作者：Geisler, E., Siebner, H., Kolton, M., Sisma-Ventura, G., Rahav, E., Arnon, S. & Bar-Zeev, E.

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 x 10(7) and 6.4 x 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.

异养固氮菌对淡水生态系统中的氮循环具有潜在重要性，但对其丰度、N₂固定速率、多样性以及与聚集体的关联仍知之甚少。本研究阐明了沿约旦河至基内雷特湖连续体中，淡水异养固氮菌作为自由生活细胞或聚集体结合细胞对总N₂固定的贡献。异养固氮菌丰度在0.4 × 10⁷ 至 6.4 × 10⁷ 个细胞/升之间，占单细胞固氮菌总数的25%–56%。异养固氮菌的N₂固定速率沿河流变化（0.1–0.4 nmol N L⁻¹ d⁻¹），贡献了总N₂固定的38%–100%。在上游，这些速率主要来自自由生活的异养固氮菌，而在下游则主要归因于与聚集体结合的固氮菌。固氮菌多样性表明，在河流不同位置，自由生活组分以非蓝藻固氮菌为主，而蓝藻主要在湖水中检出。与聚集体结合的固氮菌相比，自由生活固氮菌的多样性更易受到环境驱动因子的影响，如溶解态磷、无机氮和水温。本文结果突显了淡水异养固氮菌比以往认为的更为普遍，它们可以以自由生活细胞或聚集体结合的形式存在，并对浮游植物生产力具有显著贡献。

（来源：Limnology and Oceanography 2026 Issue 1  DOI: 10.1002/lno.70308）