Heterotrophic growth of cyanobacteria and phage-mediated microbial loop in soil: Examination by stable isotope probing (SIP) method

Cyanobacteria play an important role in maintaining soil fertility in rice fields. Some cyanobacterial members can grow heterotrophically under dark conditions, which might be an important trait for the survival of cyanobacteria during the drained period after harvest. This study aimed to elucidate the heterotrophic growth of cyanobacteria and the microbial loop mediated by cyanophages (cyanobacteria-cyanophages-dissolved organic carbon) in soil using carbon-13 (C-13)-labeled dried callus cells as a model material of plant residues. This study used the stable isotope probing (SIP) method coupled with denaturing gradient gel electrophoresis (DGGE). Although heterotrophic growth of cyanobacteria in soil was not observed, the phage-mediated microbial loop in the transformation of callus carbon was elucidated from the detection of C-13-labeled g20 genes in the heaviest fractions (buoyant density: 1.754 g mL(-1)). The closest relatives of eleven sequenced DGGE bands from the heaviest fractions were uncultured cyanophage g20 clones that had been obtained from rice field soils in Japan. Proliferation of bacteriophages having cyanophage-related g20 genes with no detectable heterotrophic growth of cyanobacteria strongly indicates that bacteriophages having these g20 genes infected bacteria other than cyanobacteria.