This diatom species, Fragilariopsis kerguelensis, photographed both alive (left) and fossilized (right), is a floating algae that is abundant in the Antarctic Ocean and was the major species in the samples collected for this study. Nitrogen isotopes in their shells vary with the amount of unused nitrogen in the surface water. 

Image of live diatom by Philipp Assmy (Norwegian Polar Institute) and Marina Montresor (Stazione Zoologica Anton Dohrn); fossilized diatoms (c) Michael Kloster, Alfred-Wegener-Institute