Trace metals in the nitrogen cycle

Nitrogen is abundant, but its most common form — N₂ gas — is too inert to be used directly by most organisms. Nitrogen has to be "fixed" — bound to hydrogen or oxygen to form more reactive compounds — before it can be incorporated into living cells. Once fixed, nitrogen compounds are transformed from one chemical form ("oxidation state") to another, as various microorganisms use them to fulfill their energy and cell-building needs. Eventually fixed nitrogen compounds are converted back to dinitrogen gas, completing the nitrogen cycle.

The key chemical transformations of the nitrogen cycle are displayed in Fig. 1. The processes of interest to Cebic are shown in blue.

In oceans, the work of fixing nitrogen is done by nitrogenase enzymes inside a particular genus of photosynthetic cyanobacteria (blue-green algae), Trichodesmium. Trichodesmium's nitrogenase enzyme probably contains both molybdenum and iron. Iron scarcity may, in fact, be the limiting factor in nitrogen fixation in oceans. Cebic aims to determine how changes in environmental molybdenum and iron concentrations affect the rate of nitrogen fixation.

Once fixed, nitrogen-containing compounds are acquired by microalgae to be used in cell-building. This is a process of great biogeochemical significance, since the acquisition rate of fixed nitrogen compounds by autotrophs is thought to limit the rate of carbon fixation in the world's oceans — hence the rate of conversion of the greenhouse gas CO₂ to organic carbon forms.

The acquisition of nitrate by microalgae is made possible by a group of enzymes called assimilatory reductases, which come in both nitrite- and nitrate-reducing varieties. Both enzymes use iron. Cebic's goal is to determine the relationship between the availability of iron and the activity of nitrate and nitrite reductase enzymes.

Another process occurring in certain bacteria converts fixed nitrogen back to N₂ gas as a side-effect of anaerobic respiration. This process — denitrification via dissimilatory reduction — is driven by dissimilatory reductase enzymes that contain molybdenum, copper, and iron. Cebic seeks a detailed understanding of the process of dissimilatory reduction — how it works, how varied the enzymes are, how the various reductase enzymes respond to changes in environmental metal concentrations.

The nitrogen cycle in detail
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