Structure of sediment-associated microbial communities along a heavy-metal contamination gradient in the marine environment

Abstract

Microbial community composition and structure were characterized in marine sediments contaminated for >80 years with cadmium, copper, lead, and zinc. Four sampling sites that encompass a wide range of sediment metal loads were compared in a Norwegian fjord (Sorfjord). HCl-extractable metals and organic matter constantly decreased from the most contaminated site (S1) to the control site (S4). All sampling sites presented low polychlorinated biphenyl (PCB) concentrations (Sigma(7)PCB < 7.0 ng g [dry weight](-1)). The biomass ranged from 4.3 x 10(8) to 13.4 x 10(8) cells g (dry weight) of sediments(-1) and was not correlated to metal levels. Denaturing gradient gel electrophoresis indicated that diversity was not affected by the contamination. The majority of the partial 16S rRNA sequences obtained were classified in the gamma- and delta-Proteobacteria and in the Cytophaga-Flexibacter-Bacteroides (CFB) bacteria. Some sequences were closely related to other sequences from polluted marine sediments. The abundances of seven phylogenetic groups were determined by using fluorescent in situ hybridization (FISH). FISH was impaired in S1 by high levels of autofluorescing particles. For S2 to S4, the results indicated that the HCl-extractable Cu, Pb, and Zn were negatively correlated with the abundance of gamma-Proteobacteria and CFB bacteria. delta-Proteobacteria were not correlated with HCl-extractable metals. Bacteria of the Desulfosarcina-Desulfococcus group were detected in every site and represented 6 to 14% of the DAPI (4',6'-diamidino-2-phenylindole) counts. Although factors other than metals may explain the distribution observed, the information presented here may be useful in predicting long-term effects of heavy-metal contamination in the marine environment.

FIG. 1.

Map of the Sørfjord in southern Norway showing the locations of the four sampling stations (S1, S2, S3, and S4). S1 is the most contaminated site and is located near the city of Odda. N, north.

FIG. 2.

DGGE banding patterns of the sediment-associated microbial communities of the Sørfjord. Three replicates are shown for each sampling site.

FIG. 3.

Maximum-likelihood tree showing the relationships among the 16S rRNA clones of the Sørfjord (in boldface) that belong to the γ-Proteobacteria. Thiobacillus denitrificans (β-Proteobacteria) served as the outgroup. A matrix of 631 nucleotides was used. GenBank accession numbers are listed for the close relatives of the clones. Bootstrap values of >50% (obtained with 100 resamplings) are shown, with upper and lower values representing those from distance and parsimony, respectively. The bar represents the expected numbers of substitutions per 10 nucleotides.

FIG. 4.

Maximum-likelihood tree showing the relationships among the 16S rRNA clones of the Sørfjord (in boldface) that belong to the δ-Proteobacteria. Thiobacillus denitrificans (β-Proteobacteria) served as the outgroup. A matrix of 615 nucleotides was used. See the legend to Fig. 3 for other information.

FIG. 5.

Maximum-likelihood tree showing the relationships between the 16S rRNA clones of the Sørfjord (in boldface) that belong to the CFB group. E. coli (γ-Proteobacteria) served as the outgroup. A matrix of 643 nucleotides was used. See the legend to Fig. 3 for other information.

FIG. 6.

Maximum-likelihood tree showing the relationships between the 16S rRNA clones of the Sørfjord (in boldface) that belong to the Nitrospira group, the α-Proteobacteria, the β-Proteobacteria, the Planctomycetales, and the high-G+C bacteria. A matrix of 635 nucleotides was used. See the legend to Fig. 3 for other explanations.

FIG. 7.

Community structure of the Sørfjord sediment-associated microbial communities as determined by FISH with rRNA-targeted oligonucleotide probes. Data are given as percentages of DAPI-stained cells (mean plus SD; n = 3). Probes used: EUB338 for eubacteria; ARC915 for archaebacteria; PLA886 for planctomycetes. Histogram bars sharing the same superscript did not differ significantly (Tukey's HSD test; α = 0.05).

FIG. 8.

Community structure of the Sørfjord sediment-associated microbial communities as determined by FISH with rRNA-targeted oligonucleotide probes. Data are given as percentages of DAPI-stained cells (mean plus SD; n = 3). Probes used: GAM42a for γ-Proteobacteria; CF319a for CFB bacteria; DSS658 for δ-Proteobacteria (Desulfosarcina-Desulfococcus group); HGC69a for high-G+C bacteria. Histogram bars sharing the same superscript did not differ significantly (Tukey's HSD test; α = 0.05).