Nutritional importance of benthic bacteria for deep-sea nematodes from the Arctic ice margin: Results of an isotope tracer experiment

A stable isotope (¹³C)-labeling experiment was performed to quantify the importance of bacterial carbon as a food source for an Arctic deep-sea nematode community. Bacterial functional groups were isotopically enriched with ¹³C-glucose, ¹³C-acetate, ¹³C-bicarbonate, and ¹³C-amino acids injected into sediments collected from 1280 m depth at 79°N, 6°E, west of Svalbard. Incorporation of the ¹³C label into bacterial phospholipid-derived fatty acids (PLFAs) and nematodes in the top 5 cm of the sediment was monitored over a 7-d period. The ¹³C dynamics of nematodes was fitted with a simple isotope turnover model to derive the importance of the different bacterial functional groups as carbon sources for the nematodes. The different substrates clearly labeled different bacterial groups as evidenced by differential labeling of the PLFA patterns. The deep-sea nematode community incorporated a very limited amount of the label, and the isotope turnover model showed that the dynamics of the isotope transfer could not be attributed to bacterivory. The low enrichment of nematodes suggests a limited passive uptake of injected ¹³C-labeled substrates. The lack of accumulation suggests that the injected ¹³C-labeled dissolved organic carbon compounds are not important as carbon sources for deep-sea nematodes. Since earlier studies with isotopically enriched algae also found limited uptake by nematodes, the food sources of deep-sea nematodes remain unclear.