Heterotrophic bacterial growth efficiency and community structure at different natural organic carbon concentrations

Abstract

Batch cultures of aquatic bacteria and dissolved organic matter were used to examine the impact of carbon source concentration on bacterial growth, biomass, growth efficiency, and community composition. An aged concentrate of dissolved organic matter from a humic lake was diluted with organic compound-free artificial lake water to obtain concentrations of dissolved organic carbon (DOC) ranging from 0.04 to 2.53 mM. The bacterial biomass produced in the cultures increased linearly with the DOC concentration, indicating that bacterial biomass production was limited by the supply of carbon. The bacterial growth rate in the exponential growth phase exhibited a hyperbolic response to the DOC concentration, suggesting that the maximum growth rate was constrained by the substrate concentration at low DOC concentrations. Likewise, the bacterial growth efficiency calculated from the production of biomass and CO(2) increased asymptotically from 0.4 to 10.4% with increasing DOC concentration. The compositions of the microbial communities that emerged in the cultures were assessed by separation of PCR-amplified 16S rRNA fragments by denaturing gradient gel electrophoresis. Nonmetric multidimensional scaling of the gel profiles showed that there was a gradual change in the community composition along the DOC gradient; members of the beta subclass of the class Proteobacteria and members of the Cytophaga-Flavobacterium group were well represented at all concentrations, whereas members of the alpha subclass of the Proteobacteria were found exclusively at the lowest carbon concentration. The shift in community composition along the DOC gradient was similar to the patterns of growth efficiency and growth rate. The results suggest that the bacterial growth efficiencies, the rates of bacterial growth, and the compositions of bacterial communities are not constrained by substrate concentrations in most natural waters, with the possible exception of the most oligotrophic environments.

FIG. 1.

Changes in bacterial numbers in dilution cultures with various concentrations of organic carbon substrates. The data are means of triplicate cultures, and the error bars indicate standard deviations.

FIG. 2.

Impact of DOC concentration on bacterial carrying capacity (a), BGE (b), and maximal intrinsic growth rate (c). The solid circles indicate values that were not corrected for the control, whereas the open squares indicate values that were corrected for the influence of extraneous carbon. The corrected values were calculated by subtracting the biomass and the dissolved inorganic carbon produced in the controls from the values obtained for the various treatments (for further description see the text). All values are averages for triplicate cultures, and the error bars indicate the standard deviations, except for the corrected BGE values. The lines indicate the best-fit regression obtained by using Sigma Plot for Windows.

FIG. 3.

Comparison of DGGE profiles of PCR-amplified bacterial 16S rRNA gene fragments derived from experimental cultures with different concentrations of natural DOM. The DOC concentrations (expressed as millimolar carbon) for the various treatments are indicated at the top. Bands that were excised cloned, and sequenced are indicated by the numbers to the left of bands (see Tables 1 and 2).

FIG. 4.

(a) Relationship between community dissimilarity and the concentration of DOC in the cultures. (b) Relationship between community dissimilarities and BGE. (c) Relationship between community dissimilarity and BGR. The degrees of dissimilarity in the DGGE patterns of the dilution cultures were converted to a binary (0/1) matrix (53). The multidimensional dissimilarity matrix was reduced to one dimension by using semistrong hybrid NMDS to visualize the relative differences in community structure between the cultures. Similar patterns are plotted close together (53, 54). Regressions were fitted by using Sigma Plot for Windows.

FIG. 5.

Fractions of total DOC utilized in the six treatments and the blank (0.04 mM DOC) (solid circles) and fractions of the aged DOC utilized (estimated by subtracting the DOC utilized in the control from the total DOC utilized in the treatments) (open squares). All values are averages for triplicate cultures, and the error bars indicate standard deviations.