Sulfur BiogeochemistryâPast and Present

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Geological Society of AmericaSpecial Paper 3792004Eukaryotes of the Cariaco, Soledad, and Santa Barbara Basins:Protists and metazoans associated with deep-water marinesulfide-oxidizing microbial mats and their possible effects onthe geologic recordJoan M. Bernhard*Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina,Columbia, South Carolina 29208, USAKurt R. BuckMonterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039, USAABSTRACTSulfide-enriched environments are not typically considered to be sites that supportabundant eukaryotes, yet it is known that plentiful and relatively diverse protistanand metazoan fauna inhabit at least one modern bathyal sulfidic site (SantaBarbara Basin, California). This contribution adds to our knowledge of eukaryoticcommunities inhabiting sulfide-enriched deep-water sediments by presenting datafrom Soledad Basin (off the western coast of Baja California, Mexico) and CariacoBasin (off Venezuela). Results indicate that, when considered at the appropriate scale,the density of eukaryotes in Soledad Basin was comparable to that of Santa BarbaraBasin. Eukaryotic biovolume and abundance were dominated by foraminifera at allthree sites. Unlike the Santa Barbara Basin assemblage, Soledad eukaryotic abundanceand biovolume were not dominated by eukaryotes with associated putativesymbionts. An undescribed polychaete found in Cariaco Beggiatoa-laden sedimentshad bacterial ectobionts. Sub-millimeter life-position analysis indicated that Soledadeukaryotes concentrated within the top 2 mm even when the bottom-water oxygenconcentration was relatively high (2.7 µM). Observations suggest that the eukaryoticfauna of a Thioploca-dominated site (Soledad) varied substantially in taxonomic compositionand sub-millimeter life positions from Beggiatoa-dominated sites (Cariacoand Santa Barbara).Keywords: Beggiatoa, Cariaco Basin, ciliate, flagellate, foraminifera, nematode,polychaete, Santa Barbara Basin, Soledad Basin, symbiosis, Thioploca.* Present address: Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA, jbernhard@whoi.edu.Bernhard, J.M., and Buck, K.R., 2004, Eukaryotes of the Cariaco, Soledad, and Santa Barbara Basins: Protists and metazoans associated with deep-water marine sulfide-oxidizingmicrobial mats and their possible effects on the geologic record, in Amend, J.P., Edwards, K.J., and Lyons, T.W., eds., Sulfur Biogeochemistry—Past andPresent: Geological Society of America Special Paper 379, p. 35–47. For permission to copy, contact editing@geosociety.org. © 2004 Geological Society of America.35
36 J.M. Bernhard and K.R. BuckINTRODUCTIONAlthough it has long been known that certain modern-daymarine basins such as the Black Sea are sulfide enriched, theuse of manned submersibles and remotely operated vehicles fordeep-ocean exploration has revealed that sulfidic environmentsin the modern ocean are relatively common. The discovery ofdeep-water hydrothermal vents in the 1970s prompted extensiveoceanic exploration to determine their extent. In the processof exploring these environments, extensive chemoautotrophiccommunities were discovered, the energy sources for whichare reduced compounds such as hydrogen sulfide and methane,rather than sunlight. Original biological studies concentrated onthe larger fauna and the symbiotic prokaryotes (i.e., cells lackingnuclei: bacteria and archaea) of these chemosynthetic communities.Only now are studies elucidating the smaller eukaryotic(cells with nuclei: single-celled protists and multi-celled metazoans)fauna of sulfide-enriched deep-sea habitats. This contributionpresents a synopsis of current knowledge of deep-water,sulfide-tolerant protistan and metazoan meiofauna, new ecologicaldata from selected sulfide-enriched sites, and a discourse onthe possible effects that these eukaryotes have on the geologicrecord. Because Earth’s early evolution and subsequent events ofoceanic anoxia were likely to have included sulfide enrichment(e.g., Canfield 1998), understanding present-day “thiobiotic”communities can help unravel past episodes in Earth’s history.Sulfide-enriched habitats in today’s oceans can occur anywherethere is organic enrichment. Besides the hard-substratehydrothermal vents and the soft-sediment associated “cold”seeps, sulfidic conditions occur in fjords, silled basins (e.g.,Santa Barbara Basin, Cariaco Basin, Black Sea), and along theopen ocean margins with well-developed oxygen minimumzones (e.g., Monterey Bay, California; off Mazatlan, Mexico). Inaddition, sulfide enrichment has been observed at large food falls,which are areas on the seafloor where the carcass of a large mammalsank to the seafloor (e.g., whale falls; Bennett et al., 1994;Deming et al., 1997).The megafaunal (e.g., bivalves, tube worms) and macrofaunal(e.g., polychaetes) chemoautotrophic communities of hydrothermalvents have been extensively studied for their physiology(e.g., Childress and Fisher, 1992), ecology (Van Dover, 2000,and references therein) and biogeography (e.g., Van Dover et al.,2002). Much information is also available regarding cold seepchemoautotrophic communities (e.g., Sibuet and Olu, 1998).Recent studies of whale-fall carcasses indicate interesting taxonomicand gene-flow patterns between these stepping stones andseeps and vents (e.g., Smith and Baco, 1998). Less is knownabout the eukaryotic fauna of silled basins and deep-water fjordsbecause many of these environments were thought to supportonly prokaryotes, given that they typically lack larger fauna.Recent studies show that high densities of protists and meiofaunalmetazoans occur in at least one bathyal oxygen-depleted, sulfide-enrichedsilled basin (Santa Barbara Basin, off California;Bernhard et al., 2000). Few studies have addressed the similarlysmall fauna of hydrothermal vents, but results suggest the presenceof ciliates (Small and Gross, 1985; Edgcomb et al., 2002)and flagellates (Edgcomb et al., 2002) at the sediment-coveredGuaymas Basin hydrothermal vent. Studies on water columnsamples collected in proximity to hydrothermal vents and theGuaymas Basin indicated flagellates capable of withstandinghigh concentrations of hydrogen sulfide (30 mM), suggestingthat these taxa may be important components of deep-waterhydrothermal vent communities (Atkins et al., 2000, 2002).Because hydrogen sulfide at micromolar concentrationsinhibits respiration, the aerobic eukaryotes of sulfide-enrichedenvironments must have physiological adaptations to allowthem to survive such conditions. The majority of physiologicalstudies on chemoautotrophic communities have been devoted tomacrofauna and megafauna; little is known about the physiologyof meiofauna (e.g., foraminifera, nematodes) and nanobiota (i.e.,ciliates, flagellates; see Gage and Tyler [1991] for more discussionon organism size classes) in any sulfide-enriched environment,and even less is known about those inhabiting deep-watersulfidic environments. In general, of the eukaryotes inhabitingsulfidic environments that have been studied, many have prokaryoticassociates (e.g., Fenchel and Finlay, 1995; Gaill 1993).For example, nematodes from shallow-water environments areknown to harbor putative symbionts (e.g., Ott et al., 1991), as dooligochaetes (Giere et al., 1991).These putative symbionts presumably provide their hostwith some metabolic byproduct(s) to promote their survival, butdemonstrating metabolic exchange between host and prokaryoteis difficult, especially in small eukaryotes such as nanobiota andmeiofauna. Prokaryotic associates can be endobionts (livinginside the host) or ectobionts (living on the host). In some cases,both endobionts and ectobionts occur (e.g., species of the ciliateMetopus; Esteban et al., 1995). Symbionts of metazoan (aerobic)hosts are typically sulfide oxidizers (e.g., nematodes, Polz et al.,1994; Hentschel et al., 1999) or methanotrophs (e.g., bivalves;Vetter and Fry, 1998). In some vent mollusks, both of thesetypes of endosymbionts can be present (e.g., Cavanaugh et al.,1992). In addition, an oligochaete species is known to harbor twotypes of endosymbionts: sulfate-reducers and sulfide-oxidizers(Dubilier et al., 2001). Symbionts of anaerobic flagellate hostsare known to be methanogens (e.g., Fenchel and Finlay, 1992)or sulfate reducers (Fenchel and Ramsing, 1992).A number of eukaryotes inhabiting sulfide-enriched habitatslack symbionts; their adaptations to sulfide exposure are varied.For example, in some animals, sulfide oxidation occurs in mitochondria.The hemoglobin of some metazoans binds sulfide toprevent or minimize its detrimental effects on respiration. Whenrespiration is inhibited, a sulfur-dependent anaerobic energymetabolism can be invoked. For details regarding these adaptations,the reader is directed to reviews by Somero et al. (1989),Vismann (1991), Childress (1995), Grieshaber and Völkel(1998), and Hagerman (1998).From the geological perspective, laminated sediments areinvaluable to studies of climate change on annual, decadal, and
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Sulfur BiogeochemistryâPast and Present