2025-09-15アメリカ合衆国・ローレンスリバモア国立研究所 (LLNL)
<関連情報>
- https://www.llnl.gov/article/53466/river-ecosystem-converts-air-fertilizer-could-hold-clues-sustainable-nitrogen-production
- https://www.pnas.org/doi/10.1073/pnas.2503108122
窒素固定原始細胞小器官の生態系への影響 Ecosystem consequences of a nitrogen-fixing proto-organelle
Jane C. Marks, Michael C. Zampini, Raina Fitzpatrick, +11 , and Mary E. Power
Proceedings of the National Academy of Sciences Published:September 8, 2025
DOI:https://doi.org/10.1073/pnas.2503108122
Significance
In major evolutionary leaps with widespread ecological consequences, mitochondria and chloroplast organelles arose from bacteria that had become endosymbionts inside eukaryotic host cells. Recent studies demonstrate endosymbiosis of N-fixing bacteria into algal hosts, but the ecological significance of such partnerships remains unexplored. In a temperate river ecosystem, we show that much of the carbon fixed by the diatom Epithemia is allocated to its nitrogen-fixing endosymbiont, fueling most summertime N-fixation in the river and accelerating energy flow up the food web. Despite selective and voracious grazing on Epithemia by riverine algivores, these diatoms can dominate summertime algal biomass in unpolluted western rivers, likely supported by abundant nitrogen from their endosymbiotic “proto-organelles.”
Abstract
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.
