The Role of Meso- and Bathypelagic Prokaryotic Organisms in the Marine Water Column:
Insights from Compound-Specific Radiocarbon Analysis.

[CO-INVESTIGATOR, Lihini I. Aluwihare, Scripps Institution of Oceanography, laluwihare@ucsd.edu]


National Science Foundation, Grant OCE-0241363


While the oceans cover more than seventy percent of the surface area of the earth, and the meso- and bathypelagic regions beneath the euphotic zone contain nearly all of the ocean’s volume, little is known about the biogeochemical processes that function in this vast environment. Recent studies of prokaryotic biodiversity highlight the lack of knowledge: bacteria of uncultivated groups (e.g., SAR11; Giovannoni et al., 1996) appear to dominate at several depths; and archaea are now considered an important population (up to 50% of all cells below 1000 m; Karner et al., 2001). Significant evidence now indicates that these archaea are autotophic producers, rather than heterotrophic consumers (Pearson et al., 2001; Sinninghe-Damsté et al., 2002), while the function of the SAR11 cluster remains unknown. These examples highlight how the biogeochemistry of the deep ocean remains largely unexplored.


Our goal is to answer the question: “What carbon substrates are used by deep-sea prokaryotes to support their growth?” This question will be addressed by measuring the radiocarbon (14C) concentration of both the carbon source pools and the resulting prokaryotic (bacterial and archaeal) biomass at (i) surface, (ii) meso- (500-600 m), and (iii) bathypelagic (> 900 m) depths.


First specific objective: Does an occasional pulse of fresh DOC to the deep ocean serve as the sole carbon source of deep-ocean heterotrophic prokaryotes? Or is “old” DOC also bioavailable?


Second specific objective: Is the chemoautotrophic fixation of inorganic carbon by planktonic archaea an important part of the deep ocean carbon cycle?

Figure 1. Radiocarbon in different reservoirs of carbon at Station M in the North Eastern Pacific Ocean. Red circles represent values for HMW DOM (Aluwihare, 1999); blue bars at 0 m and 1500 m represent the range of values observed for individual monosaccharides isolated from HMW sugars (Aluwihare, 1999). DIC, total DOC, and POC data are from Druffel et al., 1996 and Bauer et al., 1998.

 

 

 

 

 

 

 

Our study site is the North Central Pacific gyre, accessible from a shore-based pumping station located at the Natural Energy Laboratory of Hawaii (NELHA; http://www.nelha.org.) The NELHA site is unique in its ability to provide a continuous supply of seawater via two (soon to be three) high-volume pipelines – thereby eliminating the need to consume valuable ship time to obtain our target amount of 106 L of seawater (for 1 g of POC).

By measuring the radiocarbon concentrations of organic compounds, we can identify the bacterial and archaeal carbon sources.

  1. The proxy for total prokaryotic biomass is nucleic acid (NA). NA analysis provides a snapshot of the active community at each sampling depth. The carbon utilized by the total community reflects a combination of organic (heterotrophic production) and inorganic sources (autotrophic production). Because the total NA integrates many organisms, NA alone will not identify the contribution of autotrophic production to total prokaryotic biomass. Other biomarkers are required.
  2. Sterols are membrane lipid components of all phytoplankton. Pearson et al., (2000) have shown conclusively that individual sterols record the value of D14CDIC in surface waters, and therefore of photosynthetic biomass. The value of D14Csterol serves as the proxy for D14CnewDOC (i.e., the 14C value of DOC injected from dissolving POC).
  3. A wide variety of ester-linked phospholipid fatty acids (PLFAs) are found in cell membranes of bacteria (but not archaea.). PLFAs containing C18:0 (primarily bacterial), and odd numbered, methyl-branched, and monounsaturated D11 PLFAs, will be used to determine the major carbon source for heterotrophic bacteria.
  4. Crenarchaeol, an ether-linked membrane lipid consisting of two C40 isoprenoid carbon chains is accepted as a biomarker for the nonthermophilic Crenarchaeota (Hoefs et al., 1997; Schouten et al., 1998; Schouten et al., 2000). This biomarker will be used specifically to confirm that Crenarchaeota in the deep Pacific Ocean assimilate DIC as their primary carbon source, and therefore as a proxy for the D14C of total autotrophic biomass in the samples.

References
  • Aluwihare L.I. (1999) High molecular weight (HMW) dissolved organic matter (DOM) in seawater: Chemical structure, sources and cycling. Ph.D. Thesis, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution, USA.
  • Bauer J. E., Druffel E. R. M., Williams P. M., Wolgast D. M., and Griffin S. (1998) Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean. Journal of Geophysical Research 103(C2), 2867-2881.
  • Druffel E. R. M., Bauer J. E., Williams P. M., Griffin W., and Wolgast D. (1996) Seasonal variability of particulate organic radiocarbon in the northeast Pacific Ocean. Journal of Geophysical Research 101(C9), 20543-20552.
  • Giovannoni, S.J., M. S. Rappé, K. L. Vergin and N. Adair. (1996). 16S rRNA genes reveal stratified open ocean bacterioplankton populations related to the Green Non-Sulfur bacteria. Proc. Natl. Acad. Sci. U.S.A. 93:7979-7984.
  • Hoefs, M. J. L., Schouten, S., deLeeuw, J. W., King, L. L., Wakeham, S. G., and Sinninghe Damsté, J. S. (1997) Ether lipids of planktonic Archaea in the marine water column. Appl. Environ. Microbiol. 63, 3090-3095.
  • Karner, M; De Long, E. F; Karl, D. M. Archaeal dominance in the mesopelagic zone of the Pacific Ocean. Nature (2001), 409:507-510.
  • Pearson, A., T. I. Eglinton, and A. P. McNichol (2000) An organic tracer for surface ocean radiocarbon. Paleoceanography 15, 541-550.
  • Pearson, A., A. P. McNichol, B. C. Benitez-Nelson, J. M. Hayes, and T. I. Eglinton (2001) Origins of lipid biomarkers in Santa Monica Basin surface sediment: A case study using compound-specific D14C analysis. Geochimica et Cosmochimica Acta, 65(18), 3123-3137.
  • Schouten, S., Klein-Breteler, W. C. M., Blokker, P., Schogt, N., Rijpstra, W. I. C., Grice, K., Baas, M., and Sinninghe-Damsté, J. S. (1998) Biosynthetic effects on the stable carbon isotopic compositions of algal lipids; Implications for deciphering the carbon isotopic biomarker record. Geochim. Cosmochim. Acta 62, 1397-1406.
  • Schouten, S. Hopmans, E. C., Pancost, R. D., and Sinninghe Damste, J. S. (2000) Widespread occurrence of structurally diverse tetraether membrane lipids: Evidence for the ubiquitous presence of low-temperature relatives of hyperthermophiles. Proc. Nat. Acad. Sci. 97, 14421-14426.
  • Sinninghe-Damsté, J. S., Rijpstra, W. I. C., Hopmans, E. C., Prahl, F. G., Wakeham, S.G., and Schouten, S. (2002) Distribution of membrane lipids of planktonic Crenarchaeota in the Arabian Sea. Appl. Environ. Microbiol. 68, 2997-3002.