Competition and interdependence define interactions of Nostoc sp. and Agrobacterium sp. under inorganic carbon limitation.

Cyanobacteria of the Nostoc genus are capable of forming symbiotic relationships with plants but also serve as a hub for heterotrophic bacteria. By comparing the axenic strain Nostoc punctiforme PCC 73102 and the xenic strains Nostoc sp. KVJ2 and KVJ3, we were able to demonstrate an almost obligate dependence of the cyanobacteria on the heterotrophic partners under carbon-limiting conditions. A detailed analysis of the intimate relationship between N. punctiforme and the isolate Agrobacterium tumefaciens Het4 using shotgun proteomics and microscopy uncovered a complex partnership characterized by competition for iron and facilitation for carbon. The prevalent extracarboxysomal localization of the carbon-fixing enzyme RubisCO suggests that a weak carbon-concentrating mechanism in N. punctiforme enforces a dependence on heterotrophic bacteria. Our study indicates a limited autonomy of symbiotic Nostoc strains, which may also explain its preference for symbiotic interactions.