Quantifying Inorganic and Organic Carbon Assimilation in Cable Bacteria

Assimilation rates of carbon and ammonia by individual cable bacterium filaments were quantified by stable isotope probing combined with nano-scale secondary ion mass spectrometry (nanoSIMS). Parallel sediment cores with an active cable bacteria population were injected with a combination of ¹³C-bicarbonate (¹³C-DIC) and ¹⁵N-ammonia (¹⁵NH₄⁺) to target assimilation of inorganic carbon and ammonia, and with a combination of ¹³C-propionate and ¹⁵NH₄⁺ to target assimilation of organic carbon and ammonia. After 24 h of incubation, individual filaments were hand-picked from the oxic and suboxic zone of the sediment cores and analyzed by nanoSIMS. The nanoSIMS data were processed by Look@NanoSIMS (40 (link)) to determine the ¹³C-enrichment, ¹⁵N-enrichment, and relative phosphorus content (quantified as the ³¹P/[¹²C+¹³C] ion count ratio) of the individual cable bacteria. Additionally, porewater from the sediment cores was extracted and analyzed for the DIC and NH₄⁺ concentrations and for the ¹³C-labeling of the DIC pool. Using modeling, these data were finally converted to the assimilation rates of inorganic and organic carbon.