Benthic marine N2 fixation, focusing on the physiological diversity and environmental sensitivities of nitrogen fixers inhabiting the vast benthic realm, relatively understudied compared to pelagic N2 fixation. In a project generously funded by the Simons Foundation, we use culture and field studies to answer questions such as, "Why and when do benthic diazotrophs fix N2?"; "Are the environmental responses of benthic nitrogen fixers fundamentally different from pelagic nitrogen fixers?" ; "Does benthic N2 fixation contribute substantively to marine fixed N inventories? Will climate change alter this?"