The microalga Phaeodactylum tricornutum (CCMP2561, Bigelow National Center for Marine Algae and Microbiota) was imaged to illustrate the different analysis conditions. The microalga was grown on C and N labeled inorganic C and N for 48 hours, then fixed with 10% formalin overnight and filtered onto 2 µm pore size polycarbonate membranes and rinsed with double distilled water. Filters were dried overnight at 50°C prior to excising a portion to mount onto a conductive carbon tab (Ted Pella, California, U.S.A.) adhered to an aluminum stub. Mounted samples were sputter coated with ~5 nm of gold.