Influence of humic substances on bacterial and viral dynamics in freshwaters

Abstract

Bacterial and viral abundances were measured in 24 lakes with dissolved organic carbon (DOC) concentrations ranging from 3 to 19 mg of C liter(-1). In addition, a laboratory experiment was performed to test the effects of different sources of carbon (i.e., glucose and fulvic acids) and nutrients on the dynamics of viruses and bacteria. In the lake survey, no correlation was found between virus abundance and DOC concentration, yet there was a significant positive correlation between bacterial abundance and DOC concentration. A negative correlation was found between the virus-to-bacteria ratio and DOC level. These results are in agreement with our findings in the laboratory, where virus counts were significantly lower in treatments with fulvic acid additions than in a control (mean, 67.4% +/- 6.5% of the control). Virus counts did not differ significantly among the control and treatments with glucose, indicating that it was the type of organic carbon and not quantity which had an impact on viruses. Results from this study suggest that the way viruses control bacterial assemblages in humic lakes is different from the mechanism in clear water systems.

FIG. 1.

Changes in bacterial (a) and viral (b) abundances in the laboratory experiment over time. Treatments are no amendments (control), fulvic acid addition (H1 and H2; final concentrations of 5 and 11 mg of C liter⁻¹, respectively), glucose addition (G; final concentration of 11 mg of C liter⁻¹), nitrogen and phosphorus addition (NP; final concentrations of 10 and 1 μM, respectively), fulvic acid at two concentrations and nutrient addition (H1NP and H2NP), and glucose and nutrient additions (GNP). Results are given as the means ± standard deviations (number of samples, 3).

FIG. 2.

Changes in bacterial production over time. Treatments are the same as described in the legend of Fig. 1. Results are given as the means ± standard deviations (number of samples, 3). G, glucose.

FIG. 3.

Changes in VBRs over time. Treatments are the same as described in the legend of Fig. 1. Results are given as the means ± standard deviations (number of samples, 3). G, glucose.