The Case for Heterotrophic Nitrogen Fixation in Coastal Sediments

Nitrogen fixation is an energetically intense process and one that traditionally, has been thought to be unimportant in marine sediments. Studies over the last two decades however, demonstrate that heterotrophic nitrogen fixation frequently occurs in coastal sediments from brackish to euhaline habitats. Further, nitrogen fixation rates can be on par with and even exceed rates of sediment denitrification. Despite mounting evidence these data are often met with skepticism why would microbes, seemingly bathed in ammonium, conduct this energetically expensive process? In contrast to this doubt, I argue that sediments are actually ideal environments for nitrogen fixation and I will present six possible reasons for its occurrence in coastal sediments. Further I will describe the potential role nitrogen fixation plays in coastal ecosystem function.