Microbial manganese and sulfate reduction in Black Sea shelf sediments

Abstract

The microbial ecology of anaerobic carbon oxidation processes was investigated in Black Sea shelf sediments from mid-shelf with well-oxygenated bottom water to the oxic-anoxic chemocline at the shelf-break. At all stations, organic carbon (C(org)) oxidation rates were rapidly attenuated with depth in anoxically incubated sediment. Dissimilatory Mn reduction was the most important terminal electron-accepting process in the active surface layer to a depth of approximately 1 cm, while SO(4)(2-) reduction accounted for the entire C(org) oxidation below. Manganese reduction was supported by moderately high Mn oxide concentrations. A contribution from microbial Fe reduction could not be discerned, and the process was not stimulated by addition of ferrihydrite. Manganese reduction resulted in carbonate precipitation, which complicated the quantification of C(org) oxidation rates. The relative contribution of Mn reduction to C(org) oxidation in the anaerobic incubations was 25 to 73% at the stations with oxic bottom water. In situ, where Mn reduction must compete with oxygen respiration, the contribution of the process will vary in response to fluctuations in bottom water oxygen concentrations. Total bacterial numbers as well as the detection frequency of bacteria with fluorescent in situ hybridization scaled to the mineralization rates. Most-probable-number enumerations yielded up to 10(5) cells of acetate-oxidizing Mn-reducing bacteria (MnRB) cm(-3), while counts of Fe reducers were <10(2) cm(-3). At two stations, organisms affiliated with Arcobacter were the only types identified from 16S rRNA clone libraries from the highest positive MPN dilutions for MnRB. At the third station, a clone type affiliated with Pelobacter was also observed. Our results delineate a niche for dissimilatory Mn-reducing bacteria in sediments with Mn oxide concentrations greater than approximately 10 micromol cm(-3) and indicate that bacteria that are specialized in Mn reduction, rather than known Mn and Fe reducers, are important in this niche.

FIG. 1

Maps of the Black Sea and the study area on the Romanian shelf. The numbers indicate water depth (meters).

FIG. 2

Changes in the pore water constituents: total dissolved inorganic carbon (ΣCO₂), calcium, and pH during anoxic incubation of the four upper depth intervals at station III.

FIG. 3

Vertical profiles of organic matter mineralization rates in Black Sea sediments from station I (top) to station IV (bottom). Circles, ΣCO₂ production; squares, NH₄⁺ accumulation; diamonds, sulfate reduction. Error bars indicate standard errors from linear regressions (ΣCO₂ and NH₄⁺) and standard deviations of triplicate sulfate reduction rates. Note different scales for different processes. At each station, the three scales are plotted at the ratio 11.2:1:5.6 for ΣCO₂ production/NH₄⁺ accumulation/sulfate reduction, corresponding to the inferred stoichiometric ratio of these processes during sulfate reduction (see text for details).

FIG. 4

Depth distributions at stations I to IV of (left to right) extractable Mn, Mn²⁺ accumulation rates during anoxic incubations, poorly crystalline Fe(III), and Fe²⁺ accumulation rates. Concentrations are means of duplicate determinations, and error bars indicate standard errors of rates.

FIG. 5

16S rRNA-based tree reflecting the phylogenetic relationships of the clone sequences and a selection of sequences belonging to different subclasses of Proteobacteria. The tree is based on the results of a maximum parsimony analysis including complete or almost complete 16S rRNA sequences from representative bacteria of a phylogenetic branch. The topology of the tree was evaluated and corrected according to the results of distance matrix, maximum parsimony, and maximum likelihood analyses of various data sets. Branching patterns within each subclass were also evaluated by using a 50% conservation filter for the members of their corresponding subclass (41). Multifurcations indicate topologies that could not be unambiguously resolved. The bar indicates 10% estimated sequence divergence. EMBL accession numbers of the new sequences are as follows: A3Mn2, AJ271656; D1Mn, AJ271657; B4Mn1, AJ271653; A3Mn1, AJ271655; and D1Mn1, AJ271654.

FIG. 6

The relative contribution of dissimilatory Mn reduction to carbon oxidation as a function of the reactive Mn concentration in sediment from individual depth intervals from stations I to III in the Black Sea. Reactive Mn was calculated by subtraction of the average background concentration at a 4- to 10-cm depth from each station (see text for references).