Single-cell analysis of bacterial growth, cell size, and community structure in the Delaware estuary

The abundance and size of thymidine and leucine-assimilating bacteria were examined using a combination of microautoradiography and fluorescence in situ hybridization (micro-FISH) to explore the relationship between community structure and bacterial growth in the Delaware estuary. Community structure varied along the salinity gradient; -proteobacteria and Cytophaga-like bacteria dominated in freshwater parts of the estuary and -proteobacteria were the most abundant bacteria at salinities greater than about 10 PSU. The percentage of thymidine and leucine-assimilating bacteria in these groups varied substantially as well, ranging from undetectable to 50% active. The correlation between abundance and the fraction of bacteria assimilating 3 H-thymidine was significant for -proteobacteria, suggesting that the decline in the abundance of these bacteria in high salinity waters was because of slower growth. The correlation between abundance and growth, however, was not significant for the other types of bacteria we examined. Active bacteria were distributed across all size classes, and the smallest bacteria dominated cell number and cell volume. This study indicates that bacterial growth has a smaller impact on community structure than expected, and points towards the importance of other factors, such as mortality, in affecting community structure.