Microorganisms are the greatest chemists of our planet. Almost every chemical reaction that is thermodynamically feasible is exploited by these organisms to sustain growth and survival. Together, the actions of microbes comprise the biogeochemical element cycles, a vastly complicated metabolic network that is the basis of all life. Since a few years the metagenomic exploration of this network has begun. Ten years from now next-generation massive parallel sequencing technology will enable the cost-effective near-complete molecular characterization of microbial communities. This development has great applied and fundamental potential. So far metagenomes were recorded for natural communities with poorly characterized natural history. Consequently, such metagenomes are very difficult to interpret. To move forward, it is essential to record metagenomes of microbial communities at precisely defined selective pressure. That is the key innovation of my ERC application. It is a simple step forwards, and a very challenging one. It will require a combination of skills in engineering, biochemistry, microbial ecology, physiology and bioinformatics. This combination is exactly what has propelled my previous research. My project focuses on communities involved in nitrate respiration. The turnover of nitrate plays a key role in all geochemical element cycling while its natural abundance is severely affected by human activity. The environmental fate of nitrate is currently unpredictable. My project will deliver fundamental molecular understanding over the workings of natural selection for nitrate respiring communities. It will express this understanding into useful, general metagenomic markers. This is a pioneering effort; it will serve as an example to the field and show how to capitalize on the most far-reaching innovation of our time.