Comparison of vertical distributions of prokaryotic assemblages in the anoxic Cariaco Basin and Black Sea by use of fluorescence in situ hybridization

Abstract

Individual prokaryotic cells from two major anoxic basins, the Cariaco Basin and the Black Sea, were enumerated throughout their water columns using fluorescence in situ hybridization (FISH) with the fluorochrome Cy3 or horseradish peroxidase-modified oligonucleotide probes. For both basins, significant differences in total prokaryotic abundance and phylogenetic composition were observed among oxic, anoxic, and transitional (redoxcline) waters. Epsilon-proteobacteria, Crenarchaeota, and Euryarchaeota were more prevalent in the redoxclines, where previous studies reported high rates of chemoautotrophic production relative to those in waters above and below the redoxclines. Relative abundances of Archaea in both systems varied between 1% and 28% of total prokaryotes, depending on depth. The prokaryotic community composition varied between the two anoxic basins, consistent with distinct geochemical and physical conditions. In the Black Sea, the relative contributions of group I Crenarchaeota (median, 5.5%) to prokaryotic communities were significantly higher (P < 0.001; n = 20) than those of group II Euryarchaeota (median, 2.9%). In contrast, their proportions were nearly equivalent in the Cariaco Basin. Beta-proteobacteria were unexpectedly common throughout the Cariaco Basin's water column, accounting for an average of 47% of 4',6'-diamidino-2-phenylindole (DAPI)-stained cells. This group was below the detection limit (<1%) in the Black Sea samples. Compositional differences between basins may reflect temporal variability in microbial populations and/or systematic differences in environmental conditions and the populations for which they select.

FIG. 1.

Comparison of cell detection by oligo-FISH (Cy3-monolabeled probes) and CARD-FISH (HRP-modified probes) protocols for enumerating Bacteria (EUB338) and Archaea (ARCH915) in Cariaco Basin samples. The dashed horizontal line indicates the shallowest appearance of sulfide.

FIG. 2.

Vertical distributions of prokaryotes (DAPI-positive results [circles]), relative abundances of Bacteria (visualized by EUB338-HRP probe [solid bars]) and Archaea (ARCH915-HRP probe [open bars]), and concentrations of dissolved oxygen and hydrogen sulfide in the Cariaco Basin (A) and the Black Sea (B). Horizontal dashed lines approximate the shallowest appearances of sulfide.

FIG. 3.

Relative abundances of group I Crenarchaeota and group II Euryarchaeota in the Cariaco Basin (A) and the Black Sea (B). Dashed horizontal lines indicate the locations of the shallowest appearances of sulfide.

FIG. 4.

Vertical distributions of ɛ-proteobacterial cells (EPS549 positive) relative to total DAPI-positive cells in the Cariaco Basin (A) and the Black Sea (B). Dashed horizontal lines indicate the shallowest appearances of sulfide.

FIG. 5.

Vertical distributions of major bacterial clades detected by oligo-FISH in the Cariaco Basin (top row) and the Black Sea (bottom row). The x axes represent the percentages of probe-positive cells relative to DAPI counts. Dashed horizontal lines indicate the shallowest appearances of sulfide.