No abstract

To provide insights into the biogeochemistry of environments where steep chemical gradients place anaerobes, microaerophiles, and aerobes in close proximity, it might be necessary to survey biotic distributions on scales that are not possible using conventional ecological approaches. To overcome such limitations, we adapted sedimentological and cell biological methods to examine the life positions of microbes within sediments. This fluorescently labeled embedded core (FLEC) technique was used to survey the submillimeter distributions of eukaryotic nanobiota and meiofauna, plus co-occurring prokaryotes, inhabiting laminated sediments of the bathyal Santa Barbara Basin (SBB; 34Њ13ЈN, 120Њ02ЈW). Although SBB sediments were vertically structured on the scale of millimeters (i.e., as laminae), and microelectrode profiles suggested vertically distinct oxygenated and anoxic-sulfidic layers, the distributions of aerobes, microaerophiles, and sulfide-tolerant anaerobes were not concomitantly structured. Unprecedented associations were observed among microorganisms. For example, relatively deep in the sediments, where high sulfide concentrations were expected, flagellates were intimately associated with Beggiatoa. Ciliates were typically solitary, whereas flagellates were often aggregated in nearly monomorphotypic swarms of Ͼ3 ϫ 10 4 mm Ϫ3 . Such aggregations could significantly affect geochemical pore-water processes at scales Ͻ1 l. Our observations indicate that a mosaic of chemically heterogeneous microhabitats exist in both vertical and horizontal dimensions, suggesting that biogeochemical processes in the SBB are more complex than predictions based on standard biotic assessments and microelectrode profiling.Conventional methods used to study nano-, meio-and macrofauna, such as sieving or density gradient extractions, obliterate the life positions of benthic organisms. Consequently, little is known about their subcentimeter-scale spatial distributions. This information is crucial to understanding the biotic and physicochemical interactions in sediments, particularly those with steep pore-water gradients. Within the surface centimeter in such settings, for example, conditions can range from oxygen depletion to anoxia, with or without sulfide enrichment (e.g., Jørgensen 1988).1 Corresponding author (joan.bernhard@sc.edu). AcknowledgmentsAssistance from the following is gratefully acknowledged: the captain and crew of the RV Robert Gordon Sproul, as well as the resident technicians of Scripps Institution of Oceanography; all members of the 1998 and 1999 cruises' scientific parties (especially Ellen Braun-Howland, Dennis Bazylinski, Brad Dubbels, Annette Dean, Shelly Hoeft, and Dan Rogers); Sam McGee-Russell for introducing us to hot knife microtomy, Sara Meyers, Brenda Famolaro, Kat Benson, and Adam Meyer for help in adapting the technique; Monterey Bay Aquarium Research Institute for bathymetric data; Jim Barry for construction of the baseline map; Tom Chandler for LSCM access, and two anonymous reviewers for th...

A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO2 with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell suspensions readily reduced selenite to elemental selenium (Seo). Hence, SES-3 can carry out a complete reduction of selenate to Seo. Washed cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these reductions are achieved by separate inducible enzyme systems. However, both nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of ['4C] lactate to '4Co2 coupled to the reduction of selenate or nitrate by cell suspensions was inhibited by CCCP (carbonyl cyanide m-chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readily reduced to Seo by selenate-grown cells, but selenite appeared to block the synthesis of pyruvate dehydrogenase. Tracer experiments with [75Se] selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Seo. This reduction was totally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate. Cell suspensions could reduce thiosulfate, but not sulfite, to sulfide. These results suggest that reduction of selenite to Seo may proceed, in part, by some of the components of a dissimilatory system for sulfur

We tested the role of benthic infaunal functional diversity in regulating the biogeochemistry of nearshore sediments using laboratory microcosms. Single and multispecies assemblages of deposit-feeding polychaetes (Clymenella torquata, Spio setosa, and Leitoscoloplos fragilis) were used, and fluxes of oxygen and phosphate, as well as profiles of oxygen and pH in the sediment pore water, were measured. Significant differences in flux rates were found among treatments that were unrelated to polychaete abundance or biomass alone. Multispecies assemblages had lower flux rates of both oxygen and phosphate than rates calculated from the single-species treatments. Depthintegrated oxygen and pH profiles also showed significant differences between multispecies assemblages and selected single-species treatments. These differences were most likely from species-specific feeding and burrowing behavior and species-related interactions. Coefficients of variance for both oxygen and pH were highest for microcosms with no polychaetes and lowest for the assemblages, indicating a dampening effect of multispecies assemblages on porewater heterogeneity. When oxygen flux data was incorporated into a model of oxygen dynamics in Long Island Sound, results indicated that shifts in the benthic community composition could change sediment oxygen consumption rates sufficiently to disrupt the balance between the physical supply of oxygenated water and biological oxygen demand. The results of this study confirm the importance of benthic functional biodiversity to nearshore sedimentary processes and suggest that losses of functional diversity can have significant effects on ecosystem function.Over the past several decades, there has been increasing interest in the health of the global environment, with specific emphasis on the importance of elemental cycling, climate change, and biodiversity loss. Worldwide species extinction rates are comparable to those of the end of the Mesozoic era and, as such, significant efforts are underway to understand how anthropogenic influences on biodiversity might alter or impair functions or services of the earth's ecosystems.Early studies of biodiversity effects on ecosystem function focused primarily on terrestrial plant communities and revealed significant effects on physical, biogeochemical, and ecological processes. For example, Naeem et al. (1994) confirmed that diversity loss affected community respiration,

Anoxic salt marsh sediments were amended with DL-methionine and dimethylsulfoniopropionate (DMSP). Microbial metabolism of methionine yielded methane thiol (MSH) as the major volatile organosulfur product, with the formation of lesser amounts of dimethylsulfide (DMS). Biological transformation of DMSP resulted in the rapid release of DMS and only small amounts of MSH. Experiments with microbial inhibitors indicated that production of MSH from methionine was carried out by procaryotic organisms, probably sulfate-reducing bacteria. Methane-producing bacteria did not metabolize methionine. The involvement of specific groups of organisms in DMSP hydrolysis could not be determined with the inhibitors used, because DMSP was hydrolyzed in all samples except those which were autoclaved. Unamended sediment slurries, prepared from Spartina alterniflora sediments, contained significant (1 to 10 R,M) concentrations of DMS. Endogenous methylated sulfur compounds and those produced from added methionine and DMSP were consumed by sediment microbes. Both sulfate-reducing and methane-producing bacteria were involved in DMS and MSH consumption. Methanogenesis was stimulated by the volatile organosulfur compounds released from methionine and DMSP. However, apparent competition for these compounds exists between methanogens and sulfate reducers. At low (1 ,uM) concentrations of methionine, the terminal S-methyl group was metabolized almost exclusively to CO2 and only small amounts of CH4. At higher (>100 ,IM) concentrations of methionine, the proportion of the methyl-sulfur group converted to CH4 increased. The results of this study demonstrate that methionine and DMSP are potential precursors of methylated sulfur compounds in anoxic sediments and that the microbial community is capable of metabolizing volatile methylated sulfur compounds.