1. ¹Department of Biology, Portland State University, Portland, OR, USA
2. ²Glenn T. Seaborg Institute, Chemistry, Materials and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA, USA
3. ³Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA

Correspondence: Professor DG Capone, Department of Biological Sciences, University of Southern California, 3616, Trousdale Parkway, USC, Los Angeles, CA 90089-0740, USA. E-mail: capone@usc.edu

Received 5 April 2007; Revised 14 May 2007; Accepted 14 May 2007; Published online 5 July 2007.

Abstract

Filamentous nitrogen fixing cyanobacteria are key players in global nutrient cycling, but the relationship between CO₂- and N₂-fixation and intercellular exchange of these elements remains poorly understood in many genera. Using high-resolution nanometer-scale secondary ion mass spectrometry (NanoSIMS) in conjunction with enriched H¹³CO₃^- and ¹⁵N₂ incubations of Anabaena oscillarioides, we imaged the cellular distributions of C, N and P and ¹³C and ¹⁵N enrichments at multiple time points during a diurnal cycle as proxies for C and N assimilation. The temporal and spatial distributions of the newly fixed C and N were highly heterogeneous at both the intra- and inter-cellular scale, and indicative of regions performing active assimilation and biosynthesis. Subcellular components such as the neck region of heterocycts, cell division septae and putative cyanophycin granules were clearly identifiable by their elemental composition. Newly fixed nitrogen was rapidly exported from heterocysts and was evenly allocated among vegetative cells, with the exception of the most remote vegetative cells between heterocysts, which were N limited based on lower ¹⁵N enrichment. Preexisting functional heterocysts had the lowest levels of ¹³C and ¹⁵N enrichment, while heterocysts that were inferred to have differentiated during the experiment had higher levels of enrichment. This innovative approach, combining stable isotope labeling and NanoSIMS elemental and isotopic imaging, allows characterization of cellular development (division, heterocyst differentiation), changes in individual cell composition and cellular roles in metabolite exchange.