Abstract
SummaryCyanobacteria of theNostocgenus are capable of forming symbiotic relationships with plants, thus transitioning to a heterotrophic lifestyle in return for providing bioavailable nitrogen to the host. The diazotrophic photoautotrophs also serve as a hub for specialized heterotrophic bacterial communities whose physiological contributions are poorly understood. By comparing the axenic strainNostoc punctiformePCC 73102 and the related strainsNostocsp. KVJ2 and KVJ3, which still maintain their heterotrophic microbiome, we were able to demonstrate an almost obligate dependence of the cyanobacteria on the heterotrophic partners under carbon-limiting conditions. Detailed analysis of the intimate bilateral relationship betweenN. punctiformeand the isolateAgrobacterium tumefaciensHet4 using shotgun proteomics and microscopy uncovered a complex partnership characterized, among other traits, by competition for iron and facilitation for carbon. Although competitive interactions withA. tumefaciensHet4 compromise nitrogen fixation and stimulate the degradation of cyanophycin, mutualistic dependency prevails under inorganic carbon limitation. Both the absence of the high affinity bicarbonate uptake transporter SbtA and the prevalent extracarboxysomal localization of the carbon-fixing enzyme RubisCO, as detected by immunofluorescence microscopy, suggest that a weak carbon concentrating mechanism inN. punctiformeenforces a dependence on heterotrophic bacteria. Further, immunofluorescence, electron microscopic and proteomic analyses reveal a pronounced extracellular recycling of proteins under N- and C-limiting conditions. Our study shows that the pivotal influence of heterotrophic bacteria on symbioticNostocstrains should be considered when analyzing these cyanobacteria, especially in the free-living state. This work also sheds new light on howNostocbenefits from the organic carbon provided by plant hosts.
Publisher
Cold Spring Harbor Laboratory