Microorganisms as food resources at deep-sea hydrothermal vents

Transcription

1 Limnol. Oceanogr., 39(I), 1994, , by the American Society of Limnology and Oceanography, Inc. Microorganisms as food resources at deep-sea hydrothermal vents Cindy Lee Van Dover Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts Brian Fry The Ecosystems Center, Marine Biological Laboratory, Woods Hole Abstract 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 % (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 ( to %) 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 em- Acknowledgments 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. 51 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 centers- dominated 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-

2 52 Van Dover and Fry ments, and macrofauna (see Karl 1987). Numerous invertebrate suspension-feeding, grazing, and deposit-feeding species belonging to a variety of taxonomic groups have access to these microorganisms. Analysis of invertebrate consumer species from three geographically and faunistically disjunct hydrothermal vent systems (Mariana, East Pacific Rise, Mid-Atlantic Ridge) yielded a fairly consistent pattern in the use of an isotopically distinctive food resource (613C %q 615N < + 6o/oo) relative to photosynthetically derived particulate organic material (Van Dover et al. 1988; Van Dover and Fry 1989). Microorganisms that were correspondingly 13C enriched and 15N depleted were inferred to comprise the base of the invertebrate consumer food webs at these sites. Desbruy&-es et al. (1983) reached a similar conclusion about the diet of the polychaete Alvinella pompejana from a vent site at 21 N on the East Pacific Rise. The initial three sites from which we analyzed isotope compositions of consumer species were selected because of their geographical, geological, and faunistic differences. Mariana vents in the western Pacific are on a basaltic back arc spreading center and are dominated by hairy gastropods, barnacles, and limpets (Hessler et al. 1988). Hanging Gardens on the East Pacific Rise is one of the textbooktype hydrothermal vent communities, domi- nated by tubeworms (Rzjlia pachyptila and Oasisia alvinae; Van Dover pers. obs.) and located on a basaltic midocean ridge with an intermediate spreading rate. In the Atlantic, shrimp (Rimicaris exoculata) dominate vents at two known hydrothermal sites on the slowspreading Mid-Atlantic Ridge (Rona et al. 1986). Despite site-specific faunistic and geological differences, the stable isotope compositions of most consumer invertebrate species at these three locations were remarkably similar, suggesting a uniformity in biogeochemical processes. To explore the generality of this observation, we collected additional consumer invertebrate species from four microhabitats at Gorda Ridge and from a sulfide flange at the Endeavour Site on the Juan de Fuca Ridge. Unlike most hydrothermal settings on midocean ridges where hydrothermal fluids exit from sulfide chimneys or cracks in fields of lava, the Gorda hydrothermal system is com- prised of a series of low sulfide mounds surrounded by sediment (McMurray 1990). The mounds support hydrothermal vents of varying temperatures; three Gorda collection sites are designated by the maximum temperature of the effluent water: Gorda 22O C, Gorda 11 O C, and Gorda 15 C (Van Dover et al. 1990). Samples from these sites consisted of tubeworm clumps (Ridgeia sp.) and associated macrofauna and bacterial mats from low-temperature regions. A fourth Gorda site was located in sediment at the base of one mound, where we found a small bed of vesicomyid clams and associated anemones and orbiniid polychaetes (designated Gorda clam bed). Samples from the Juan de Fuca site were collected from a sulfide flange-a remarkable hydrothermal environment where trapped, 350 C hydrothermal fluids pool beneath sulfide outgrowths from a large sulfide mound (Delaney et al. 1992). These irregularly semicircular flanges resemble shelf mushrooms in habit, but can reach dimensions of > l-m diameter and lo-20 cm thick. Dense communities of tubeworms (Ridgeia sp.), limpets (Lepetodrilus fucensis), and polychaetes (Paralvinella) colonize the top surfaces of the flanges. Methods Specimens were collected in 1988 during A fvin dives at sites along the Gorda Ridge (Northern Escanaba Trough; N, W; 3,200-3,250 m) and the Juan de Fuca Ridge (Endeavour Segment; N, W; 2,200 m). For each sampling site, the species analyzed (Table 1) represent the complete suite of dominant macrofaunal invertebrate species in the microhabitat. All specimens were stored frozen until prepared for isotope analyses. Protein-rich muscle tissue was preferentially analyzed; for small species, however, pooled samples, of organic tissues from multiple individuals were prepared. Frozen tissues were thawed and acidified with 0.1 N HCl to remove contaminating carbonates, dried at 60 C powdered, and analyzed for carbon and nitrogen stable isotope compositions following the methods of Minagawa et al. (1984). CO, and N2 gases were analyzed separately with a Finnigan MAT 25 1 isotope-ratio mass spectrometer. Isotope compositions are expressed in terms of per mil

3 Food resources at vents 53 Table C and 615N values of vent organisms (microbial mats, symbiotic species, and invertebrate consumers) from Gorda and Juan dc Fuca Ridges. Only organic tissues were analyzed. -151, Gcfds Clambed T 1 l-f21 Gorda 220 C c Fig. 1. cy7c vs. 615N for suites ofinvertebrate consumer species from different locations. Isotope compositions of nonvent invertebrates (ambient Gorda and ambient East Pacific Rise, EPR) are provided for reference. Where more than two specks were analyzed, values plotted reprcscnted mean + 1 SD. Mean 613C and 615N for two species-*$ C and 615N for one species-**. differences from a standard, where 6X = [(R(sample)lR(standard)) - l] x 103. X = 13C or l5n; R = 13C: 12C or 15N: 14N. Standard reference materials are carbon in the Pee Dee Belemnite (PDB) and nitrogen gas in the atmosphere (air). Lighter or more negative 6 values are enriched in the lighter isotope and reciprocally depleted in the heavier isotope. Carbon and nitrogen compositions were obtained from each sample and reported as paired, nonindependent data sets in Table 1. Our strategy of analyzing composite and individual samples did not allow us to determine isotope variance within populations. Results and discussion Although the carbon and nitrogen isotope compositions of a variety of invertebrates from Mariana, Hanging Gardens, and Mid-Atlantic Ridge invertebrate consumers are relatively uniform, this uniformity is not observed at Gorda Ridge and Juan de Fuca sites (Fig. 1). The range in 613C values of consumers at these latter sites varies from an 8.7?&~ difference between two species at the Gorda clam bed site to a 19%0 difference among four species at the Gorda 110 C site. Carbon isotope compositions of most consumer species from the Juan Vent site Gorda 220 C Paralvinella sp. 1 Limpet Coiled gastropod Polynoid sp. 1 (polychacte) Bacterial mat vcstimentum) Gorda 110 C Anemone sp. 1 Anemone sp. 2 Simrothiella (aplacophoran) Limpet Bacterial mat vestimentum) Gorda 15 C Amphisamytha sp. Polynoid sp. 1 Paralvinella sp. 1 (polychaetc) Bacterial mat vestimentum) Gorda clam bed Orbiniid Anemone sp. 3 Vesicomyid (clam with symbi onts; mantle tissue) Juan de Fuca flange Hydroids Paralvinella sp. Amphisamytha sp. Polynoid sp. Nicomache sp. Lepetodrilus fucensis (limpet) Buccinum viridum (coiled gastropod) Provanna sp. (coiled gastropod) vestimentum) * Uncounted bulk samples. - No. Of spccimens 6 C b >N pooled Vd (~ * * * * I

4 54 Van Dover and Fry de Fuca flange, Gorda 220 C and Gorda 15 C sites (6 C = to -21.5o/oo) overlap 613C compositions of ambient deep-sea invertebrates not associated with hydrothermal or seep systems. Consumer species from two other Gorda microhabitats (Gorda 110 C and Gorda clam bed) have isotopically light 613C values (-24.7 to -43.7%) relative to ambient deepsea invertebrates. From the carbon isotope compositions of consumers, we can make some inferences regarding the carbon isotope composition of the free-living microorganisms upon which we think the consumers ultimately depend. We suggest that in situ populations of free-living microorganisms at vents may have heterogeneous carbon isotope compositions and that this heterogeneity may be microhabitat-dependent. We infer that extremes in the ranges of carbon isotope compositions of vent consumers within a microhabitat correspond to two dietary sources of isotopically distinctive microorganisms. Intermediate 613C compositions of consumers in a microhabitat are ambiguous and might reflect consumption of a mixed diet of two isotopically extreme (or light and heavy) sources of microbial carbon, the presence of yet a third dietary source of microorganisms with intermediate 6l 3C values, reliance on photosynthetic detritus, or some combination of these possibilities. Given the pronounced carbon isotope diversity among consumer invertebrates at Gorda and Juan de Fuca sites, it is remarkable that 613C values of invertebrates cluster so closely at Mariana, Hanging Gardens, and Mid-Atlantic Ridge sites. We are not certain of the causes of this tight clustering, but note that the Mariana, Hanging Gardens, and Mid-Atlantic Ridge vent consumers were collected from the sides of classic C mineral chimneys. These chimneys may provide a uniform environment for microbial production. In contrast to smoker chimney environments, the Gorda Ridge sites occur on a sediment-hosted ridge segment where the hydrothermal fluid chemistry becomes complex by reaction with sediments and yields hydrocarbons and other exotic products. Assimilation of these compounds by microorganisms may contribute to microbial isotopic diversity. At the Juan de Fuca site, isotopic diversity may reflect, in part, abundant methanotrophic mi- croorganisms. Millimolar levels of CH4 have been measured in the high temperature fluids collected from this site (M. D. Lilley cited by de Angelis et al. 199 l), and de Angelis et al. (199 1) implicate methanotrophic bacteria as significant in the diet of L. fucensis, one of the dominant invertebrates in the flange community. They found elevated uptake of 14Clabeled CH, by microorganisms living on invertebrate surfaces presumably browsed by limpets. The carbon isotope composition of CH4 from the Juan de Fuca vents is not known but may be similar to that measured by Welhan and Craig (1983) for crustal CH4 at a site on the East Pacific Rise (- 15 to %~). de Angelis et al. found 613C composition of a pooled sample of limpets to be o/oo, which would be consistent with some dependence on methanotrophs if the methane 6 C value of Juan de Fuca vent water is - 15 to - 1 Sa/oo and some fractionation occurs during microbial growth on methane (Claypool and Kaplan 1974). Large ranges in 613C values of consumers within some microhabitats (e.g. 19%~ at the Gorda 110 C site) suggest that multiple sources of isotopically distinct microbial C may occur at the same site and can be selectively consumed. In at least one instance there is evidence that this selectivity is correlated with feeding type: suspension-feeding invertebrates might consume a different isotope pool of carbon than grazers. At the Gorda 110 C site, for example, two species of presumed suspensionfeeding anemones have 613C values of and -26.2%, while presumed grazers (a limpet and an aplaophoran) have relatively 13Cdepleted values of and -43.7%~ Nitrogen isotope compositions are also variable among Gorda and Juan de Fuca sites but are always depleted in 15N relative to nonvent deep-sea invertebrates (Fig. 1); the most negative values are found at the Gorda 220 C vent (mean 615N = - - 7a/oo). As at Mariana, EPR, and Mid-Atlantic Ridge vents, 615N values of consumers (615N < lo?&) indicate that organic N is of local, microbial origin. If a 3.4% increment in 615N with each trophic level is assumed (Minagawa and Wada 1984) 615N compositions of bacterial populations at vent sites are inferred to range from a maximum of + 5a/oo (MAR, EPR, Mariana) to a minimum of ~ (Gorda 220 C) based on 615N values of

5 Food resources at vents 55 primary consumers. The inference of extremely negative 6l 5N values for some free-living microbial populations at vents precludes the possibility that N2 fixation is important in these populations and implies elevated ambient inorganic N substrate concentrations (e.g. [NH, ]) relative to biological demand. Substantial nitrogen isotope fractionations of 4-27YL0 can occur during microbial uptake of ammonium (Hoch et al. 1992). Although no single species occurs at all of the vents studied, carbon and nitrogen isotope compositions of representatives of the polychaete genus Paralvinella were analyzed from Hanging Gardens, Mariana, Gorda 220 C Gorda 15 C, and Juan de Fuca sites (Table 2). These polychaetes appear to share similar functional morphologies and presumably belong to the same feeding guild (grazing depositfeeders). Gut contents of specimens collected from the Endeavour Site on the Juan de Fuca Ridge consist of filamentous bacteria, mucus, and diatomaceous debris (Tunnicliffe et al. 1985). Isotope compositions are site-specific rather than uniform throughout the genus (Table 2), which contrasts with the uniformity of 613C compositions of symbiont tubeworm or bivalve species across diverse vent sites (Kennicutt et al. 1992) and indirectly underscores the role of the host invertebrate in providing a relatively homeostatic culture environment for its endosymbionts. At three Gorda sites, we analyzed the carbon and nitrogen isotope composition of bacterial mats (Table 1). At two of the sites (Gorda 220 C and Gorda 15 C), the mats were closely associated with paralvinellid polychaetes. There is good agreement between the carbon isotope composition of the bacterial mats (S13C = and o/oo) and the paralvinellid polychaetes (613C = and - 19.lo/oo, respectively), especially at the Gorda 220 C vent. 615N values of the bacterial mats at these two sites were ~ lighter than the polychaetes. These carbon and nitrogen isotope data are consistent with a bacterial mat diet for the paralvinellid polychaete. Bacterial mat from the Gorda 110 C site has a 13C-depleted isotope composition (-4 1.6%) consistent with an inferred light pool of microbial organic C based on consumer isotope compositions, especially those species likely to be grazers [i.e. the aplacophoran (- 37.lYm) and Table C and 615N values of Parahinella species from different locations, compiled from Van Dover and Fry (1989) and Table 1. Each pair of values was obtained from the same individual. Hanging Gardens, EPR Mariana Gorda 220 C Gorda 15 C Juan de Fuca, Endcavour 6 C b N ( N ( s $4.1 the limpet (- 43.7o/oo)] rather than suspension feeders. But comparison of the 615N values of this mat and its likely consumers is not so satisfactory- the presumptive diet being as much as 5.5o/oo heavier than its consumer. We are forced to conclude that the bacterial mat sampled is not the principal diet of consumers at the Gorda 110 C site or that the mat may be spatially heterogeneous with respect to 6 5N. Alternatively, a microbial source of organic C not collected in our limited sampling program may be available to consumers at the site. The diversity of 613C values in three samples of bacterial mat reported here may only hint at the isotope diversity of free-living microorganisms at vent sites. Jannasch and his colleagues have repeatedly emphasized the potential for diversity of microbial metabolic types at vents (e.g. Jannasch 1985; Jannasch and Mottl 1985); there is likely to be a correlative diversity in microbial isotope composition. A summary of measured and inferred 6 l 3C values for chemoautotrophic bacteria (Fig. 2) supports this view and suggests that microorganisms at deep-sea hydrothermal vents span an even broader range of 613C values than shallow-water microbial mats (Schidlowski et al. 1984). 13C-rich values in the - 10 to - 12% range are consistent with autotrophic CO, fixation under conditions of C limitation (Fisher et al. 1990). Although the range of 613C compositions may be broad across all types of vents, individual vent microhabitats appear to be biased toward much smaller ranges of 613C. Conclusions Results presented here suggest that the most conspicuous components of vent communities-the large standing crops of adult tubeworms, bivalves, and other invertebrate hosts

6 56 Van Dover and Fry 00 Beggiatoa mat Seggiatoa mat Thiomicrospira sp. Tubeworm symbionts Bivalve symbionts Bacterial mats Inferred free-living microorganisms Inferred free-living microorganisms shallow petroleum seep (a) shallow petroleum seep (b) vent isolate, laboratory culture (c) various vents (d) various vents (d) Gorda Ridge vents (e) Han ing Gardens, Mariana and R id-atlantic Ridge vents(f) Gorda Ridge and Juan de Fuca vents (e) I I I I I v3cplj~%o Fig C values inferred and measured in chemoautotrophic microorganisms at vents and related habitats. Data from: a-spies and DcsMarais 1983; b-sasscn et al. 1993; c-ruby et al. 1987; d-fisher et al. 1990, Kcnnicutt et al. 1992; c-this study; f-van Dover and Fry For laboratory-cultured vent isolate (Thiomicrospira), plotted value is extrapolated from published value cultured with a DIC source P3C of - 10% to one of O%O. For inferred carbon isotope composition of free-living microorganisms at Gorda and Juan de Fuca vents, dashed line represents ambiguous data. where consumer values mav result either from a mixed diet of light and heavy microbial C or from consumption of an undetermined, isotopicallywintcrmcdiatc C source. to endosymbiotic bacteria-play an unexpectedly minor role as nutritional resources in vent food webs. This view derives from the large carbon isotope differences between symbiotic species and consumer invertebrates in some locales. A specific example is the Juan de Fuca flange habitat, where the symbiotic tubeworm had a 613C composition of % while consumer invertebrates were generally < - 20%~. A significant direct trophic link between tubeworms and other invertebrates within the community thus seems tenuous. Similar isotope differences between dominant symbiotic species and consumer invertebrates also occur in Gorda microhabitats. Carbon isotope values of most vent consumers compel us to look for significant sources of primary production in the vent food web outside of invertebratebacteria endosymbioses. Organic C fixed by the free-living autotrophic microbial community appears to be more readily available to higher trophic levels within vent food webs than endosymbiont production. The ultimate fate of endosymbiont production is not entirely clear but may include in situ respiration and consumption by vent crabs, fish, and nonvent deep-sea scavengers (Tunnicliffe et al. 1990). Data in this report contribute to accumulating evidence that a dual approach by means of carbon and nitrogen isotope analyses of or- ganisms is useful in distinguishing chemosynthetically fixed organic material from photosynthetically fixed organic material. This approach may be useful for tracing the export and contribution of chemosynthetic organic material to the surrounding benthic and pelagic environments, thereby establishing a spa- tial scale for the role of hydothermal primary production in the food-limited deep sea. Tsotope measurements may also be helpful screening tools for identifying benthic species

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