Ecosystem consequences of a nitrogen- fixing proto- organelle

• 成果类型: Article
• 署名作者: 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; Ridge, Jennifer Pett -; Power, Mary E.
• 署名单位: Northern Arizona University; Northern Arizona University; University of Alabama System; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Lawrence Livermore National Laboratory; University of California System; University of California Merced; University of California System; University of California Berkeley; University of California System; University of California Berkeley
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-15214
• DOI: 10.1073/pnas.2503108122
• 发表日期: 2025-09-16
• 关键词: food-web; inorganic nitrogen; life-history; phosphorus; Fixation; ecology; limitation; rhopalodia; EVOLUTION; epithemia

摘要

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 Cladophora glomerata uses nutrients in spring runoff to grow streamers up to 10 m long. During summer flow recession, riverine N concentrations wane and Cladophora becomes densely epiphytized by three species of Epithemia, diatoms with N-fixing endosymbionts (proto-organelles) descended from a free-living Crocosphaera cyanobacterium. Over summertime epiphyte succession on Cladophora, N-fixation rates increased as Epithemia spp. became dominant, Cladophora C-fixation declined to near zero, and Epithemia C-fixation increased. Carbon transfer to caddisflies grazing on Cladophora with high densities of Epithemia was 10-fold higher than C transfer to caddisflies grazing Cladophora with low Epithemia loads. In response to demand for N, Epithemia 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.