Ecotypes of planktonic actinobacteria with identical 16S rRNA genes adapted to thermal niches in temperate, subtropical, and tropical freshwater habitats

Abstract

Seven strains with identical 16S rRNA genes affiliated with the Luna2 cluster (Actinobacteria) were isolated from six freshwater habitats located in temperate (Austria and Australia), subtropical (People's Republic of China), and tropical (Uganda) climatic zones. The isolates had sequence differences at zero to five positions in a 2,310-nucleotide fragment of the ribosomal operon, including part of the intergenic spacer upstream of the 16S rRNA gene, the complete 16S rRNA gene, the complete 16S-23S internal transcribed spacer (ITS1), and a short part of the 23S rRNA gene. Most of the few sequence differences found were located in the internal transcribed spacer sequences. Two isolates obtained from habitats in Asia and Europe, as well as two isolates obtained from different habitats in the People's Republic of China, had identical sequences for the entire fragment sequenced. In spite of minimal sequence differences in the part of the ribosomal operon investigated, the strains exhibited significant differences in their temperature response curves (with one exception), as well as pronounced differences in their temperature optima (25.0 to 35.6 degrees C). The observed differences in temperature adaptation were generally in accordance with the thermal conditions in the habitats where the strains were isolated. Strains obtained from temperate zone habitats had the lowest temperature optima, strains from subtropical habitats had intermediate temperature optima, and a strain from a tropical habitat had the highest temperature optimum. Based on the observed temperature responses, we concluded that the strains investigated are well adapted to the thermal conditions in their home habitats. Consequently, these closely related strains represent different ecotypes adapted to different thermal niches.

FIG. 1.

Neighbor-joining tree showing the phylogenetic relationships of Actinobacteria affiliated with the Luna2 cluster. The tree was calculated by using homologous 1,427-nucleotide sequence fragments (E. coli positions 51 to 1499) of the 16S rRNA genes. Bootstrap values (1,000 replicates) that are >60% are indicated at the nodes. Scale bar = 1% estimated sequence divergence. The numbers in brackets are accession numbers.

FIG. 2.

Temperature response curves for temperature-acclimatized genotype 1 strains. The graphs are sorted (top to bottom) by increasing optimum temperature. (A) Strain MWH-Aus1 (isolated from a habitat in a temperate climatic zone); (B) strain MWH-Wo1 (temperate zone); (C) strain MWH-Mo1 (temperate zone); (D) strain MWH-HuqW11 (subtropical zone); (E) strain MWH-Ta1 (subtropical zone); (F) strain MWH-Uga2 (tropical zone). In panels A to D the crossing points of the linear regressions of the increasing and decreasing parts of the curves are indicated by open triangles. The vertical bar at the top indicates the range of optimum temperatures determined for the sixstrains. The gray part of the bar represents the temperature range in which the crossing points (A to D) and the optimum temperatures for strains MWH-Ta1 (E) and MWH-Uga2 (F) fall. The error bars indicate the standard deviations of the average growth rates from repeated experiments (only for 30.0 and 32.5°C data).

FIG. 3.

(Top) Distribution of growth rate data measured for the six genotype 1 strains at different temperatures. Each symbol indicates the mean and standard deviation measured for one strain at one temperature. The data for tropical strain MWH-Uga2 are indicated by open squares. (Bottom) Pairwise comparison of strain-specific growth rates for significant (P < 0.05) differences by a one-way ANOVA (Tukey test). The percentage of pairs with significant differences increased with temperature. At lower temperatures the tropical zone isolate MWH-Uga2 is involved in the majority of significantly different pairs.

FIG. 4.

Water temperatures in the nonstratified Lake Taihu (▾) and surface water (0 m) of Lake Mondsee (▪) for 1999 to 2001. The maximum water temperatures reported for these lakes were observed during other years.

FIG. 5.

Yearly maximum water temperatures of habitats plotted versus the optimum temperatures of genotype 1 strains obtained from these habitats. The horizontal bars indicate the ranges of yearly maximum water temperatures observed in different years. The average of the lowest and highest yearly maximum temperatures was plotted versus the optimum temperatures determined (triangles). The dotted line indicates a linear regression of the data plotted. Data for the strain isolated from an artificial pond in Sydney Royal Botanic Gardens are indicated by an open triangle. Water temperature data for several years were not available for ponds in the area of Jinja. Therefore, only the maximum temperature measured by Asiyo (1) in a field study lasting from May to August 2003 is plotted. No water temperature records for the Huqiu pond were available.