Award: OCE-0960806

  

The ocean’s microbial realm is the largest and arguably most complex ecosystem on Earth.  It plays a central role in controlling global element cycles and is tightly linked to climate-feedback processes.  Nitrogen (N) is an essential element for all life on Earth.  Organisms need N to form proteins and deoxyribonucleic acids (DNA) and will compete vigorously for N to survive and grow.  The total concentrations of the inorganic and organic N pools in the ocean are fairly well understood.  What is not well known are the identities of the microbes that take up the different forms of N and the rates with which they do it.  Not capturing the ‘who’ and the ‘how much’ dimensions of the N cycle can have large consequences for our understanding of global geochemical fluxes.  This grant used a stable isotope of N (¹⁵N) to trace the incorporation of N into microbial (phytoplankton, bacteria, and archaea) DNA and to determine how quickly different N compounds (for example, nitrate or urea) were taken up.  Our goal was to specifically link N uptake from different sources to individual microbial populations.

 

Isotopes of N can be distinguished between one another by their molecular weight.  For example, ¹⁵N is heavier than the more common ¹⁴N isotope that accounts for 99.6% of all global N.  Since ¹⁵N is heavier, we can determine whose DNA has incorporated the ¹⁵N from the added substrate and whose DNA still contains the naturally abundant ¹⁴N and thus did not use the added N.  This work is done through stable isotope probing (SIP) techniques.  SIP has been used as a powerful tool for the investigation of carbon metabolism in microbial communities but technical challenges limited its use in N studies. 

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