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Quantifying dissolved organic matter degradation in filtering shelf sands

 

Markus Huettel, Joel E. Kostka, Thorsten Dittmar, William T. Cooper, Carol Arnosti.

(Funded by NSF Award OCE-0726754)

In this project, we investigate degradation of dissolved organic matter (DOM) in filtering shelf sediments.

Oceanic DOM (~700 x 10^15 g) contains approximately as much carbon as the entire global vegetation or the CO2 pool of the atmosphere and this DOM is one of the most mobile forms of non-gaseous carbon. DOM provides a nutrient source to primary producers far from its original source and represents a temporary sink in the global carbon cycle. Even small perturbations in the production or sink terms of the oceanic DOM pool could strongly impact the balance between oceanic and atmospheric CO2, or change the storage of carbon in marine deposits (Hedges 2002). Thus, the dynamics of DOM is a central factor in the global carbon cycle, with implications for global energy flows and climate change. Understanding the causes and mechanisms that control DOM production and mineralization are essential for the calculation of global carbon budgets, climate predictions and nutrient cycles.

An unresolved question in the global carbon cycle is that only a small fraction of the organic matter dissolved in deep ocean seawater and preserved in marine sediments is of terrestrial origin (Hedges 1992) . Because most of the riverine DOM is highly refractory, one would expect that this material is widely distributed and accumulates. The small concentrations of lignin in the ocean, and the 13 C-enriched composition of marine DOC, however, suggest relatively rapid degradation of the terrestrial DOM. Sediments may play an important role for terrestrial DOM mineralization.

Dissolved organic matter reaches highest concentrations in the shallow continental shelf, where river inflow, terrestrial materials, and marine organisms contribute large amounts of organic matter to the DOM poo (Fig. 1)l. About half of the carbon fixed by phytoplankton is routed through DOM during the decomposition process, and a large fraction of this mineralization process takes place in the shelf sediments. Bacterial abundance per unit volume s ediment is two to six orders of magnitude higher than in the overlying water column boosting the degradation potential. Enzymatic hydrolysis rates in sediments exceed those in the water column by far.

 

  Fig. 1. Benthic chamber deployed at our study site in the Northern Gulf of Mexico. The water is relatively clear but dissolved organic matter, carried by Apalachicola River into the coastal zone, stains the water brown.

 

Despite this role of sediments for the mineralization process, the mechanisms by which the large volumes of DOM released to coastal waters are rapidly degraded following discharge are not understood.

Because most of the shelf beds are covered with permeable sands, understanding DOM degradation in permeable sediments is central for the modeling of the cycles of matter. Knowledge of the sizes and turnover times of discrete carbon pools is essential for accurate accounting and modeling of global ocean carbon dynamics. Production or sink terms of the oceanic DOM pool, one of the largest exchangeable carbon reservoir s on earth, can strongly impact the balance between oceanic and atmospheric CO2 or change the storage of carbon in marine deposits. Excess labile DOC can cause oxygen deficiencies, and sedimentary DOM mineralization may mobilize nutrients impacting water quality and fisheries yield.

The study combines laboratory experiments (Huettel, Arnosti), detailed DOM characterization (Dittmar, Cooper, Huettel), microbial community structure and activity analysis (Kostka), and in-situ measurements to validate the laboratory results (Huettel, Dittmar, Fig. 2). 13C-labeled labile and refractory DOM are used as traceable substrates for the experiments. Using Fourier transform ion cyclotron resonance mass spectrometry (Cooper), we can identify molecular changes in the DOM caused by the passage through permeable sediment and distinguish biodegradable and refractory portions. (Tremblay et al. 2008) demonstrated the application of this technique for studying DOM cycling in the coastal zone. Specific molecular-level changes, as detected by FT-ICR MS , can be related to processes in the sediment.

While FT-ICR MS is unsurpassed in resolving complex organic mixtures and providing molecular formulae for thousands of molecules in DOM, structural information are limited. Molecular-level lignin analyses add a different dimension to DOM characterization. Using a molecular lignin approach, the major sources of DOM and sedimentary diagenetic process affecting the composition of DOM can be determined (Dittmar and Lara 2001) . Novel molecular technique are used to link DOM decomposition directly to the phylogenetic structure of the microbial community catalyzing the degradation . Total DNA and RNA could be successfully extracted permeable sediment samples (Mills et al. 2008) . Amplicons are processed for terminal restriction fragment length polymorphism (t-RFLP) fingerprint analysis (Kostka).

 

Fig 2. Location of the study site at St. George Island in the northern Gulf of Mexico . Images from Google maps/NASA. Surface sediments here consist of well sorted quartz sands (median ~ 230 µm, porosity 30%-36%, permeability ~3x10 -11 m 2 ). Salinities range from 30 to 34, and water temperatures from 14 to 34 o C.  

 

  Through the quantification of DOM degradation in shelf sands, this project will produce data that will improve our current models of the coastal and global carbon cycles.

 

 

 

Dittmar, T., and R. J. Lara. 2001. Molecular evidence for lignin degradation in sulfate-reducing mangrove sediments (Amazonia, Brazil). Geochimica Et Cosmochimica Acta 65: 1417-1428.

Hedges, J. I. 1992. Global Biogeochemical Cycles - Progress and Problems. Marine Chemistry 39: 67-93.

---. 2002. Why dissolved organic matter, p. 1-35. In D. A. Hansell, Carlson A. C. [ed.], Biogeochemistry of Marine Dissolved Organic Matter. Elsevier, Academic Press.

Mills, H. J. and others 2008. Characterization of nitrifying, denitrifying, and overall bacterial communities in permeable marine sediments of the northeastern Gulf of Mexico. Appl. Environ. Microbiol. 74: 4440-4453.

Tremblay, L. B., T. Dittmar, A. G. Marshall, W. J. Cooper, and W. T. Cooper. in press (web-available). Molecular Characterization of Dissolved Organic Matter in a North Brazilian Mangrove Porewater and Mangrove-Fringed Estuaries by Ultrahigh Resolution Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry and Excitation/Emission Spectroscopy. Marine Chemistry.