A field of mud diapirs and mud volcanoes situated in the Barbados trench at 13°50′N and extending along an old oceanic fracture zone (Mercurus) was investigated during the Manon cruise using both surface ship and Nautile submersible sampling and measurements. The entire zone from 13°50′N up to 14°20′N has an anomalously high heat flow which implies that fluids are drained into it from a segment of the accretionary wedge a few hundred kilometers wide. Two structures interpreted as diatremes, Atalante and Cyclops, expell large amounts of water and methane. We propose that they were formed from the release of a light fluid when gas hydrates were dissociated in the sediment as the result of the circulation of warm fluid in the area. However they expell only a small fraction of the incoming fluid, implying that disperse flow is the dominant mode of expulsion in this area. The chemoautotrophic communities on the surface of the structures rely mostly on sulfides. Submersible observations, temperature measurements in the sediment, and the chemistry of the pore fluid indicate that convection of seawater occurs within the first few meters of sediment through high‐permeability channels, such as cemented carbonate conduits. We propose that this convection is driven by the density difference between the pore fluid and seawater, but fresh water released by the dissolution of shallow hydrates may also contribute. This shallow convection may be a frequent process in cold seep environments.

TWO deep-sea gastropods, Ifremeria nautilei and Alr~iniconcha hessleri, collected on a hydrothern~al site of the North Fiji Basin (Southwestern Pacific) were analysed for polar and neutral lipids using gas-liquid chromatography/mass spectrometry. A high level of monounsaturated fatty acids (MUFAs) and a low level of 0 3 series polyunsaturated fatty aclds (PUFAs) indicated that nutrition of both gastropods was related to a food web based malnly on bacterial supply. From differences In MUFA distribution between the 2 gastropods, it appeared that most of the energy requirements of A hessleri were supplied by sulfur-oxidizing endobacteria whereas 1. nautllei probably had a mixotroph~c diet based on endogenous as well as exogenous bacteria. Given the relat~vcly high level of linoleic acid, which represented from 2 to 8"b of the phospholipid fatty acids, hydrothermal gastropods did not appear to be depleted in m6 PUFAs. It was hypothesized that they obtain linoleic acid from a pathway different to that in heterotrophic marine molluscs. In contrast to 0 6 PUFAs, both hydrothermal gastropods appeared to b e depleted in 0 3 PUFAs, indicating the limited importance of photosynthesisbased food supplies. Some non-methylene-interrupted dienes, particularly 20:209,15 which represented from 9 to 18% of the phospholipid fatty acids, may be synthesized by deep-sea symbiotic molluscs in order to restore the depleted w3 PUFAs considered a s essential for animals. Gills of both gastropods had high levels of neutral lipids, mainly MUFAs that may have originated from degradation of endobacterial phospholipids.

ABSTRACT-Two species of deep-sea mussels belonging to the genus Bathymodiolus were collected from 5 vent fields distributed over 2 basins of a hydrothermal system in the South Pacific. Free and bound primary amino compounds were analysed in gills and mantles using OPA-HPLC. The method allowed the detection of 32 amino acids and related compounds (AARCs). An undetermined free chemical was found at hlgh levels (up to 30% of the total free AARCs) In vent mussel tlssues This amino compound is presumed to be an end-product from specific metabolism occurnng in vent n~ussels. Except for this unknown compound, comparlson of the biochemical composition between vent and littoral mussels revealed that the general nitrogenous requirement as well as the endogenous metabolic pathways of the main amino acids seem to be similar for the 2 mussel types. Ultrastructural observations indicated that these deep mussels harbour bacteria in their gills. Indices of high metabolic rates found in these organs appear to be related to the presence of endobacterial. It seems that symbionts could provide a preponderant fraction of essential amino acids to their host. In particular, free thiotaurine displayed high concentrations (up to 13% of the total free AARCs). This sulphur-containing compound could be involved in a specific metabolism related to toxic forms of sulphur. Variations of the AARC composition between the 2 vent mussel species do not appear to be species related but appear to be due to environmental constraints. As shown by stress indices, deterioration of the physiological condition in these mussels is directly hnked to the quantity and/or the quality of the hydrothermal vent emissions.

Two bacteria-bearing mussels,Bathymodiolus breviorandB. elongatus, collected at five deep-sea hydrothermal sites from Lau and Fiji back arc basins (south-west Pacific), were analysed for polar and neutral lipids using gas-liquid chromatography mass spectroscopy (GLC-MS). The respective fatty acid distributions found in gills and mantles were compared to data from a Mediterranean littoral filter-feeding mussel,Mytilus galloprovincialis. The main characteristic common to the gills of the two vent mussels was the high level of (n-7) monounsaturated phospholipid fatty acids, which are supposed to originate from sulphur-oxidizing symbionts. Using palmitoleic, 16:1 (n-7), and vaccenic, 18:1 (n-7), phospholipid fatty acids as bacterial markers, endobacterial biomass was estimated to account for 2–10% of the vent mussel gill volume. Vent mussel tissues were poor in plant-derived polyunsaturated fatty acids, indicating the limited importance of the photosynthesis based material in the metabolism of these hydrothermal vent bivalves. It is suggested that some compounds, such as nonmethylene-interrupted dienoic fatty acids and specific trienoic fatty acids originating from elongation and desaturation of oleic and palmitoleic acids, respectively, were synthesized in order to substitute exogenous plant-derived polyunsaturated fatty acids considered as essential in marine invertebrates. The high level of neutral lipids in vent mussel gills suggests that these compounds are used as a major energy storage material. The neutral lipid/phospholipid ratio in gills emphasized the preponderance of the endosymbionts in vent mussel nutrition. Using this ratio as an index of the physiological state, the symbiosis patterns in different hydrothermal fields were compared.