Roots as a site of hydrogen sulfide uptake in the hydrocarbon seep vestimentiferan Lamellibrachia sp.

Vestimentiferan tubeworms have no mouth or gut, and the majority of their n utritional requirements are provided by endosymbiotic bacteria that utilize hydrogen sulfide oxidation to fix CO2 into organic molecules. It has been assumed that ail vestimentiferans obtain the sulfide, O-2 acid CO2 needed b y the bacteria across the plume (gill) surface, but some live in locations where very little sulfide is available in the sea water surrounding the plu me. We propose that at least some of these vestimentiferans can grow a post erior extension of their body and tube down into the sea-floor sediment, an d that they can use this extension, which we call the 'root', to take up su lfide directly from the interstitial water. In this study of the vestimenti feran Lamellibrachia sp., found at hydrocarbon seeps in the Gulf of Mexico at depths of approximately 700 m, we measured seawater and interstitial sul fide concentrations in the hydrocarbon seep habitat, determined the structu ral characteristics of the root tube using transmission electron microscopy , characterized the biochemical composition of the tube wall, and measured the sulfide permeability of the root tube. We found that, while the sulfide concentration is less than 1 mu mol l(-1) in the sea water surrounding the gills, it can be over 1.5mmol l(-1) at a depth of 10-25 cm in sediment ben eath tubeworm bushes, The root tube is composed primarily of giant beta-chi tin crystallites (12-30 % of total mass) embedded in a protein matrix (50 % of total mass). Root tubes have a mean diameter of 1.4mm, a mean wall thic kness of 70 mu m and can be over 20 cm long. The tubeworm itself typically extends its body to the distal tip of the root tube, The root tube wall was quite permeable to sulfide, having a permeability coefficient at 20 degree s C of 0.41x10(-3) cm s(-1), with root tube being 2.5 times more permeable to sulfide than trunk tube of the same diameter. The characteristics of the root suggest that it reaches down to the higher sulfide levels present in the deeper sediment and that it functions to increase the surface area avai lable for sulfide uptake in a manner analogous to a respiratory organ.