Biology subjects

Arnon, S.

Publications and source records attributed to Arnon, S..

3 recordsLinked to original sources

bioRxiv2026

Lifestyle modulates riverbed diazotrophy: from abundance and activity to diversity of free-living and biofilm-associated N2 fixers

Despite the critical role that biological N2 fixation plays in controlling primary and secondary production in aquatic ecosystems, freshwater subsurface diazotrophy has received minimal attention. Here, we quantified N2 fixation rates, diazotroph abundance, and diversity in the hyporheic zone across seasons and redox regimes, distinguishing between free-living and biofilm-associated lifestyles. Samples were collected from aerobic and suboxic strata of the Jordan River streambed and incubated in microcosms for 24 h with dissolved 15N2 under ambient conditions. Immunolabeling the nitrogenase enzyme coupled with flow-cytometry revealed that diazotrophs accounted for [≤]9.6 % of total bacterial abundance but were consistently enriched in biofilms. These biofilms supported 377-fold higher N2 fixation rates per-cell than free-living diazotrophs. Confocal laser scanning microscopy captured thicker (several tens of microns) complexes of extracellular polymeric substances (EPS) encasing aerobic biofilms than suboxic, especially during winter. Biofilm-associated communities retained high abundances ([≤]21 %) of facultative and obligate anaerobes in oxic zones. Compared to those in biofilms, the abundance, N2 fixation rates, and diversity of the free-living fraction varied much more across both aerobic and suboxic zones. nifH sequencing unveiled the dominance of Pseudomonadota and Thermodesulfobacteriota across all communities. Overall N2 fixing activity, when converted to volumetric units, was 1.5 x 105-fold greater in the streambed than the overlying water. These findings identify the hyporheic zone as an active hotspot for benthic diazotrophy and intimate the centrality of microbial lifestyle in determining how diazotrophs persist and remain highly active in fluctuating redox and nutrient conditions in the freshwater ecospace.

microbiology↗

bioRxiv2024

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.

microbiology↗

bioRxiv2024

Meta-metabolome ecology reveals that geochemistry and microbial functional potential are linked to organic matter development across seven rivers

Rivers receive substantial dissolved organic matter (DOM) input from the land and transport it to the ocean. As DOM travels through watersheds, it undergoes biotic and abiotic transformations that impact biogeochemical cycles and any subsequent CO2 release into the atmosphere. While recent research has increased our mechanistic knowledge of DOM composition within watersheds, DOM development across broad spatial distances and within divergent biomes is under investigated. Here, we combined DOM characterization, geochemical analyses, and shotgun metagenomics to analyze samples from seven rivers ranging from the U.S. Pacific Northwest to Berlin, Germany. Initial analyses revealed that many DOM properties were distinguished by river type (e.g., wastewater, headwater) and that geochemistry often explained variation across rivers. At a global scale, analyses rooted in meta-metabolome ecology indicated that DOM was structured overwhelmingly by deterministic selection. When controlling for scale, however, analyses indicated that ecological assembly dynamics were again partially structured by river type. Finally, microbial analyses revealed that many riverine microbes from our systems shared core metabolic functional potential while differing in peripheral capabilities in across the rivers. Further analysis of the carbon degradation potential for recovered metagenomically assembled genomes indicated that the sampled rivers had strong taxonomically conserved niche differentiation and that carbon degradation potential diversity was significantly related to organic matter diversity. Together, these results help us uncover interconnections between the development of DOM, riverine geochemistry, and microbial functional potential.

ecology↗