NanoSIMS instrument clarifies nutrient cycle

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Scientists at the University of Southern California have applied a nanoscale imaging method to a biological system, helping to clear up an old puzzle of the global carbon and nitrogen cycle.

Their study appears July 5 as an advanced online publication of The ISME Journal, at www.nature.com/ismej.

A state of the art NanoSIMS instrument (short for nanometer-scale, secondary ion mass spectrometry) located at Lawrence Livermore National Laboratory can image and measure minute amounts of chemical elements.

Douglas Capone and Kenneth Nealson, USC Wrigley Professors of Environmental Biology and Geology, respectively, used NanoSIMS to track the flow of carbon and nitrogen inside two types of cells in the freshwater organism Anabaena oscillarioides.

"This instrument is allowing us to look at how these two elements are being taken up and are being transported around these different types of cells," Capone said.

"It's basically allowing us to image elements at very high resolution."

The organism "fixes," or pulls from the atmosphere, both carbon and nitrogen. For decades biologists wondered how the organism could fix both elements, since carbon fixation associated with photosynthesis produces oxygen, while nitrogen fixing needs an oxygen-free environment.

Previous studies have shown that some photosynthetic cells can differentiate into heterocysts: thick-walled relatives that fix nitrogen but do not produce oxygen.

Using NanoSIMS, Capone and Nealson followed nitrogen as it is fixed in heterocysts and then transported to the other cells, where it is needed as a nutrient.

Capone and Nealson also were able to observe cellular differentiation: as the single-stranded organism grows, the nitrogen concentration in the cell halfway between two existing heterocysts falls below a threshold.

The drop in nitrogen starts a process that turns the cell into a heterocyst.

Nealson described the study as a technology demonstration in one of the toughest systems to study in the world."

He predicted that NanoSIMS would find general applications in biology and medical research. The technology produces time-series observations of chemical composition, and doubles as an electron microscope to allow researchers to overlay chemical and physical images.

"It opens up a whole world of studies," Nealson said."You can use this technology to look at things going on inside the cell. This is going to change the way that we do a lot of microbiology."

Capone added that the structural imaging capability of NanoSIMS could allow medical researchers to study metabolic function in distinct cell types, in cells at different stages of development and in cancerous versus non-cancerous cells."

Gunther Dennert, professor of molecular microbiology at the USC Keck School of Medicine, said he could see many interesting questions, which could perhaps be attacked experimentally by this technique.

"For example, how do tissues react upon implantation of a metastasizing tumor cell, and what does the tumor cell synthesize in order to make implantation possible"

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