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Applications  |  Ecology: NanoSIMS and protist-bacterial symbioses

¹³C-transfer between a protist host and attached bacteria revealed by NanoSIMS

Introduction

The hindguts of lower termites harbour highly diverse and endemic microbial biotas that are essential to the termite’s ability to digest wood. However, the ecological roles of many of these microbes are unknown, partly because almost none can be cultivated. Many of the protists associate with thousands of bacterial symbionts (Carpenter et al., 2011), but hypotheses for their respective roles in carbon and nutrient exchange are based on genomic data from a very small sampling of such bacteria. Direct evidence of nutrient exchange between the symbionts is lacking and difficult to obtain.

Stable Isotope Solution : HR-MAS NMR

To provide direct evidence for carbon transfer between protist host cells and attached bacteria, Carpenter et al. (2012) developed an in-situ, culture-independent approach of stable isotope labelling (¹³C-cellulose) of live insects. Imaging-analysis of isotopic ratios in fixed hindgut microbes using high resolution imaging mass spectrometry (NanoSIMS) was related to cell ultrastructure using TEM imaging to infer carbon transfer.

Results

Isotopic data (¹³C/¹²C ratios) from NanoSIMS indicate that the oxymonad protist Oxymonas dimorpha transfers carbon to its bacterial surface symbionts.

Figure 1. A. TEM micrograph of an ultramicrotome section of an Oxymonas dimorpha protist cell with attached bacterial cells. B. A NanoSIMS ion micrograph of the same cell showing consistently higher ¹³C enrichment in the O. dimorpha host than in the attached bacteria (Carpenter et al, 2012).

The authors conclude that in-situ stable isotope labelling combined with NanoSIMS techniques is very useful to study prokaryote-eukaryote interactions (e.g. host-pathogen interactions).

References

Carpenter KJ, A Horak, L Chow, PJ Keeling 2011.
Symbiosis, Morphology, and Phylogeny of Hoplonymphidae (Parabasalia) of the Wood-Feeding Roach Cryptocercus punctulatus.
Journal of Eukaryotic Microbiology 58: 426 – 436.

Carpenter KJ, PK Weber, ML Davisson, J Pett-Ridge, MI Haverty, M Ohkuma, V Tai, PJ Keeling. 2012.
Protist-bacterial symbioses and nitrogen fixation in the hindgut of a lower termite: stable isotopic tracers and correlated SEM, FIB-SEM, TEM, and NanoSIMS analysis.
The 15th European Microscopy Congress, Manchester Central, UK, September 16 – 21.