Viral activity in two contrasting lake ecosystems

Area of Science:

• Aquatic microbiology
• Viral ecology
• Freshwater systems

Background:

• Limited data exists on long-term and depth-related viral variability in freshwater systems.
• Understanding viral impact on bacterial communities is crucial for aquatic ecosystem dynamics.

Purpose of the Study:

• To investigate virus-induced bacterial mortality over 10 months in two French lakes.
• To compare viral mortality with non-viral mortality sources (heterotrophic nanoflagellate and ciliate bacterivory).
• To assess the long-term and depth-related variability of viral infection parameters.

Main Methods:

• Long-term monitoring of bacterial production and mortality over 10 months.
• Estimation of virus-induced mortality and bacterivory using microscopy and enumeration.
• Analysis of frequency of visibly infected cells (FVIC) and burst sizes via transmission electron microscopy.

Main Results:

• Viral infection parameters (FVIC, burst size) showed less variability than bacterial production.
• FVIC peaked in late spring and early autumn; higher in oligotrophic Lake Pavin than eutrophic Lake Aydat.
• Flagellate grazing was the primary bacterial mortality source, though viral lysis prevailed in specific conditions (e.g., anoxic hypolimnion).

Conclusions:

• Viral lysis is a relatively constant mortality source for bacteria, buffering variations in growth and grazing rates.
• Viruses play a more significant role in bacterial mortality in less productive freshwater systems.
• Seasonal peaks in viral infection suggest distinct viral dynamics influencing bacterial populations.