Viral lysis and nutrient availability as drivers of bacterial carbon cycling in a hypereutrophic freshwater ecosystem.

Hypereutrophic freshwater systems, characterized by elevated bacterial biomass and activity, efficiently process organic matter through bacterial production and respiration, summarized as bacterial growth efficiency (BGE). Yet, the factors controlling BGE, particularly viral influences, remain underexplored. This study examined the interactive effects of bottom-up (nutrient-driven) and top-down (viral) controls on BGE over a one-year period in the pelagic zone of Lake Fargette (France). BGE exhibited a clear seasonal pattern, ranging from 23% to 60% (mean = 40.0 ± 9.9%), and was significantly correlated with organic carbon, total nitrogen, and C: N ratio, indicating control by substrate availability. Significant relationship between bacterial production and respiration suggests a stable and tightly coupled balance between anabolic and catabolic processes under resource-rich conditions. Increased input of labile organic matter sustained an active, high nucleic acid bacterial community and enhanced elevated viral abundance and production. Viral mediated lysis rate was positively correlated (p < 0.001) with BGE, indicating that viral activity was resource-modulated rather than host-limited. Elevated substrate supply appeared to favor rapid bacterial growth that outpaced viral infection cycles, maintaining high BGE despite strong lytic pressures. These findings underscore the complex interplay between resource availability and viral lysis in regulating bacterial carbon dynamics within hypereutrophic freshwater ecosystems.