Assessment of bacterioplankton viability by membrane integrity

The present paper reports the variability of bacterial viability, assessed by membrane integrity (BacLight probe), observed in the western Seto Inland Sea of Japan. The average concentrations of bacteria (1.54.2×106 ml−1 in January and 1.1–1.7×106 ml−1 in September) were typical of eutrophic coastal waters. The percentage of viable bacteria were 40.5–80.3% (mean of 59.6%) in September 1996, and 65.2–86.4% (mean of 79.6%) in January 1997. The present investigation shows that: 1. BacLight provides valuable information for studies in microbial ecology; 2. lower bacterial viability was not necessarily associated with higher or lower bacterial concentrations, or with a particular water layer; 3. temperature and concentration of dissolved oxygen might play a role in influencing the viability ratio of bacterioplankton, since higher viability were observed at lower temperature and higher oxygen concentration. Further work is required in order to understand the relationship between bacterial viability and environmental parameters.