ABSTRACT
As reported in many aquatic environments, recent studies in terrestrial ecosystems implicate a role for viruses in shaping the structure, function, and evolution of prokaryotic soil communities. However, given the heterogeneity of soil and the physical constraints (i.e., pore-scale hydrology and solid-phase adsorption of phage and host cells) on the mobility of viruses and bacteria, phage-host interactions likely differ from those in aquatic systems. In this study, temporal changes in the population dynamics of viruses and bacteria in soils under different land management practices were examined. The results showed that bacterial abundance was significantly and positively correlated to both virus and inducible prophage abundance. Bacterial and viral abundance were also correlated with soil organic carbon and nitrogen content as well as with C:N ratio. The seasonal variability in viral abundance increased with soil organic carbon content. The prokaryotic community structure was influenced more by land use than by seasonal variation though considerable variation was evident in the early plant successional and grassland sites. The free extracellular viral communities were also separated by land use, and the forest soil viral assemblage exhibiting the most seasonal variability was more distinct from the other sites. Viral assemblages from the agricultural soils exhibited the least seasonal variability. Similar patterns were observed for inducible prophage viral assemblages. Seasonal variability of viral assemblages was greater in mitomycin-C (mitC) induced prophages than in extracellular viruses irrespective of land use and management. Taken together, the data suggest that soil viral production and decay are likely balanced but there was clear evidence that the structure of viral assemblages is influenced by land use and by season.
ABSTRACT
Cell-density dependent quorum sensing (QS) is fundamental for many coordinated behaviors among bacteria. Most recently several studies have revealed a role for bacterial QS communication in bacteriophage (phage) reproductive decisions. However, QS based phage-host interactions remain largely unknown, with the mechanistic details revealed for only a few phage-host pairs and a dearth of information available at the microbial community level. Here we report on the specific action of eight different individual QS signals (acyl-homoserine lactones; AHLs varying in acyl-chain length from four to 14 carbon atoms) on prophage induction in soil microbial communities. We show QS autoinducers, triggered prophage induction in soil bacteria and the response was significant enough to alter bacterial community composition in vitro. AHL treatment significantly decreased the bacterial diversity (Shannon Index) but did not significantly impact species richness. Exposure to short chain-length AHLs resulted in a decrease in the abundance of different taxa than exposure to higher molecular weight AHLs. Each AHL targeted a different subset of bacterial taxa. Our observations indicate that individual AHLs may trigger prophage induction in different bacterial taxa leading to changes in microbial community structure. The findings also have implications for the role of phage-host interactions in ecologically significant processes such as biogeochemical cycles, and phage mediated transfer of host genes, e.g., photosynthesis and heavy metal/antibiotic resistance.
ABSTRACT
Release of antibiotics into the environment has caused ecological and human health concerns in recent years. However, little is known about their transport behaviors in chemically heterogeneous porous media. In this study, we investigated the coupled effects of surface chemistry and soil colloids on the transport of ciprofloxacin and tetracycline through sand under steady state saturated flow conditions. Both antibiotics had a much higher capacity of adsorption on soil colloids (17,500 mg/kg for ciprofloxacin and 8600 mg/kg for tetracycline) than on sand (5.11 mg/kg for ciprofloxacin and 2.80 mg/kg for tetracycline). However, ciprofloxacin adsorption increased to 8.91 mg/kg after the sand was coated with iron oxide and to 8.73 mg/kg after the sand was coated with humic acid. Tetracycline, adsorption increased to 7.99 mg/kg after sand was coated with iron oxide coated sand and to 8.35 mg/kg after the sand was coated with humic acid coated The high adsorption capacity of ciprofloxacin led to a recovery rate of <4% in the effluents of the columns containing 0%, 20% and 50% of iron oxide/humic acid coated sand. The surface coating decreased the recovery rates of tetracycline from 35.4% (in uncoated sand) to 12.0% (in column containing 50% iron oxide coated sand) and 0.010% (in column containing 50% humic acid coated sand), respectively. Once adsorbed to soil colloids, the recovery rate of ciprofloxacin increased by 26.7% in uncoated sand column, 21.1% in iron oxide coated sand column, and 32.7% in humic acid coated sand column. Similarly, the presence of the colloids increased the recovery rate of tetracycline from 13.8% to 33.2% after the sand was coated with humic acid. Colloids did not significantly influence the transport and recovery of tetracycline in the uncoated sand and iron oxide coated sand due likely to its lower adsorption affinity.
