Heterotrophic diazotrophy along a river to lake continuum: lifestyle and contribution to N2 fixation

Heterotrophic diazotrophs are potentially important agents in freshwater ecosystems, yet they remain poorly understood. This study elucidates the contribution of freshwater heterotrophic diazotrophs as free-living or aggregate-associated cells to total N2 fixation along the continuum from the Jordan River to Lake Kinneret, Israel. Heterotrophic diazotrophs accounted for 25%-56% of the total diazotrophs and commonly found as free-living cells or attached to aggregates in the river. N2 fixation by heterotrophic diazotrophs associated with aggregates varied along the river, while accounting for ~50% of N2 fixation in the lake. Non-cyanobacterial diazotrophs dominated the free-living fraction in the river, and cyanobacteria were mostly identified in the lake. The diversity of free-living diazotrophs was sensitive to environmental changes, as the aggregates have likely provided a protected micro-environment. The results reveal the dynamic lifestyle of heterotrophic diazotrophs along a river-lake continuum and highlight their contribution to total N2 fixation and primary production. TeaserFreshwater heterotrophic diazotrophs are more ubiquitous than previously thought, can be found as free-living cells or associated with aggregates, and significantly contribute to primary production.