Comparison of the frequency of visibly infected bacterial cells between the soil and the floodwater in two Japanese rice fields

Abstract

The frequency of visibly infected bacterial cells (FVIC) by phages was reported so far to be larger in soil environments than in aquatic environments. However, it was evaluated with a transmission electron microscope (TEM) and might be biased depending on the criterion of judgment of phage infection by researchers. This study directly compared FVIC with a TEM in soils and in the overlying floodwaters in two Japanese rice fields (Oryza sativa L. Japonica). Soil and floodwater samples were collected three times from three sites in each field during the rice cultivation period. Proportions of bacterial morphology were long-bacteria > short-rod bacteria > coccoid bacteria in this order in both floodwater and soil samples. FVIC was higher in soil samples than in floodwater samples with a range from 4.2 to 11.4% (average 7.4%) and from non-detectable to 5.2% (average 1.2%), respectively. The frequency of phage-infected bacterial cells calculated from FVIC ranged from 32 to 52%, which indicates that the microbial loop mediated by phage infection seems to be far more active in soils than in the floodwaters in these rice fields.