Seasonal dynamics of bacterioplankton community structure in a eutrophic lake as determined by 5S rRNA analysis

Abstract

Community structure of bacterioplankton was studied during the major growth season for phytoplankton (April to October) in the epilimnion of a temperate eutrophic lake (Lake Plusssee, northern Germany) by using comparative 5S rRNA analysis. Estimates of the relative abundances of single taxonomic groups were made on the basis of the amounts of single 5S rRNA bands obtained after high-resolution electrophoresis of RNA directly from the bacterioplankton. Full-sequence analysis of single environmental 5S rRNAs enabled the identification of single taxonomic groups of bacteria. Comparison of partial 5S rRNA sequences allowed the detection of changes of single taxa over time. Overall, the whole bacterioplankton community showed two to eight abundant (>4% of the total 5S rRNA) taxa. A distinctive seasonal succession was observed in the taxonomic structure of this pelagic community. A rather-stable community structure, with seven to eight different taxonomic units, was observed beginning in April during the spring phytoplankton bloom. A strong reduction in this diversity occurred at the beginning of the clear-water phase (early May), when only two to four abundant taxa were observed, with one taxon dominating (up to 72% of the total 5S rRNA). The community structure during summer stagnation (June and July) was characterized by frequent changes of different dominating taxa. During late summer, a dinoflagellate bloom (Ceratium hirudinella) occurred, with Comamonas acidovorans (beta-subclass of the class Proteobacteria) becoming the dominant bacterial species (average abundance of 43% of the total 5S rRNA). Finally, the seasonal dynamics of the community structure of bacterioplankton were compared with the abundances of other major groups of the aquatic food web, such as phyto- and zooplankton, revealing that strong grazing pressure by zooplankton can reduce microbial diversity substantially in pelagic environments.

FIG. 1

High-resolution gel electrophoresis of the LMW RNA fraction obtained directly from bacterioplankton at a depth of 1 m from Lake Plußsee in 1989. Bands were detected by autoradiography after 3′-end labeling of the RNA with ³²P. The black arrow indicates a major decrease in the number of 5S rRNA bands at the end of the spring phytoplankton bloom.

FIG. 2

Comparison of selected gel scans from the autoradiogram of environmental 5S rRNA shown in Fig. 1 from samples drawn in April (A) and September (B). The numbers of nucleotides were determined from the molecular weight markers in Fig. 1. OD, optical density.

FIG. 3

Relative amounts of single 5S rRNA bands compared with the total amount of 5S rRNA as quantified by gel scanning and PhosphorImaging from the gel shown in Fig. 1. Only bands with a relative abundance higher than 4% are shown. Different colors indicate the different size classes of the individual 5S rRNA bands.

FIG. 4

Comparison of the total number of abundant 5S rRNA bands (>4% of the total 5S rRNA) of the bacterioplankton (open squares), its Shannon index H (filled triangles), and chlorophyll a (filled diamonds) concentration of samples from Lake Plußsee collected at a depth of 1 m in 1989. Arrows indicate the three minima of diversity on 17 May, 19 June, and 21 August.

FIG. 5

Relative abundance of major single 5S rRNA bands of the bacterioplankton during different seasons compared to total chlorophyll a concentration as an indicator of algal biomass. (A) Chlorophyll a concentration (filled diamonds) and relative abundance of the 116-nt 5S rRNA band (open squares). (B) Relative abundance of the 118-nt 5S rRNA band (open squares) and relative abundance of both the 116- and 118-nt 5S rRNA bands (filled triangles).

FIG. 6

Partial sequences of 5S rRNA bands of 116 nt obtained after RNase T₁ digestion. Sources of the 5S rRNA are as follows: isolate C. acidovorans PX54 (lane 1) and bacterioplankton from Lake Plußsee from 10 April (lane 2), 2 May (lane 3), 22 May (lane 4), 24 June (lane 5), 2 October (lane 6), and 24 October (lane 7). Numbers along the left side of the gel indicate the positions of the G in relation to the 5′ end of the 5S rRNA sequence.

FIG. 7

Phylogenetic position of the C. acidovorans 5S rRNA sequence, identical to the sequence obtained from the 116-nt bands of bacterioplankton from Lake Plußsee, within the β subclass of the class Proteobacteria. The phylogenetic tree is based on comparison of full-length 5S rRNA sequences of the reference strains indicated, by using the Jukes and Cantor algorithm and Thiobacillus thioparus as the outgroup. Species names in capital letters represent sequences from the 5S rRNA sequence database (11a). Species names in italics represent 5S rRNA sequences that were generated by us and were submitted to the EMBL database under the accession numbers provided in Materials and Methods.

FIG. 8

Seasonal dynamics of total bacterial numbers (filled circles), numbers of HNFs (open triangles), and chlorophyll a concentrations (filled diamonds) from samples of Lake Plußsee collected at a depth of 1 m in 1989.