Cross‐Feeding of Carbon and Nitrogen Between Aquificales and <i>Thermus</i> in Hot Springs

Acquisition and cycling of carbon and nitrogen among members of hot spring communities are not well understood. Metagenomic analyses of 105 communities inhabiting high temperature hot springs across Yellowstone and Iceland showed a co-distribution pattern of putatively autotrophic and/or diazotrophic (nitrogen-fixing) Aquificales and Thermus populations. Targeted enrichment of autotrophic and diazotrophic populations in an Icelandic hot spring produced a co-culture of Pampinifervens (Aquificales) that encoded carbon dioxide and nitrogen fixation pathways and Thermus (Thermales). Growth experiments revealed Pampinifervens could support the fixed carbon and nitrogen demands of Thermus, enabling growth. Interestingly, growth of Thermus was enhanced in co-cultures when Pampinifervens was forced to fix both carbon and nitrogen versus just carbon (ammonia-amended cultures). Further experimentation with Thermus, when grown in isolation, showed it preferred amino acids over ammonia as a nitrogen source. These findings demonstrate the importance of metabolic interactions among populations that can dictate the co-distribution of taxa in hot springs, drive community assembly, and maintain biodiversity. Further, these results highlight the fundamental role of Aquificales in the functioning of hot spring ecosystems, particularly those limited in organic carbon and fixed nitrogen like those in Iceland and to a lesser extent Yellowstone.