Chemoautotrophic bacteria–marine invertebrate symbioses at deep-sea hydrothermal vents, cold seeps and in other

at deep-sea hydrothermal vents, cold seeps and in other reducing environments derive all or part of their nutrition from symbiont chemoautotrophy (Fisher, 1990; Cavanaugh, 1994). To support growth based on sulfide oxidation and carbon dioxide fixation, inorganic nitrogen (e.g. nitrate and ammonia) must also be assimilated. How this occurs has not been well studied and is potentially complicated by the possibility that both host and symbiont are involved in the assimilation of nitrogen. Chemoautotrophic symbioses have a high demand for nitrogen because of their high biomass (Nix et al., 1995; Lutz and Kennish, 1993) and growth rates (Lutz et al., 1994), and this is matched by the high level of availability of environmental nitrate and, in some cases, ammonia (Johnson et al., 1988; Lilley et al., 1993; Conway et al., 1992). Ammonia and nitrate are likely to be important sources of nitrogen for these symbioses. This is supported by host anatomy, in situ water chemistry measurements and physiological studies. In contrast to conventional invertebrate feeding modes, while particulate sources of nitrogen can be utilized by the vent mussel Bathymodiolus thermophilus because it is a competent suspension-feeder (Page et al., 1991), these sources are less important to the vent clam Calyptogena magnifica, which has a reduced apparatus for feeding (Boss and Turner, 1980; Fiala-Médioni and Métivier, 289 The Journal of Experimental Biology 202, 289–300 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JEB1702