Microorganisms as food resources at deep-sea hydrothermal vents

We used stable isotopes of carbon and nitrogen to examine the diversity of microbial populations consumed as foods at deep-sea hydrothermal vents. Invertebrate consumers at Gorda and Juan de Fuca Ridge vent sites had variable carbon isotope compositions, implying the use of more than one microbial food resource. 6’C values for consumer invertebrates at Gorda ranged between 13.2% (polynoid polychaete) and -43.7% (limpet); within Gorda microhabitats, fi13C compositions of invertebrate species were also not uniform, differing by as much as 8-19%. At the Juan de Fuca site, 6’C values showed a wide range (14.6 to 33.9%) for nine invertebrate species collected from a dense community colonizing the surface of a sulfide flange. Carbon isotope differences between tubeworm symbioses and consumer invcrtebratcs within microhabitats suggest that these symbioses may play a minor role as nutritional resources in vent food webs. Nitrogen isotope compositions of consumer spccics from vents were consistently depicted in 15N relative to animals collected away from vents. 615N compositions of some vent individuals are among the lowest measured in any organism (< 10%) and likely reflect relatively abundant supplies of inorganic nitrogen compounds used by microbial populations at vents. The deep sea is generally believed to be a food-limited environment (Gage and Tyler 199 1) with organisms subsisting on the rain of organic material derived from surface primary productivity. Localized enrichments in invertebrate biomass and abundance are found at deep-sea hydrothermal vents and cold seeps where autochthonous organic material is generated through chemosynthetic processes. Stable isotope techniques have been used extensively as tools for understanding trophic relationships at these sites, and the results have been well reviewed (e.g. Fisher 1990; Kennicutt et al. 1992; Conway et al. in press). Within hydrothermal vent communities, research emAcknowledgments WC thank Bob Michener for analysis of the isotope compositions reported here. WC are also grateful to Fred Grassle and John Delaney, Chief Scientists of the Alvin/Atlantis II expeditions to the Gorda and Juan dc Fuca hydrothermal fields. Fred Grasslc also shared his knowledge of hydrothcrmal systems and provided encouragement and inspiration. We thank Rose Petrecca, other members of the scientific parties ofthcse expeditions, the Alvin group, and the crew of the Atlantis II, all of whom helped us secure specimens. This paper benefited from two anonymous rcviewers. The work was supported by grants from the National Science Foundation, the WHO1 Education Office, the WHO1 Ocean Ventures Fund, and the WHO1 Biology Department. phasis has been on characterization of the carbon and nitrogen isotope compositions of invertebrates that contain endosymbiotic bacteria (hereafter referred to as “symbiotic species”). Studies were initially done at the Galapagos Spreading Center and at 2 1”N on the East Pacific Rise by Rau and Hedges (1979) and Rau (198 1 a,b) and subsequently by Fisher and his colleagues (see Fisher 1990). From these works, variability in isotope compositions of vent symbiotic species at the population level has been measured and related to physiological and microhabitat characteristics. “Textbook” vent communities of the Galapagos and East Pacific Rise spreading centersdominated by giant tubeworms, clams, and mussels-underscore the novel role that invertebrate-bacteria symbioses play in deepsea chemosynthetic communities but lead, perhaps, to an underemphasis of the importance of free-living microorganisms as primary producers in these environments. Van Dover and Fry (1989) and Van Dover et al. (1988) found isotope data useful in examining the role of free-living microorganisms in hydrothermal vent food webs. Free-living microorganisms at vents are found as suspended populations associated with the discharge of low-temperature hydrothermal fluids and as microbial mats growing on surfaces of basalt, sulfides, sedi-