Many microorganisms inhabit rice paddy fields, and they are responsible for nutrient cycling. We studied the involvement of microorganisms in environmental transfer of 14C. Microorganisms in batch cultures were treated with autoclaving (121 °C, 15 min), mixing with glutaraldehyde [final concentration of 2.5 % (vol/vol)], and mixing with cycloheximide (final concentration, 250 ^g ml-1). Autoclaving and expose to glutaraldehyde inactivate bacteria and fungi, but expo­sure to cycloheximide only inhibits fungi. The partitioning ratios of 14C into solid, liquid, and gas phases for each treatment sample are listed in Table 26.1. When microorganisms were treated by autoclaving and exposing to glutaraldehyde, almost all the 14C added remained in the liquid phase; that is, negligible transfor­mation of 14C occurred. On the other hand, the 14C atoms in the control and the cycloheximide-treated sample were partitioned into solid, liquid, and gas phases at certain ratios, and these ratios were similar between the control and the cyclohex — imide samples. We confirmed fungi made no contribution to partitioning of 14C

Fig. 26.2 Relationships between pH and partitioning ratios of 14C into the liquid phase (scatter plots). Solid line shows the solubility curve of total carbonic acid in water

Fig. 26.3 Effect of pH on the partitioning of 14C into the liquid phase

based on these results. We concluded that environmental transfer of 14C in rice paddy fields was driven by bacteria, not by fungi.

To confirm incorporation of 14C into bacteria cells, bacteria that were isolated from a flooding water of a paddy soil sample were cultivated on agar plates containing [1,2-14C] sodium acetate [3]. After cultivation, bacterial colonies were formed, and their autoradiography images showed that all colonies had the ability to take up 14C (Fig. 26.4). In our experimental procedure, bacterial cells were conse­quently partitioned into the solid phase, and thus the solid phase contains the 14C incorporated by bacteria, which could be one of the reasons for the relatively high Kd values.