New Guinea

Abstract: Infaunal macroinvertebrates were characterized along an environmental gradient from a shallow-water hydrothermal vent located at Tutum Bay, Ambitle Island, Papua New Guinea. Samples were collected at three sites located at 7.5, 60, and 150 m from the vent and from a nonhydrothermal reference site located to the north. Temperature and arsenic concentration were found to decrease and pH increased with distance away from the vent. At each site, five replicate core samples were taken randomly from a 1 m2 sampling grid. All infaunal invertebrates > 500 µm were sorted, identified to the lowest practical taxonomic level and counted. Results from the macrofauna data show a strong trend of increasing abundance, species richness and diversity relative to distance away from the vent, but even at 150 m the benthic macrofauna appeared to be depressed relative to the reference site. Mollusks were completely absent 7.5 m from the vent, rare at 60 m, and abundant at 150 m, suggesting that the low pH values asso...

Meiofaunal diversity and assemblage structure in a shallow-water hydrothermal vent in the Pacific Ocean

Despite their ubiquitous distribution in tectonically active coastal zones, shallow-water vents have been much less explored than deep-sea vents in terms of biodiversity and adaptations to extreme conditions. We investigated the meiofaunal biodiversity and environmental variables at dis- tances of 10, 100 and 200 cm from a shallow-water hydrothermal vent in the equatorial Pacific (Sulawesi, Indonesia). Meiofaunal abundance and the richness of higher taxa increased from the site of fluid-vent emission (where temperatures of the fluids and sediments reached approximately 90°C) to the control sediments (200 cm from the vent, with no sign of effects from the vent fluids). Nema- tode species richness was also high in the intermediate station, where bottom sediment temperature reached 55°C. These data suggest that some nematode species were able to survive in conditions typically hostile to metazoan life. Gas emissions also influenced the biochemical composition of the sediment organic matter in proximity to the vent and favoured the growth of a large photo- and/or chemo-autotrophic prokaryotic biomass. This biomass represented a potentially important food source for predator/omnivore nematodes and influenced the trophic structure of benthic assem- blages. Since the metazoan species found in proximity to the vent were a subset of those inhabiting control sediments, but were characterised by lower abundances, it might be hypothesized that the populations close to the vent are the result of colonization from adjacent areas.

Bioavailable metals and cellular effects in the digestive gland of marine limpets living close to shallow water hydrothermal vents

The pressure exerted by shallow water hydrothermal vents on edible gastropods and their cellular responses triggered by these stresses are almost unknown. The aims of this study were to evaluate the bioavailability of metals in the Macaronesian endemic limpet Patella candei gomesii living close to shallow water hydrothermal vents, and the structural differences in their digestive gland as well as the levels of apoptosis in that organ. Limpets were sampled in four sites, two with the presence of hydrothermalism and the other two without it. Whole body concentrations of several metals (Ca, Cd, Cs, Co, Cu, Fe, Hg, Mg, Mn, Pb, Rb, Se, Sr, and Zn) were obtained, morphometry analysis of the digestive gland and TUNEL test for apoptosis were also performed. Results revealed that the presence of shallow water hydrothermal vents is a source of chronic metal stress to limpets, imposing modifications in the morphometry and cell composition of the digestive gland of those limpets that may constitute cell and tissue adaptations to the environment they live in. This study sets up new baseline data for further research on the influence of shallow water hydrothermal vents over communities living in these habitats.

Prokaryote Diversity and Virus Abundance in Shallow Hydrothermal Vents of the Mediterranean Sea (Panarea Island) and the Pacific Ocean (North Sulawesi-Indonesia)

Despite their ubiquitous distribution in tectonically active coastal zones, shallow water hydrothermal vents have been less investigated than deep-sea vents. In the present study, we investigated the role of viral control and fluid emissions on prokaryote abundance, diversity, and community structure (total Archaea, total Bacteria, and sulphate-reducing bacteria) in waters and sediments surrounding the caldera of four different shallow-water hydrothermal vents (three located in the Mediterranean Sea and one in the Pacific Ocean). All vents, independent of their location, generally displayed a significant decrease of benthic prokaryote abundance, as well as its viable fraction, with increasing distance from the vent. Prokaryote assemblages were always dominated by Bacteria. Benthic Archaea accounted for 23-33% of total prokaryote abundance in the Mediterranean Sea and from 13 to 29% in the Pacific Ocean, whereas in the water column they accounted for 25-38%. The highest benthic bacterial ribotype richness was observed in close proximity of the vents (i.e., at 10-cm distance from the emissions), indicating that vent fluids might influence bacterial diversity in surrounding sediments. Virioplankton and viriobenthos abundances were low compared to other marine systems, suggesting that temperature and physical-chemical conditions might influence viral survival in these vent systems. We thus hypothesize that the high bacterial diversity observed in close proximity of the vents is related with the highly variable vent emissions, which could favor the coexistence of several prokaryotic species.

Molecular identification of symbiotic dinoflagellates (Symbiodinium spp.) from Palythoa spp. (Anthozoa: Hexacorallia) in Japan

In Japan, zooxanthellate Palythoa tuberculosa Klunzinger and Palythoa mutuki Verrill (Anthozoa: Hexacorallia: Zoantharia) are found over a 1,000 + km latitudinal range, often in environments where most other zooxanthellate anthozoans are not found (i.e. tidal lagoon pools, around shallow water hydrothermal vents, subtropical rocky shorelines). Sequences of internal transcribed spacer of ribosomal DNA (ITS-rDNA) of the symbiotic dinoflagellate genus Symbiodinium (zooxanthellae) Freudenthal (Order Suessiales) from P. tuberculosa and P. mutuki from several locations in Japan (34°11′N–24°16′N) were analysed. Unexpectedly, despite the ability of the genus Palythoa to be flexible in association with different Symbiodinium subclades, most (35 of 36) Palythoa investigated here specifically associate with subclade C1 and closely related types. Symbiodinium subclade C1 has been characterized as a “generalist” in terms of the ability to associate with a range of hosts, but present results suggest that subclade C1 may also be a “generalist” in terms of being able to live in a variety of environments over a latitudinal range.

“CandidatusThiobios zoothamnicoli,” an Ectosymbiotic Bacterium Covering the Giant Marine CiliateZoothamnium niveum

Zoothamnium niveum is a giant, colonial marine ciliate from sulfide-rich habitats obligatorily covered with chemoautotrophic, sulfide-oxidizing bacteria which appear as coccoid rods and rods with a series of intermediate shapes. Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization showed that the ectosymbiont of Z. niveum belongs to only one pleomorphic phylotype. The Z. niveum ectosymbiont is only moderately related to previously identified groups of thiotrophic symbionts within the Gammaproteobacteria, and shows highest 16S rRNA sequence similarity with the free-living sulfur-oxidizing bacterial strain ODIII6 from shallow-water hydrothermal vents of the Mediterranean Sea (94.5%) and an endosymbiont from a deep-sea hydrothermal vent gastropod of the Indian Ocean Ridge (93.1%). A replacement of this specific ectosymbiont by a variety of other bacteria was observed only for senescent basal parts of the host colonies. The taxonomic status "Candidatus Thiobios zoothamnicoli" is proposed for the ectosymbiont of Z. niveum based on its ultrastructure, its 16S rRNA gene, the intergenic spacer region, and its partial 23S rRNA gene sequence.

Distribution, speciation and bioavailability of arsenic in a shallow-water submarine hydrothermal system, Tutum Bay, Ambitle Island, PNG

Shallow-water hydrothermal vent systems can introduce large amounts of potentially toxic elements, such as arsenic (As), into coastal marine environments. The first step in understanding and describing the potential impact of these elements throughout hydrothermally influenced coastal ecosystems is to determine the element's distribution and speciation, which in turn influences the availability of the toxin for biological uptake. Shallow submarine hot springs near Ambitle Island, Papua New Guinea, are discharging as much as 1.5 kg per day of arsenic directly into a coral-reef ecosystem. We have investigated the bioavailability of the As throughout Tutum Bay by studying vent fluid, seawater, pore water, precipitates, and sediments. In addition to measuring As abundance, As speciation (As(III), As(V), and the methylated species DMA and MMA) was determined in various waters. The As concentration for discrete mineral phases in vent precipitates and sediments was determined by sequentially extracting arsenic from the easily extractable, carbonate, Fe-oxyhydroxide or hydrous ferric oxide (HFO), and residual fractions, each of which have a different bioavailability. Diffuse venting seems to play a critical role on the distribution of As throughout Tutum Bay surface sediments, which have a mean As concentration of 527 ppm while excluding the vent precipitates (range = 1483 to 52 ppm). Up to 54 ppm As were extracted from the easily extractable fraction of surface sediments (mean = 19.7 ppm), using a K 2HPO 4/KH 2PO 4 buffer at pH = 7.2. Arsenic from this fraction is considered to be the most available for biological processes, and therefore the most dangerous for biota. However, sequential extraction shows that 98.6% of the As in vent precipitates, and a mean of 93.3% in surface sediments (range = 88.2% to 96.3%), is coprecipitated with the hydrous ferric oxide (HFO) fraction. Thus, the bulk of the As being discharged into Tutum Bay is scavenged by the HFO, and should remain stable unless the physicochemical conditions surrounding the oxides change. In surface seawaters of Tutum bay, we found as much as four times the average seawater concentration of As (8.4 μg/L compared to ∼2 μg/L). The abundance of As in seawater just above the sediment/water interface is near normal, although As(III) in both surface and bottom seawater throughout Tutum Bay is substantially enriched compared to average seawater. Hydrothermal venting therefore provides bioavailable As by two major pathways throughout Tutum Bay: 1) easily-exchangeable As from hydrothermally influenced sediments to as far away as 200 m from focused venting, and 2) in surface seawaters, which may allow for biological uptake by phytoplankton and transfer up the food web.

Deep-sea and shallow-water hydrothermal vent communities: Two different phenomena?

Hydrothermal vent communities from 76 areas lying at depths from 0 to 4100 m were split into two groups — “deep” and “shallow-water”, based on the occurrence of vent obligate (restricted to hydrothermal vents) taxa of a high rank (genus and family). The two groups were then compared based on a large set of environmental, faunistic and community parameters. The degree of obligacy of fauna sharply changes at the depth of approximately 200 m. In 21 shallow-water areas, located at depths < 200 m, vent obligate taxa are absent or their taxonomic rank does not exceed a species. The total number of vent obligate species in shallow water is 8, and their obligacy is uncertain. Data from 55 areas lying at depths from 200 to 4100 m indicate there are ca. 400 obligate species. In 47 of these areas, obligate vent taxa of the genus or the family rank are present. Obligate taxa of the genus rank appear at 200 m depth, and of the family rank — at 386 m. The depth ca. 200 m corresponds well with differences in community structure and composition and environmental parameters. Deep-sea (> 200 m) hydrothermal communities differ from shallow-water ones (< 200 m) in a much higher ratio of vent obligate taxa. Deep-sea vent communities are also distinguished by the lack of diatom and algae-bacterial mats, phytoplankton, lower species richness, the development of large sulphide structures, higher role of active vents in structuring communities in space, replication of a set of dominant taxa in communities within large regions reproduced through the succession at newly formed vent fields, more pronounced concentric and vertical zonality, higher biomass of macrofauna, the important role of symbiotrophic forms dominating the biomass. The ratio of species with a “point” species range (endemic to small areas) is higher in the deep sea. There are no significant differences between the vent and non-vent biogeography both in shallow-waters and the deep ocean. Modern deep-sea hydrothermal vent communities have a longer evolutionary history than their shallow-water counterparts. To explain major differences between deep and shallow-water hot vent communities, evolutionary processes need to be analysed over long geological periods and within global vertical zones. It is suggested that the proportion of organic matter derived from chemosynthesis and photosynthesis plays a crucial role in evolution. When calculated for global zones this proportion increases with depth to a certain critical level that separates deep and shallow-water hot vent communities and fauna. Vertical zonation in the distribution of obligate taxa in hydrothermal vent communities indicates that this level is reached at approximately 200 m depth. Differences between deep and shallow-water hydrothermal vent communities are significant enough to distinguish the two different phenomena. The principle differences are a higher ratio of vent obligate taxa and a dominance of symbiotrophic forms in deep-sea hydrothermal vent communities.

Periodicities in sediment temperature time-series at a marine shallow water hydrothermal vent in Milos Island (Aegean Volcanic arc, Eastern Mediterranean)

Time-series data sets of total bottom pressure (tidal plus atmospheric), seawater temperature and sediment temperature from a marine shallow hydrothermal vent (Milos, Hellenic Volcanic Arc, Aegean Sea) were studied to determine factors influencing periodicity at the vents. Bottom pressure and vent temperature were mainly opposite in phase, with the main fluctuations of vent temperature occurring at tidal frequencies. Although the fluctuations in atmospheric pressure were of the same order as those due to tidal pressure, the contribution of atmospheric pressure was considerably weaker at diurnal frequencies. Some sudden discontinuities in sediment temperature were recorded, at least one of these may have been caused by seismic events. Seawater temperature changes were not reflected in the sediment temperature record. Transient loadings, such as tidal loadings, barometric pressure and earth tides, may affect the pore pressure in sediments, influencing fluid expulsion and sediment temperature as a consequence. Most of the contribution to the fluctuations in sediment temperature depends on tidal loadings. Gravitational forces, in the form of earth tides, can also be involved and barometric pressure is probably responsible for long period temperature oscillations.

Shallow-water hydrothermal vents in the Mediterranean sea: stepping stones for Lessepsian migration?

The present study was carried out in Palaeochori Bay, south eastern coast of Milos, where outflow of hot gas bubbles and hydrothermal water seepage was observed. The study tests hypotheses about relationships between seawater temperature and species with warm-water affinities in areas close to shallow-water vents. We predicted that if temperature plays a major role in influencing abundance of species with warm-water affinities, then there should be more thermophilous species in areas with higher seawater temperature. Time series of seawater temperature were recorded in Palaeochori Bay and Pollonia Bay where hot emission was not observed. Temperature, water pressure and conductivity were measured every 30 min from 19 June 1996 to 16 June 1997. Fourteen qualitative samples of benthic flora were collected by SCUBA diving in six rocky sites. Photosamples of benthic assemblages were collected using an UW camera equipped with a wide-angle lens (15 mm) and an electronic flash unit. A constant area of 0.7 m2 was sampled. Quantitative estimates of abundance of algal species were performed as percent cover. This paper shows that patterns of fluctuation in seawater temperature in Palaeochori were different from the control where no venting was found. In Palaeochori, algal assemblages were dominated by species with warm-water affinities that were not found elsewhere. These results are consistent with the hypothesis that temperature plays a major role in influencing abundance of species with warm-water affinities. According to our results, hydrothermal vents are potential oases that experience biological-physical coupling not yet clearly understood. They are important in affecting migration of alloctonous thermophilous species suggesting a possible role of vent areas as stepping stones for species migration.

Identification of 16S ribosomal DNA-defined bacterial populations at a shallow submarine hydrothermal vent near Milos Island (Greece).

In a recent publication (S. M. Sievert, T. Brinkhoff, G. Muyzer, W. Ziebis, and J. Kuever, Appl. Environ. Microbiol. 65:3834-3842, 1999) we described spatiotemporal changes in the bacterial community structure at a shallow-water hydrothermal vent in the Aegean Sea near the isle of Milos (Greece). Here we describe identification and phylogenetic analysis of the predominant bacterial populations at the vent site and their distribution at the vent site as determined by sequencing of DNA molecules (bands) excised from denaturing gradient gels. A total of 36 bands could be sequenced, and there were representatives of eight major lineages of the domain Bacteria. Cytophaga-Flavobacterium and Acidobacterium were the most frequently retrieved bacterial groups. Less than 33% of the sequences exhibited 90% or more identity with cultivated organisms. The predominance of putative heterotrophic populations in the sequences retrieved is explained by the input of allochthonous organic matter at the vent site.

Relative abundance of Archaea and Bacteria along a thermal gradient of a shallow-water hydrothermal vent quantified by rRNA slot-blot hybridization.

Slot-blot hybridization of rRNA with domain-specific oligonucleotide probes targeting the 16S rRNA of Archaea and Bacteria was utilized to assess the relative abundance of these domains along a thermal gradient at a shallow submarine hydrothermal vent near Milos Island (Greece). The highest prokaryotic rRNA concentrations (defined as the sum of bacterial and archaeal rRNA) were found in the uppermost sediment surface (0-20 mm), decreasing strongly with depth. This indicates that the microbial activity was mainly occurring in the surface layer of this hydrothermal vent. Furthermore, rRNA concentrations were higher in regions closer to the vent, suggesting that the hydrothermal activity stimulated microbial activity. Archaea seemed to be a minor component of the microbial community at this vent site, even in the zones with higher temperatures. Bacteria made up at least 78% (mean 95%) of the prokaryotic rRNA. However, along the steepest temperature gradient, the proportion of archaeal rRNA increased. Nevertheless, even in the hottest sediment layer where a quantification was possible (in situ temperature 82 degrees C) archaeal rRNA made up only 11.9% of the prokaryotic rRNA. This suggests that Archaea were generally of minor importance at this vent site and were probably restricted to a narrow niche. The factors that allow Bacteria to dominate in a high temperature environment that was once believed to be the realm of Archaea remain elusive.

Halothiobacillus kellyi sp. nov., a mesophilic, obligately chemolithoautotrophic, sulfur-oxidizing bacterium isolated from a shallow-water hydrothermal vent in the Aegean Sea, and emended description of the genus Halothiobacillus.

A new mesophilic, chemolithoautotrophic, sulfur-oxidizing bacterium, strain Milos-BII1T, was isolated from a sediment sample taken from a shallow-water hydrothermal vent in the Aegean Sea with thiosulfate as electron donor and CO2 as carbon source. Based on the almost complete sequence of the 16S rRNA gene, strain Milos-BII1T forms a phylogenetic cluster with Thiobacillus hydrothermalis, Thiobacillus neapolitanus, Thiobacillus halophilus and Thiobacillus sp. W5, all of which are obligately chemolithoautotrophic bacteria. Because of their phylogenetic relatedness and their physiological similarities it is proposed to transfer these organisms to a newly established genus within the gamma-subclass of the Proteobacteria, Halothiobacillus gen. nov. (Kelly and Wood 2000). Strain Milos-BII1T represents a new species of this genus, named Halothiobacillus kellyi. Cells were Gram-negative rods and highly motile. The organism was obligately autotrophic and strictly aerobic. Nitrate was not used as electron acceptor. Chemolithoautotrophic growth was observed with thiosulfate, tetrathionate, sulfur and sulfide. Growth was observed between pH values of 3.5 and 8.5, with an optimum at pH 6.5. The temperature limits for growth were 3.5 and 49 degrees C, with an optimum between 37 and 42 degrees C. Growth occurred between 0 and 2 M NaCl, with an optimum NaCl concentration between 400 and 500 mM. The mean maximum specific growth rate on thiosulfate was 0.45 h(-1).

Desulfacinum hydrothermale sp. nov., a thermophilic, sulfate-reducing bacterium from geothermally heated sediments near Milos Island (Greece).

A thermophilic, sulfate-reducing bacterium, strain MT-96T, was isolated from an active, marine, shallow-water hydrothermal vent system. It used a large variety of substrates, ranging from simple organic compounds to long-chain fatty acids, as electron donors. Autotrophic growth was possible with H2 and CO2 in the presence of sulfate. Sulfate, thiosulfate and sulfite were used as electron acceptors. Sulfur and nitrate were not reduced. Fermentative growth was obtained with pyruvate, but not with fumarate or malate. Substrate oxidation was usually complete, leading to production of CO2, but at high substrate concentrations acetate accumulated. The oval-shaped cells were 0.8-1.0 microm in width and 1.5-2.5 microm in length. Cells were motile during the early-exponential-growth phase, but motility rapidly declined during later growth phases. Spores were not produced and cells stained Gram-negative. The temperature limits for growth were between 37 and 64 degrees C, with an optimum at 60 degrees C. Growth was observed at salinities ranging from 15 to 78 g NaCl l(-1), with optimum growth in the presence of 32-36 g NaCl l(-1). This might reflect an adaptation to the elevated salinity of the hydrothermal fluid. The G+C content of the DNA was 59.5 mol%. Vitamins or other supplements were not required. Based on the 16S rRNA gene sequence, strain MT-96T belonged in the delta-subclass of the Proteobacteria. Strain MT-96T was found to be phenotypically and phylogenetically related to Desulfacinum infernum (< 95.3% sequence identity) and represents a new member of the genus Desulfacinum. The name Desulfacinum hydrothermale is proposed for this strain; the type strain is MT-96T (= DSM 13146).

In situ microsensor studies of a shallow water hydrothermal vent at Milos, Greece

Abstract The microenvironment and microcirculation of a shallow water hydrothermal vent system was studied together with the benthic primary production at Milos, Greece. In situ microprofiles of O 2 , pH, H 2 S and temperature were obtained using a miniaturised version of a profiling instrument. The sediment temperature increased toward the centre of the vent system, reaching a surface maximum of 100°C in the central yellow coloured sulfidic area. The oxygen penetration depth decreased from the unaffected sediment surrounding the vent system towards the vent centre; however, at the inner vent area the O 2 penetration increased again. Similar results were obtained during laboratory measurements. H 2 S concentrations increased rapidly beneath the oxygenated zone in the different vent areas and reached values of approximately 900 μM at sediment depths of 7–17 mm in the central vent areas. The microprofiles resolved a microcirculative pattern where local pressure differences caused by outflowing seep fluids induced a downward transport of oxygenated water, creating small convective cells which efficiently reoxidised H 2 S of the seep fluid. Patches of benthic diatoms covered the sediment surface in the areas surrounding the vent system. The net photosynthesis of this community increased from 25 to 41.8 mmol O 2 m −2 d −1 from early morning to midday. The amount of carbon fixed daily, as calculated from the in situ oxygen microprofiles, accounted for 0.67 mmol C m −2 d −1 . Laboratory incubations indicated that photosynthesis was not carbon limited and consequently the excess dissolved inorganic carbon contained in the vent fluids presumably had no effect on benthic primary production.

Sulphide tolerance of the marine nematode Oncholaimus campylocercoides-a result of internal sulphur formation?

The free-living, marine nematode Oncholaimus campylocercoides occurs in high abundance (up to 600 ind. 10 cm -2 ) at the fringe area of shallow-water hydrothermal vents off the Greek island of Milos in the Aegean Sea. It was found to have a sulphide tolerance (LT 50 ) of 4.5 d at 500 pM sulphide concentration and of 4 d at 1 mM sulphide. Light- and electron-microscopical inspections showed that the non-symbiotic O. campylocercoides, when exposed to sulphidic conditions, develops oily to viscous inclusions in the epidermis consisting of elemental sulphur in the form of S 8 -rings and polysulphur chains. The longer the exposure to sulphidic conditions, the more sulphur was formed, which disappeared after re-introduction of the nematodes in normoxic conditions for 12 h. Based on these results and on tolerance experiments with hydrogen sulphide, we suggest a model of sulphide metabolism in O. campylocercoides which could relate to its occurrence in sulphidic, hydrothermal sediments.

Spatial heterogeneity of bacterial populations along an environmental gradient at a shallow submarine hydrothermal vent near Milos Island (Greece).

The spatial heterogeneity of bacterial populations at a shallow-water hydrothermal vent in the Aegean Sea close to the island of Milos (Greece) was examined at two different times by using acridine orange staining for total cell counts, cultivation-based techniques, and denaturing gradient gel electrophoresis (DGGE) analysis of PCR-amplified 16S rRNA gene fragments. Concurrent with measurements of geochemical parameters, samples were taken along a transect from the center of the vent to the surrounding area. Most-probable-number (MPN) counts of metabolically defined subpopulations generally constituted a minor fraction of the total cell counts; both counting procedures revealed the highest cell numbers in a transition zone from the strongly hydrothermally influenced sediments to normal sedimentary conditions. Total cell counts ranged from 3.2 x 10(5) cells ml(-1) in the water overlying the sediments to 6.4 x 10(8) cells g (wet weight) of sediment(-1). MPN counts of chemolithoautotrophic sulfur-oxidizing bacteria varied between undetectable and 1.4 x 10(6) cells g(-1). MPN counts for sulfate-reducing bacteria and dissimilatory iron-reducing bacteria ranged from 8 to 1.4 x 10(5) cells g(-1) and from undetectable to 1.4 x 10(6) cells g(-1), respectively. DGGE revealed a trend from a diverse range of bacterial populations which were present in approximately equal abundance in the transition zone to a community dominated by few populations close to the center of the vent. Temperature was found to be an important parameter in determining this trend. However, at one sampling time this trend was not discernible, possibly due to storm-induced disturbance of the upper sediment layers.

Distribution and diversity of sulfur-oxidizing Thiomicrospira spp. at a shallow-water hydrothermal vent in the Aegean Sea (Milos, Greece).

A shallow-water hydrothermal vent system in the Aegean Sea close to the island of Milos (Greece) was chosen to study the diversity and distribution of the chemolithoautotrophic sulfur-oxidizing bacterium Thiomicrospira. Cell numbers in samples from different regions around a solitary vent decreased toward the center of the vent (horizontal distribution), as well as with depth (vertical distribution), corresponding to an increase in temperature (from ca. 25 to 60 degrees C) and a decrease in pH (from ca. pH 7 to 5). Thiomicrospira was one of the most abundant culturable sulfur oxidizers and was even dominant in one region. Phylogenetic analysis of Thiomicrospira spp. present in the highest most-probable-number (MPN) dilutions revealed that most of the obtained sequences grouped in two new closely related clusters within the Thiomicrospira branch. Two different new isolates, i.e., Milos-T1 and Milos-T2, were obtained from high-dilution (10(-5)) enrichments. Phylogenetic analysis indicated that isolate Milos-T1 is related to the recently described Thiomicrospira kuenenii and Hydrogenovibrio marinus, whereas isolate Milos-T2 grouped with the MPN sequences of cluster 2. The predominance of strain Milos-T2 was indicated by its identification in several environmental samples by hybridization analysis of denaturing gradient gel electrophoresis (DGGE) patterns and by sequencing of one of the corresponding bands, i.e., ML-1, from the DGGE gel. The results shown in this paper support earlier indications that Thiomicrospira species are important members of hydrothermal vent communities.