Fluid venting in the eastern Aleutian subduction zone

Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over‐steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane‐ or sulfide‐dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5±0.7 L m−2 d−1 for tectonics‐induced water flow [Wallmann et al., 1997b]. A geophysical estimate of dewatering from the same area [von Huene et al., 1997] based on sediment porosity reduction shows a fluid loss of 0.02 L m−2 d−1 for a 5.5 km wide converged segment near the deformation front. Our video‐guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m−2 d−1 is estimated from the biogeochemical approach. The two estimates for tectonics‐induced water flow from the accretionary prism are in surprisingly good agreement

Phylogeny and Diversification Patterns among Vesicomyid Bivalves

Vesicomyid bivalves are among the most abundant and diverse symbiotic taxa in chemosynthetic-based ecosystems: more than 100 different vesicomyid species have been described so far. In the present study, we investigated the phylogenetic positioning of recently described vesicomyid species from the Gulf of Guinea and their western Atlantic and Pacific counterparts using mitochondrial DNA sequence data. The maximum-likelihood (ML) tree provided limited support for the recent taxonomic revision of vesicomyids based on morphological criteria; nevertheless, most of the newly sequenced specimens did not cluster with their morphological conspecifics. Moreover, the observed lack of geographic clustering suggests the occurrence of independent radiations followed by worldwide dispersal. Ancestral character state reconstruction showed a significant correlation between the characters “depth” and “habitat” and the reconstructed ML phylogeny suggesting possible recurrent events of ‘stepwise speciation’ from shallow to deep waters in different ocean basins. This is consistent with genus or species bathymetric segregation observed from recent taxonomic studies. Altogether, our results highlight the need for ongoing re-evaluation of the morphological characters used to identify vesicomyid bivalves

On the evolutionary ecology of symbioses between chemosynthetic bacteria and bivalves

Mutualistic associations between bacteria and eukaryotes occur ubiquitously in nature, forming the basis for key ecological and evolutionary innovations. Some of the most prominent examples of these symbioses are chemosynthetic bacteria and marine invertebrates living in the absence of sunlight at deep-sea hydrothermal vents and in sediments rich in reduced sulfur compounds. Here, chemosynthetic bacteria living in close association with their hosts convert CO2 or CH4 into organic compounds and provide the host with necessary nutrients. The dominant macrofauna of hydrothermal vent and cold seep ecosystems all depend on the metabolic activity of chemosynthetic bacteria, which accounts for almost all primary production in these complex ecosystems. Many of these enigmatic mutualistic associations are found within the molluscan class Bivalvia. Currently, chemosynthetic symbioses have been reported from five distinct bivalve families (Lucinidae, Mytilidae, Solemyidae, Thyasiridae, and Vesicomyidae). This brief review aims to provide an overview of the diverse physiological and genetic adaptations of symbiotic chemosynthetic bacteria and their bivalve hosts

Carrier of reduced sulfur is a possible role for thiotaurine in symbiotic species from hydrothermal vents with thiotrophic symbionts

Experiments supporting the possible role of the free sulfur-containing amino acid thiotaurine, as a transport and storage compound for sulfide in invertebrates with thiotrophic symbionts are described. The free-living chemotrophic sulfur-oxidising bacterium, Thiobacillus hydrothermalis (strain DSMZ 7121), was used as a model for the symbionts as the actual symbionts have not been obtained in culture. Thiotaurine contains two sulfur atoms, namely the inner sulfone and the outer sulfane sulfur; the latter presents a potential source of reducing equivalents for the symbiont. Nevertheless, we observed no oxidation of thiotaurine when this compound was added to a culture of T. hydrothermalis pre-grown on sulfide. In contrast, when thiotaurine was added to the culture together with an extract of the trophosome of a vestimentiferan tubeworm from the Manus basin, we observed that thiotaurine was oxidised to hypotaurine with concomitant acidification and formation of bacterial biomass. Thus, the trophosome contains an unknown catalytic factor. We suggest that thiotaurine requires reduction prior to oxidation by T. hydrothermalisand that the host may catalyse the conversion of thiotaurine through the glutathione redox couple. This way, the host can accurately control energy delivery (as reduced sulfur) to the symbionts and can therefore control their symbiont biomass

Fatty acid composition in deep hydrothermal vent symbiotic bivalves.

Fatty acids in deep hydrothermal vent bivalves have been analyzed. Their composition is completely different from that of a littoral mussel collected in the Mediterranean sea. The distribution of fatty acids in the littoral mussel is characterized by a predominance of polyunsaturated fatty acids (20:5n-3, 22:6n-3) reflecting the planktonic origin of the food. Vent bivalve fatty acid distribution is dominated by an abundance of the monounsaturated acids (double bond in the n-7 position) 16:1n-7, 18:1n-7, and 20:1n-7 which are clearly of bacterial origin and give an indication of the symbiotic bacterial activity in the bivalves. Differences between the fatty acid composition of the bivalves from two hydrothermal sites (13 degrees N and Galapagos) and differences between the mantle and the gill were observed and are discussed with respect to vent activities at the two sites and species metabolic capacities as a function of ecological conditions

Structure and distribution of cold seep communities along the Peruvian active margin:relationship to geological and fluid patterns

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