radiocarbon signature of microorganisms in the mesopelagic ocean

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 106; no. 16; pp. 6513 - 6518
• Main Authors: Hansman, Roberta L, Griffin, Sheila, Watson, Jordan T, Druffel, Ellen R.M, Ingalls, Anitra E, Pearson, Ann, Aluwihare, Lihini I
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 21.04.2009; National Acad Sciences
• Subjects: Archaea; Archaea - metabolism; Bacteria; Bacteria - metabolism; Biological Sciences; Biomarkers; Carbon; Carbon Radioisotopes; Complex Systems: From Chemistry to Systems Biology Special Feature; Crenarchaeota; Dissolved organic carbon; DNA, Bacterial - genetics; Hawaii; Marine; Microorganisms; Molecular Sequence Data; Nitrates - analysis; Nitrites - analysis; Oceans; Oceans and Seas; Oxygen - analysis; Physical Sciences; Polymerase Chain Reaction; Radiocarbon; Radiometric Dating; Sample Size; Sea water; Signatures; Studies; Water Microbiology; Hawaii; Pacific Northwest; IN, North Pacific, North Pacific Subtropical Gyre
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0810871106

Several lines of evidence indicate that microorganisms in the meso- and bathypelagic ocean are metabolically active and respiring carbon. In addition, growing evidence suggests that archaea are fixing inorganic carbon in this environment. However, direct quantification of the contribution from deep ocean carbon sources to community production in the dark ocean remains a challenge. In this study, carbon flow through the microbial community at 2 depths in the mesopelagic zone of the North Pacific Subtropical Gyre was examined by exploiting the unique radiocarbon signatures (Δ¹⁴C) of the 3 major carbon sources in this environment. The radiocarbon content of nucleic acids, a biomarker for viable cells, isolated from size-fractionated particles (0.2-0.5 μm and >0.5 μm) showed the direct incorporation of carbon delivered by rapidly sinking particles. Most significantly, at the 2 mesopelagic depths examined (670 m and 915 m), carbon derived from in situ autotrophic fixation supported a significant fraction of the free-living microbial community (0.2-0.5 μm size fraction), but the contribution of chemoautotrophy varied markedly between the 2 depths. Results further showed that utilization of the ocean's largest reduced carbon reservoir, ¹⁴C-depleted, dissolved organic carbon, was negligible in this environment. This isotopic portrait of carbon assimilation by the in situ, free-living microbial community, integrated over >50,000 L of seawater, implies that recent, photosynthetic carbon is not always the major carbon source supporting microbial community production in the mesopelagic realm.