Bacteriophage-host depth distribution patterns in soil are maintained after nutrient stimulation in vitro.

Previous research has revealed the ecological importance of viruses in different ecosystems. However, bacteriophage-host distribution patterns in soil depth profiles have not been investigated. Environmental factors such as nutrient availability and physiological stress can impact the mode (either lytic or lysogenic) of viral reproduction and subsequent influence of virus infection on ecological processes. Soil depth profiles with distinct geochemical properties are ideal models to investigate the virus-host relationships as a function of environmental trophic status and cell abundance. Batch enrichment experiments using soil collected at varying depths (0-140 cm) as inoculum were performed to explore the interactions between viruses and co-occurring microbial hosts under nutrient stimulation. Both viral and bacterial abundance increased in the nutrient media compared with those in the original soils. Bacterial abundance was similar in mixed-cultures of soils regardless of sampling depth, whereas viral abundance was negatively correlated with the depth of soil samples which caused a decreasing virus-to-bacteria ratio. The lysogenetic fraction increased with soil depth in a similar manner as in the original soils assessed directly without nutrient stimulation. The bacterial diversity decreased with soil depth, and was influenced primarily by soil type, viral abundance, and virus-to-bacteria ratio. The bacterial communities were dominated by Bacilli, Beta-, Gamma-Proteobacteria, and Clostridia after nutrient stimulation. Viral and bacterial community structure also varied with soil horizons (i.e., depth). The results showed that the patterns for virus-host interactions shaped by the geochemical properties in the original environment were conserved or similar after in vitro nutrient stimulation. These findings suggest that short-term changes in trophic status alone may not significantly alter the balance of viral reproductive strategies in terrestrial ecosystems as in the antecedent environmental conditions that the host community has long adapted to, and other factors such as stress, host diversity or adaptation may be necessary to trigger community-level shifts in the interactions between viruses and hosts that responded most favorably to nutrient addition.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.