Detecting metabolic activities in single cells, with emphasis on nanoSIMS

Investigating the contribution of microbial populations to biochemical processes of global significance is challenging as there are few approaches that can detect microbial metabolic activities on single-cell level. Given the widespread distribution and importance of microorganisms in elemental transformations, improved methods for measuring microbial activities in naturally occurring microbial communities is essential. In this article, microautoradiography (MAR), Raman microspectroscopy, and Secondary Ion Mass Spectrometry (SIMS) and their combination with isotope labeling and molecular genetic methods for cell identification (i.e. FISH and related methods) are reviewed. We focus our review on the application of MAR-FISH, Raman-FISH, and FISH-SIMS to environmental samples, with a more detailed description of the use of nanoSIMS-based methodologies to identify, quantify, and visualize the incorporation of labeled substrates of single microorganisms in complex microbial communities. We highlight examples from the marine habitat. In addition, relevant technical aspects as well as important considerations concerning sample preparation and handling are presented. We conclude with a perspective on the usefulness of such tools to study the role of microorganisms in biogeochemical cycling from micron to global scales.