Person:
Kostka,
Joel E.
Kostka,
Joel E.
Permanent Link
Associated Organization(s)
Organizational Unit
ORCID
ArchiveSpace Name Record
4 results
Publication Search Results
Now showing
1 - 4 of 4
-
ItemComparisons of Structural Iron Reduction in Smectites by Bacteria and Dithionite: II. A variable-temperature Mössbauer spectroscopic study of Garfield nontronite(Georgia Institute of Technology, 2009-07) Ribeiro, Fabiana R. ; Fabris, José D. ; Kostka, Joel E. ; Komadel, Peter ; Stucki, Joseph W.The reduction of structural Fe in smectite may be mediated either abiotically by reaction with chemical reducing agents or biotically by reaction with various bacterial species. The effects of abiotic reduction on clay surface chemistry are much better known than the effects of biotic reduction, and differences between them are still in need of investigation. The purpose of the present study was to compare the effects of dithionite (abiotic) and bacteria (biotic) reduction of structural Fe in nontronite on the clay structure as observed by variabletemperature Mössbauer spectroscopy. Biotic reduction was accomplished by incubating Na-saturated Garfield nontronite (sample API 33a) with Shewanella oneidensis strain MR-1 (FeII/total Fe achieved was ~17 %). Partial abiotic reduction (FeII/total Fe ~23 %) was achieved using pH-buffered sodium dithionite. The nontronite was also reduced abiotically to FeII/total Fe ~96 %. Parallel samples were reoxidized by bubbling O2 gas through the reduced suspensions at room temperature prior to Mössbauer analysis at 77 and 4 K. At 77 K, the reduction treatments all gave spectra composed of doublets for structural FeII and FeIII in the nontronite. The spectra for reoxidized samples were largely restored to that of the un - altered sample, except for the sample reduced to 96 %. At 4 K, the spectrum for the 96 % reduced sample was highly complex and clearly reflected magnetic order in the sample. When partially reduced, the spectrum also exhibited magnetic order, but the features were completely different depending on whether reduced biotically or abiotically. The biotically reduced sample appeared to contain distinctly separate domains of FeII and FeIII within the structure, whereas partial abiotic reduction produced a spectrum representative of FeII–FeIII pairs as the dominant domain type. The 4 K spectra of the partially reduced, fully reoxidized samples were virtually the same as at 77 K, whereas reoxidation of the 96 % reduced sample produced a spectrum consisting of a magnetically ordered sextet with a minor contribution from a FeII doublet, indicating significant structural alterations compared to the un - altered sample.
-
ItemSeasonal composition and activity of sulfate-reducing prokaryotic communities in seagrass bed sediments(Georgia Institute of Technology, 2004-11-11) Smith, April C. ; Kostka, Joel E. ; Devereux, Richard ; Yates, Diane F.Sulfate-reducing prokaryotes (SRP) play a key role in the carbon and nutrient cycles of coastal marine, vegetated ecosystems; however, the interactions of SRP communities with aquatic plants remain little studied. The abundance, activity, and community composition of SRP were studied in relation to sediment geochemical gradients and plant growth state in a Thalassia testudinum seagrass bed and in adjacent unvegetated areas. Geochemical analyses indicated significantly higher concentrations of microbial respiration products in vegetated sediments during summer than during winter. Depth-integrated sulfate reduction rates were 3 to 5 times higher in vegetated (52.5 mmol m-2 d-1 in summer and 20.4 to 26.5 mmol m-2 d-1 in winter) compared to unvegetated sediments (10.7 mmol m-2 d-1 in summer and 3.6 to 7.6 mmol m-2 d-1 in winter), and depth-integrated activities further showed a strong correlation with seagrass biomass. Most probable number (MPN) counts of SRP were 10 times higher in vegetated compared to unvegetated sites in the summer during the period of maximum growth for seagrasses, whereas no difference was observed for counts between sites during the winter. The community composition of SRP was determined using restriction fragment length polymorphism (RFLP) screening and amino acid sequence comparisons inferred from partial dissimilatory bisulfite reductase (dsrA and B) genes that were PCR-amplified and cloned from DNA extracted from sediment samples. The majority of unique DSR sequences were not affiliated with any known SRP group, and clustered at levels indicative of new SRP. Some DSR sequences grouped on the basis of originating from vegetated or unvegetated sediments, although the relationship did not appear to be strong. The diversity of SRP in seagrass bed sediments, as indicated by dsr analysis, was high and did not appear to covary with the other environmental parameters tested. Our results indicate that seagrass growth state enhances the abundance and activity of SRP, while SRP community composition remains relatively stable across the environmental parameters tested.
-
ItemImpact of fiddler crabs and plant roots on sediment biogeochemistry in a Georgia salt marsh(Georgia Institute of Technology, 2003-09-12) Gribsholt, Britta ; Kostka, Joel E. ; Kristensen, ErikThe influence of macrofauna and macrophytes on sediment biogeochemistry was quantified in a Spartina alterniflora (Loisel) saltmarsh, with emphasis on sulfur and iron cycling. Vertical profiles of sediment geochemistry and rates of microbial metabolism at 3 sites with different abundances of fiddler crab Uca pugnax burrows, vegetation coverage and hydrology were supplemented with high-resolution radial profiles around burrow walls and S. alterniflora roots. Carbon oxidation was measured as sulfate reduction using the 35S technique, as total anaerobic CO2 production, and as Fe(III) reduction by monitoring Fe(II) evolution. Depth-integrated (0 to 10 cm) sulfate reduction was 25% lower, while total Fe and Fe(III) concentrations were 1.5 and 6 times higher, respectively, in bioturbated than in nonbioturbated sediment. Low sulfate-reduction rates adjacent to burrow walls (3% of those in bulk sediment) were counteracted by very high Fe(III) reduction rates. Thus, Fe(III) reduction accounted for 54 to 86% of the total carbon oxidation within 4 cm distance of burrows, decreasing in importance with distance from the burrow wall. Overall, S. alterniflora roots showed a greater impact on sediment biogeochemistry than crab burrows. Sulfate reduction was almost absent in the rhizosphere, whereas Fe(III) reduction rates (6.2 µmol Fe cm-3 d-1) were among the highest reported for marine sediments, accounting for >99% of carbon oxidation. Our results confirm the universal relationship between the contribution of Fe(III) respiration to total carbon oxidation and solid Fe(III) concentrations that has been suggested based on studies of subtidal marine sediments. The importance of Fe(III) respiration was strongly dependent on Fe(III) concentrations below levels of 30 µmol cm-3, whereas above this level almost all anaerobic respiration was mediated by Fe(III) reduction in saltmarsh sediments.
-
ItemRates and Pathways of Carbon Oxidation in Permanently Cold Arctic Sediments(Georgia Institute of Technology, 1999-05-03) Kostka, Joel E. ; Thamdrup, Bo ; Nohr Glud, Ronnie ; Canfield, Donald E.We report here a comprehensive study of the rates and pathways of carbon mineralization in Arctic sediments. Four sites were studied at 115 to 329 m water depth in fjords on Svalbard and in coastal Norway. The Svalbard coastal region is characterized by permanently cold bottom water temperatures of -1.7 to 2.6 °C. Carbon oxidation (avg = 20 to 400 nmol d-') and sulfate reduction rates (avg = 10 to 350 nmol cm-3 d-l) were measured at high resolution to 10 cm depth in sediment incubation~ T. he distribution of oxidants available for microbial respiration was determined through porewater and solid phase geochemistry. By comparing the distribution of potential oxidants to the depth-integrated mineralization rates, the importance of various respiratory pathways to the oxidation of organic C could be quantified. Integrated C oxidation rates measured in sediment incubations (11 to 24 mm01 m-2 d-') were comparable to within a factor of 2 to dissolved inorganic carbon (DIC) fluxes measured in situ using a benthic lander. Sulfate reduction was the dominant microbial respiration pathway (58 to 92% of total C oxidation) followed by Fe(II1) reduction (10 to 26%), oxygen (5 to 14 %), and nitrate respiration (2 to 3%). At sediment depths where sulfate reduction was dominant, C oxidation equivalents, calculated from independently measured sulfate reduction rates, matched DIC production rates in incubations. Sediment geochemistry revealed that the same vertical sequence of oxidants is reduced/respired in these Arctic sediments as in temperate continental shelf sediments of equivalent water depths. Microbial communities in permanently cold Arctic sediments exhibited mineralization rates and pathways comparable to temperate nearshore environments. This study completely partitioned C oxidation pathways, showing a predominance of sulfate respiration and a substantial contribution of Fe(II1) reduction to organic matter mineralization in Arctic sediments for the first time. Microbial communities in cold sediments exposed to relatively high C deposition appear to respond to the input or availability of organic matter rather than to temperature.