Heterotrophic diazotrophy along a river–lake continuum: Lifestyle and contribution to dinitrogen fixation

Abstract Heterotrophic diazotrophs are potentially important to the nitrogen cycle in freshwater ecosystems, yet their abundance, N 2 fixation rates, diversity, and association with aggregates remain poorly understood. This study elucidates the contribution of freshwater heterotrophic diazotrophs as free‐living or aggregate‐associated cells to total N 2 fixation along the Jordan River‐to‐Lake Kinneret continuum. Heterotrophic diazotrophs ranged between 0.4 × 10 7 and 6.4 × 10 7 cells L −1 , accounting for 25–56% of the total unicellular diazotrophs. N 2 fixation rates by heterotrophic diazotrophs varied along the river (0.1–0.4 nmol N L −1 d −1 ), contributing between 38% and 100% of total N 2 fixation. The rates were mostly ascribed to free‐living heterotrophic diazotrophs upstream while attributed to those associated with aggregates downstream. Diazotrophs diversity indicated that non‐cyanobacterial diazotrophs dominated the free‐living fraction along different river locations, while cyanobacteria were mostly identified in lake water. Compared to aggregates‐associated N 2 ‐fixers, the diversity of free‐living diazotrophs was highly affected by environmental drivers, such as dissolved phosphorus, inorganic nitrogen, and water temperature. Our results highlight that freshwater heterotrophic diazotrophs are more ubiquitous than previously thought, can be found as free‐living cells or associated with aggregates, significantly contributing to phytoplankton productivity.