Single-Cell Measurements of Fixation and Intercellular Exchange of C and N in the Filaments of the Heterocyst-Forming Cyanobacterium sp. Strain PCC 7120.

Under diazotrophic conditions, the model filamentous, heterocyst-forming cyanobacterium sp. strain PCC 7120 develops a metabolic strategy based on the physical separation of the processes of oxygenic photosynthesis, in vegetative cells, and N fixation, in heterocysts. This strategy requires the exchange of carbon and nitrogen metabolites and their distribution along the filaments, which takes place through molecular diffusion via septal junctions involving FraCD proteins. Here, was incubated in a time course (up to 20 h) with [C]bicarbonate and N and analyzed by secondary ion mass spectrometry imaging (SIMS) (large-geometry SIMS [LG-SIMS] and NanoSIMS) to quantify C and N assimilation and distribution in the filaments. The C/C and N/N ratios measured in wild-type filaments showed a general increase with time. The enrichment was relatively homogeneous in vegetative cells along individual filaments, while it was reduced in heterocysts. Heterocysts, however, accumulated recently fixed N at their poles, in which the cyanophycin plug [multi-l-arginyl-poly(l-aspartic acid)] is located. In contrast to the rather homogeneous label found along stretches of vegetative cells, C/C and N/N ratios were significantly different between filaments both at the same and different time points, showing high variability in metabolic states. A mutant did not fix N, and the C/C ratio was homogeneous along the filament, including the heterocyst in contrast to the wild type. Our results show the consumption of reduced C in the heterocysts associated with the fixation and export of fixed N and present an unpredicted heterogeneity of cellular metabolic activity in different filaments of an culture under controlled conditions. Filamentous, heterocyst-forming cyanobacteria represent a paradigm of multicellularity in the prokaryotic world. Physiological studies at the cellular level in model organisms are crucial to understand metabolic activities and qualify specific aspects related to multicellularity. Here, we used stable isotopes (C and N) coupled to LG-SIMS and NanoSIMS imaging to follow single-cell C and N fixation and metabolic dynamics along the filaments in the model heterocyst-forming cyanobacterium sp. strain PCC 7120. Our results show a close relationship between C and N fixation and distribution in the filaments and indicate that wild-type filaments in a culture can exhibit a substantial variability of metabolic states. This illustrates how some novel properties can be appreciated by studying microbial cultures at the single-cell level.