Abundance of bacteriophages of common heterotrophic bacteria in the floodwater of a Japanese paddy field

Abstract

Bacteriophages of 18 heterotrophic bacterial strains (four Cytophaga–Flavobacterium–Bacteroides bacteria, six α-Proteobacteria, two β-Proteobacteria, two γ-Proteobacteria, one Firmicute and three Actinobacteria) that were isolated from floodwater were enumerated in the floodwater of four plots (no fertilization, N, P, and K fertilizers, N, P, K and Ca fertilizers, and N, P, K and Ca fertilizers with compost) in a Japanese paddy field under a long-term fertilizer trial throughout the rice cultivation period. All floodwater samples contained phages of some of the tested bacteria. In general, phage abundance was large at the first sampling, 6 days after transplanting rice, and decreased with time until the midseason drainage, except for two bacterial strains. Abundance tended to be more numerous after the midseason drainage than before drainage. There was no clear plot preference for phages of respective bacteria, and no relationship was recognized between the rice growth stage when the studied bacteria were isolated and the stage of abundant phages of respective bacteria. Phages of a Sphingomonas sp., an Enterobacter sp., a Cytophaga sp. and a Microbacterium sp. occurred commonly and abundantly during the rice cultivation period among the bacteria examined. And phage communities in the floodwater appeared to consist of phages of physiologically broad hosts without any appreciable deflection of specific phage groups at the specific rice cultivation stage. There was no difference in phage infectivity between bacterial strains with and without prophage in their chromosomes. Phages of Sphingomonas spp. in the floodwater were strain dependent, especially before the midseason drainage. No relationship was observed between viral abundance in the floodwater and the incidence of phages infectious to bacteria. In contrast, significant correlations were observed between phage abundances of 18 bacterial strains in 34 out of 153 combinations.