Activity and distribution of aggregate-associated bacteria along the Jordan River, Israel

The upper Jordan River flows into the Sea of Galilee and is considered as an important source of nutrients for the lake ecosystem. Currently, little is known on aggregate-associated bacteria in such freshwater systems, despite their potential ecological importance as nutrient recyclers. During the WHONDRS project we followed the biomass, activity, and diversity associated with bacterial aggregates along a trophic gradient at the upper Jordan River, Israel. Our results indicate that bacteria were mostly confined to aggregates larger than 150 μm in the upstream stations (1.8-3.5x108 cells L-1), while planktonic bacteria were more abundant downstream (3.6x108-1.2x109 cells L-1). Similarly, bacterial activity associated with aggregates was higher upstream compared to downstream sampling sites (380-1747 mg C agg-1 L-1 d-1). Differently, planktonic activity was found to be similar along the river (105-120 μg C L-1d-1). Visualization of aggregates by confocal microscopy along the different sampling locations showed that bacteria formed dense clusters around different phytoplankton species, suggesting a tight relationship between autotrophic and heterotrophic microorganisms. Moreover, heterotrophic diazotrophs were identified within these aggregates using a newly developed immunolocalization approach. We propose that bacteria, especially those associated with aggregates play a key role in the carbon and nitrogen cycle along the Jordan River. Together with analyses of next generation sequencing, this study provides new insights on the interactions of microbes associated with aggregates and their contribution to nutrient cycling in freshwater environments.