Competition and interdependence: The multifaceted interaction of symbiotic Nostoc sp. and Agrobacterium sp. under inorganic carbon limitation

Nostoc cyanobacteria are capable to form symbiotic relationships with plants, transitioning to a heterotrophic lifestyle in return for providing bioavailable nitrogen to the host. The diazotrophic photoautotrophs also serve as a hub for a specialized heterotrophic bacterial community whose physiological contributions are poorly understood. By comparing the axenic strain N. punctiforme PCC 73102 and the related strains Nostoc sp. 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 between Nostoc punctiforme and the isolate Agrobacterium tumefaciens Het4 using multi-omics technologies and microscopy uncovered a complex partnership characterized, among other traits, by competition for iron and facilitation for carbon. Although competitive interactions with A. tumefaciens Het4 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 a weak carbon concentrating mechanism in N. punctiforme that enforces 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. The pivotal influence of heterotrophic bacteria on symbiotic Nostoc strains should be considered when analyzing these strains, especially in the free-living state, and also sheds new light on the benefit to Nostoc of the provision of organic carbon by plant hosts.

念珠藻属蓝细菌（Nostoc cyanobacteria）可与植物形成共生关系：它们会切换至异养生活方式，以此为宿主换取可生物利用的氮源。这类固氮光合自养生物同时也是一类特化异养细菌群落的栖息枢纽，目前学界对该群落的生理贡献尚知之甚少。本研究通过对比无菌菌株点形念珠藻（N. punctiforme）PCC 73102，与仍保留异养微生物组的相关菌株念珠藻属（Nostoc sp.）KVJ2及KVJ3，证实了在碳限制条件下，蓝细菌几乎完全依赖异养共生伙伴。借助多组学技术与显微镜成像手段，对点形念珠藻与分离株根癌农杆菌（Agrobacterium tumefaciens）Het4之间的紧密双边互作关系展开详细分析后，研究团队揭示了二者间复杂的共生模式——其特征包括铁资源竞争与碳代谢互助等多项性状。尽管与根癌农杆菌Het4的竞争性互作会削弱固氮作用并促进蓝藻素的降解，但在无机碳限制条件下，二者的互利依赖关系仍占主导。免疫荧光显微镜检测结果显示，点形念珠藻既缺乏高亲和力碳酸氢盐摄取转运蛋白SbtA，其固碳关键酶核酮糖二磷酸羧化酶-加氧酶（RubisCO）也普遍定位于羧酶体之外。这些特征均表明，点形念珠藻的碳浓缩机制（carbon concentrating mechanism）较为薄弱，这也迫使它必须依赖异养细菌。此外，免疫荧光、电子显微镜与蛋白质组学分析显示，在氮限制与碳限制条件下，点形念珠藻会显著发生胞外蛋白质循环回收过程。在分析共生念珠藻菌株（尤其是其自由生活状态下的菌株）时，应当考虑异养细菌对其产生的关键影响；该研究同时也为理解植物宿主为念珠藻提供有机碳的益处提供了新视角。