Marks Jane C, Zampini Michael C, Fitzpatrick Raina, Kariunga Saeed H, Sitati Augustine, Samo Ty J, Weber Peter K, Thomas Steven, Hungate Bruce A, Ramon Christina E, Wulf Michael, Leshyk Victor O, Schwartz Egbert, Pett-Ridge Jennifer, Power Mary E
Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011.
Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ 86011.
Proc Natl Acad Sci U S A. 2025 Sep 16;122(37):e2503108122. doi: 10.1073/pnas.2503108122. Epub 2025 Sep 8.
Microscale symbioses can be critical to ecosystem functions, but the mechanisms of these interactions in nature are often cryptic. Here, we use a combination of stable isotope imaging and tracing to reveal carbon (C) and nitrogen (N) exchanges among three symbiotic primary producers that fuel a salmon-bearing river food web. Bulk isotope analysis, nanoSIMS (secondary ion mass spectrometry) isotope imaging, and density centrifugation for quantitative stable isotope probing enabled quantification of organism-specific C- and N-fixation rates from the subcellular scale to the ecosystem. After winters with riverbed-scouring floods, the macroalga uses nutrients in spring runoff to grow streamers up to 10 m long. During summer flow recession, riverine N concentrations wane and becomes densely epiphytized by three species of , diatoms with N-fixing endosymbionts (proto-organelles) descended from a free-living cyanobacterium. Over summertime epiphyte succession on , N-fixation rates increased as spp. became dominant, C-fixation declined to near zero, and C-fixation increased. Carbon transfer to caddisflies grazing on with high densities of was 10-fold higher than C transfer to caddisflies grazing with low loads. In response to demand for N, allocates high levels of newly fixed C to its endosymbiont. Consequently, these endosymbionts have the highest rates of C and N accumulation of any taxon in this tripartite symbiosis during the biologically productive season and can produce one of the highest areal rates of N-fixation reported in any river ecosystem.
微观共生关系对生态系统功能可能至关重要，但这些相互作用在自然界中的机制往往难以捉摸。在这里，我们结合稳定同位素成像和追踪技术，揭示了为一条有鲑鱼的河流食物网提供能量的三种共生初级生产者之间的碳（C）和氮（N）交换。大量同位素分析、纳米二次离子质谱（nanoSIMS）同位素成像以及用于定量稳定同位素探测的密度离心技术，能够从亚细胞尺度到生态系统尺度对特定生物体的C和N固定率进行量化。在经历了河床冲刷洪水的冬季后，大型藻类利用春季径流中的养分生长出长达10米的藻体。在夏季流量减少期间，河流中的N浓度下降， 被三种 密集附生， 是具有固氮内共生体（原细胞器）的硅藻，其起源于一种自由生活的蓝细菌。在夏季 上的附生植物演替过程中，随着 属物种成为优势种，N固定率增加，C固定率下降至接近零，而 的C固定率增加。向高密度 的 上觅食的毛翅目幼虫的碳转移比向低 负荷的 上觅食的毛翅目幼虫的碳转移高10倍。为了响应N的需求， 将高水平的新固定C分配给其内共生体。因此，在生物生产季节，这些内共生体在这种三方共生关系中具有任何分类群中最高的C和N积累率，并且能够产生任何河流生态系统中报道的最高面积N固定率之一。
