Sulfur Biogeochemistry—Past and Present

Eukaryotes of the Cariaco, Soledad, and Santa Barbara Basins 39laden Cariaco sediments. The single gastrotrich morphotype ismost likely a new Urodasys species because it is smaller thanits Santa Barbara Basin congener U. anorektoxys (Todaro et al.,2000). The Cariaco polychaete is a nerillid, but not the speciespresent in Santa Barbara Basin (Xenonerilla bactericola, Mülleret al., 2001). Most likely, this Cariaco polychaete is new to science(M. Müller, 2002, personal commun.). A single oligochaetewas also observed in the Cariaco Beggiatoa sample; additionalspecimens are required for its identification.Prevalence of Putative SymbiontsEach of the major eukaryotic biomass contributors in SantaBarbara (Nonionella stella) and Cariaco (Virgulinella fragilis)exhibit the unusual characteristic of sequestering chloroplasts(Bernhard and Bowser, 1999; Bernhard, 2003). Although theseforaminifers live in darkness, the sequestered plastids are presumablycrucial to their dominance in these sulfide-enrichedenvironments (Grzymski et al., 2002). The ultrastructure offoraminifers from Soledad has not been analyzed, so it isunclear if its dominant species, Bolivina subadvena, similarlysequesters chloroplasts.Neither the Soledad flagellate nor ciliate community wasdominated by specimens with associated putative symbionts(in terms of either abundance or biomass; Fig. 3). This contrastswith those communities of Santa Barbara Basin (Fig. 3;Bernhard et al., 2000). The flagellate and ciliate community ofCariaco’s Beggiatoa-laden sediments also appeared to be dominated(in terms of abundance and biovolume) by eukaryotes withprokaryotic associates. Although the available non-quantitativeCariaco sample only allows relative comparisons, ciliates withputative symbionts comprised 96.5% of the ciliate populationwhile 57.7% of the flagellates similarly had putative symbionts.As in Santa Barbara’s Xenonerilla bactericola (Bernhardet al., 2000; Müller et al., 2001), the Cariaco polychaete hadrod-shaped bacterial ectobionts (Fig. 4). The epibionts had welldevelopedattachments to the polychaete epidermis (Fig. 4B).A specimen of a Cariaco gastrotrich appeared to have bacterialassociates when examined using DAPI to identify nuclei andprokaryotes. Ultrastructural analysis of this species is pending.In the few specimens examined ultrastructurally to date, putativesymbionts were not observed in either of the other Cariaco metazoans(i.e., nematode, oligochaete). Although the ultrastructureof Soledad nematodes has not yet been examined, DAPI resultssuggest the presence of bacterial ectobionts.Sub-Millimeter Life Positions in Laminated Soledad andSanta Barbara SedimentsFigure 3. Histograms of flagellate (A, B) and ciliate (C, D) abundanceand biovolume data, presented for specimens with and without putativesymbionts. SBB—Santa Barbara Basin.Eukaryotes in Soledad sediments were concentrated inthe surface ~2 mm, even when bottom water O 2was relativelyhigh (2.7 µM; Fig. 5). When such “high” oxygen concentrationsoccurred in Santa Barbara, the eukaryotes were also concentratedin the top few mm, but they also occurred in relatively high abundancesto depths of at least 8 mm (2.4 µM O 2; Bernhard et al.,2003). In the Soledad FLEC material examined to date, veryfew protists (foraminifera, flagellates, ciliates) were observedat a depth >2mm, although nematodes were noted to depths of~1.5 cm. Life-position data is not available for Cariaco sediments.Soledad FLEC sections show that the laminae in the top2 mm were substantially disrupted (Fig. 6). The disruptionswere not caused by sampling because prokaryotes appeared toselectively inhabit lighter-colored layers rather than darker layers(Fig. 6A). Because gravity cores and piston cores show thatSoledad subsurface sediments are laminated to a depth of >5 m(van Geen et al., 2001), it is possible that disrupted laminae“realign” during compaction due to sediment burial or that modernenvironmental conditions differ from those that produced thewell-preserved laminae.Higher magnification examination of Soledad FLECmaterial shows some of the abundant foraminifera, flagellates,ciliates, and filamentous bacteria other than Thioploca (Fig. 7).Specimens of the agglutinated foraminifer Leptohalysis sp. andthe tectinous foraminifer Nodellum sp. were easily identified(Fig. 7A). The presence of Nodellum sp. in Soledad samples is,