A heat-induced molecular signature in marine dissolved organic matter

nature geoscience | VOL 2 | MARCH 2009 | www.nature.com/naturegeoscience 175

progress articlePublished online: 27 February 2009 | doi: 10.1038/ngeo440

Dissolved organic matter (DOM) in the ocean is vital for life on Earth, forming a link between the production and decay of organic matter in the oceanic water column. A delicate

balance exists between photosynthesis and respiration, and algal products are only available to microbial consumers as DOM1,2. Most DOM is quickly turned over by bacteria within hours or days after production3,4, however, a small fraction resists microbial degradation. This refractory portion of DOM has persisted in the ocean for thousands of years5,6

, and has accumulated to become one of the

largest pools of reduced carbon on the Earth’s surface7. The overall quantity of carbon residing in DOM is well constrained8,9, but it is not known whether the size of this carbon pool has fluctuated over time as no historical record of DOM exists. There are indications that the formation of refractory DOM in the oceanic water column occurs on the timescale of months10, but it has also been proposed that the release of fossil compounds from marine sediments contributes to the oceanic DOM pool11,12. DOM may thus link very dynamic carbon pools to almost inert carbon pools in a unique way13.

The amount of carbon residing in DOM far exceeds the difference in atmospheric carbon dioxide concentrations observed between glacial and interglacial periods, and the amount of carbon dioxide released by human activities14. Thus, minor changes in the DOM pool could considerably impact atmospheric carbon dioxide concentrations and the radiation balance on Earth. Indeed, it has been speculated that large-scale oxidation of DOM may have prevented a dramatic global glaciation (‘snowball earth’) hundreds of millions years ago in the Neoproterozoic era15.

Despite its potential climatic significance, DOM is arguably the most mysterious carbon pool on the planet. It is an enigma how reduced organic molecules persist in an oxygen-rich environment in the deep ocean for millennia16. Bacteria acquire chemical energy and essential elements, such as nitrogen and phosphorous, from the oxidation of DOM, and most refractory DOM is small enough in molecular size to be taken up by bacteria17. Our lack of knowledge about DOM is closely related to our inability to understand it on the molecular level. There are no organism remains that can be visually examined. The only indication of the source, history and turnover

a heat-induced molecular signature in marine dissolved organic matterthorsten dittmar1 and Jiyoung Paeng2

The bulk of sea water is an aqueous solution of inorganic salts and gases. However, if it was just this, life as we know it would not exist. In addition to this inorganic component, at least tens of thousands of organic molecules — collectively known as dissolved organic matter — exist in picomole amounts in each litre of sea water. Dissolved organic matter is important for aquatic food webs and, integrated over the entire volume of the world’s oceans, contains roughly as much carbon as all living biota on land and in the ocean combined. Yet, the cycling of dissolved organic matter in the ocean is not well understood. Recent progress in analytical chemistry has allowed the characterization of dissolved organic matter at the molecular level in unprecedented detail, revealing that a significant proportion has been thermally altered, either in deep sediments or through combustion on land with later delivery to the sea. Thermal alteration may explain, at least in part, the resistance of oceanic dissolved organic matter to microbial decomposition.

of DOM is imbedded in its molecular and isotopic composition. For instance, the existence of a terrigenous component in deep-sea DOM was demonstrated with the help of lignin-derived phenols18,19. Lignin is produced by vascular plants on land, but not by algae, therefore it is one of the few unambiguous molecular indicators for the source of oceanic DOM. The need for detailed compositional information on DOM was recognized more than half a century ago20, and much of our knowledge on DOM remains dependent on a limited set of molecular tools. All established molecular approaches target operationally-defined subunits, such as lignin-derived phenols, in the refractory component of DOM21. On the whole, the molecular structure of DOM is unknown22.

new advances in the molecular-level analysis of doMDOM is one of the most complex organic mixtures on Earth, and as such is one of the largest challenges in analytical chemistry. The isolation of individual molecules from the intricate DOM cocktail, a prerequisite for detailed molecular characterization, is not yet possible. Analytical chemistry has seen enormous technological advances over the past few years23, but even modern chromatographic separation techniques are not capable of separating intact individual molecules from DOM22,24,25 (Fig. 1a). The development of soft ionization techniques, in particular electrospray ionization, has made it possible to perform mass spectrometry on complex organic mixtures in aqueous solution26, but still conventional mass spectrometry cannot detect the mass of individual molecules in DOM27 (Fig. 1b). Only the recent advent of ultrahigh-resolution mass spectrometry by means of Fourier transform ion cyclotron resonance (FT-ICR-MS) has made it possible to obtain information on individual molecules in the ocean28,29 (Fig. 1c). Using this technique, thousands of molecular masses can be determined simultaneously in complex DOM mixtures30. The mass of an individual DOM molecule can be measured with an accuracy of less than 0.2 mDa. For comparison, the mass of an electron is approximately 0.5 mDa. At this high mass accuracy, the mass defect (the slight deviation from an integer mass) is precisely known, and molecular formulae can then be computed for each detected molecule in DOM28,29.

1Max Planck Research Group for Marine Geochemistry, Carl von Ossietzky University, ICBM, PO Box 2503, 26111 Oldenburg, Germany, 2Florida State University, Department of Oceanography, 117 N Woodward Avenue, Tallahassee, Florida 32306, USA. e-mail: [email protected]; [email protected]

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progress article NaTuRe geoscIeNce doi: 10.1038/ngeo440

Molecular formulae are the first step towards understanding DOM on the molecular level, but many structural isomers exist for each formula. Only structural information can provide unambiguous evidence for the source and history of DOM. The assignment of specific structures to molecular formulae is still a major challenge in the case of DOM22,30,31. Nuclear magnetic resonance (NMR) spectroscopy provides helpful complementary information in this context32. The application of high-resolution, multi-element NMR (1H and 13C) allows the identification of functional groups and the assignment of possible structures to a fraction of molecular formulae in marine DOM29 (Fig. 1d).

New analytical techniques enable us to unravel the molecular information in DOM in unprecedented detail. By reading the molecular archive that is dissolved in the ocean, new insights into global elemental cycles are being obtained. The first FT-ICR-MS

analyses of marine DOM revealed molecular structures in the remote abyssal ocean off Antarctica that could not have been produced by any known organism12. The polycyclic aromatic structures of these compounds (Fig. 2) are undoubtedly heat-induced (thermogenic) and can be identified from molecular formulae alone because the high degree of unsaturation results in low hydrogen to carbon ratios31.

Preservation through heatingHeat-processed carbon compounds could enter the ocean by escape through the sea floor. The fossil remains of organisms are exposed to geothermal heat and elevated pressure in deep sediments. A variety of chemical reactions occur under these conditions, including condensation reactions that produce polycyclic aromatic structures. These structures are particularly resistant

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Figure 1 | The advancement in the molecular analysis of oceanic DoM. a, Reversed-phase chromatography provides bulk molecular information and is not capable of separating intact individual molecules. b, The nominal (integer) mass distribution of DOM can be determined with conventional mass spectrometry combined with soft ionization techniques. c, Ultrahigh-resolution mass spectrometry (FT-ICR-MS) separates molecules with the same nominal mass, but with different molecular formulae. For clarity only one nominal mass is shown. d, The structures shown are proposed constituents of DOM and are based on the combination of FT-ICR-MS with experimental evidence30 and NMR29.

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progress articleNaTuRe geoscIeNce doi: 10.1038/ngeo440

to biodegradation, and degraded crude oils and asphaltenes are therefore rich in this compound class33. It has been proposed that a portion of this thermogenic carbon may become soluble over time and escape into the marine DOM pool through fluids from marine sediments12. The bulk of the polycyclic aromatic compounds in asphaltenes contains aliphatic side-chains33. Microbial oxidation of these side chains increases the water solubility of these compounds, enabling them to migrate with aqueous fluids through sediments.

Marine thermogenic DOM could also originate from wildfires and fossil-fuel burning on land. The organic products of biomass and fossil-fuel burning are referred to as black carbon, and range from slightly charred biomass to charcoal and soot. Similar to the processes in deep sediments, the resulting polycyclic aromatic structures are particularly stable in the environment, and black carbon accumulates in soils34 and marine sediments35. Biomass combustion may therefore be crucial in the long-term sequestration of organic carbon36. The actual stability of black carbon in soils depends on a variety of environmental factors34, but it has been shown that a portion of black carbon becomes soluble over time and enters the DOM pool of rivers and estuaries37,38.

In the context of global biogeochemical cycles it is important to know whether thermogenic DOM in the ocean interior is derived primarily from wildfires on land or from geothermal processes in deep sediments. It is intriguing that the polycyclic aromatic compounds in marine DOM12 and asphaltenes33 are identical in their core structures and differ only in their side chains. This structural similarity indicates a common origin that implies a link between fossil sedimentary organic carbon, which is locked into the rock cycle, and more active carbon cycles on the Earth’s surface. In addition, steep land–ocean gradients of thermogenic DOM37,39 point towards continental runoff as another potentially significant source. Information on the origin of thermogenic DOM, its turnover rates and the resulting implications for global biogeochemical cycles cannot be obtained from FT-ICR-MS data alone. The exact quantification of thermogenic DOM on a large number of samples is required for global budget and turnover estimates. FT-ICR-MS is unsurpassed in the unambiguous identification of thermogenic DOM, but the method is considered at best semi-quantitative, and the large instrumental effort prevents routine application on a large number of samples.

Quantification of thermogenic doMA new method for quantifying thermogenic DOM39,40 was used along an oceanographic section from South Africa to Antarctica in 2008. This method quantifies specific molecular building blocks of DOM, and therefore provides information that is complementary to FT-ICR-MS. The Antarctic region is well suited to such work because the ocean off Antarctica is one of the regions least impacted by humans. Conversely, the subtropical surface ocean off Africa is strongly influenced by continental runoff and human activity. Several water masses join in the abyssal zone of the Southern Ocean.

Deep water that originates from the North Atlantic is considered ‘old’ because it has not been in contact with the atmosphere for many decades. In contrast, Antarctic intermediate and bottom waters are continuously being formed through the sinking of surface waters off Antarctica, and are considered ‘young’ because of their recent contact with the atmosphere. The young water masses of the Southern Ocean are enriched in persistent industrial products, such as the Freon CFC-11 (ref. 41); the older water masses do not yet carry this industrial signal.

First results from this research (Fig. 3) show a surprisingly homogeneous distribution of thermogenic DOM in the deep ocean. The concentration of carbon that resides in polycyclic aromatic compounds varies between 610 and 800 nM carbon — approximately 2% of DOM (on a carbon basis). For comparison, the concentration of unsubstituted polycyclic aromatic hydrocarbons (PAHs) in the open ocean is more than four orders of magnitude lower42, which is probably due to their low solubility in water. To explain the lack of major gradients in the deep ocean, thermogenic DOM is assumed to be virtually inert in this environment, at least on the timescale of oceanic turnover, which is about one millennium43. Bioactive compounds, such as inorganic nutrients or labile DOM, have strong gradients from their source to sink, which is most often reflected in sharp vertical concentration-gradients in the water column and clear differences between water masses. Inactive compounds, such as chloride or sodium, whose turnover rates exceed the mixing rates of the oceanic water column, are more homogeneously distributed in the deep ocean. It is remarkable that the oldest water masses, which are not yet enriched in Freon41 (in particular North Atlantic Deep Water), show slightly higher concentrations of thermogenic DOM than the younger water masses (Antarctic bottom and intermediate waters), which are enriched in Freon41. The pre-industrial background of thermogenic DOM clearly predominates over more recent sources in the deep ocean, even in the youngest water masses that otherwise carry a strong industrial signal. This is surprising, because at present human activity is the largest source of organic combustion products on Earth36. In particular, airborne soot particles are distributed globally as aerosols, and are deposited on the surface ocean causing elevated concentrations of PAHs at the sea surface along some of the main dust distribution pathways42.

These recent impacts are reflected in slightly elevated concentrations of thermogenic DOM in the subtropical waters off the coast of Africa; soot deposition and continental runoff are probably responsible (Fig. 3). It is interesting that this surface signal has not been carried down into deeper water masses. On the contrary, the surface ocean of the studied region seems to be a sink for thermogenic DOM, rather than a source. Cold Antarctic surface waters are slightly depleted in thermogenic DOM, and this surface depletion has been preserved deeper down in Antarctic intermediate waters (Fig. 3) that originate from the sinking of surface waters off Antarctica. This is consistent with previous observations

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Figure 2 | Two examples of probable structures for thermogenic DoM in the deep ocean. The structures were proposed based on exact molecular masses obtained from FT-ICR-MS12. The same fused ring structures are probably a major component of petroleum asphaltenes33. In the case of asphaltenes, the carboxylic functional groups found in DOM are substituted with aliphatic side-chains.

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of a surface depletion of thermogenic DOM in the Weddell Sea12. Photochemical reactions or an affinity to sinking particles may explain the observed surface depletion off Antarctica.

The global pool of polycyclic aromatic compounds in DOM exceeds one petamole of carbon. This estimate seems to be robust because of the homogenous distribution of this compound class in the ocean interior. The total amount of thermally altered DOM exceeds this estimate, because not all thermogenic organic matter has a polycyclic aromatic structure. The amount of recognizable polycyclic aromatic compounds exceeds the concentration of recognizable lignin structures19, or any other compound group that has been determined on the molecular level in the deep sea21. Radiocarbon dating of this compound class in the deep sea will provide further evidence as to whether thermogenic DOM is derived primarily from sedimentary fossils or biomass burning.

establishing structure–function relationshipsIt is envisioned that organic geochemistry will develop into a predictive science. At present, our understanding of the organic carbon cycle is largely phenomenological and descriptive. Only if we understand the molecular structure of the individual players can we understand their functioning44, similar to the progress made in medicine where structure–function relationships led to the development of genomics and proteomics, and the capacity to predict and treat concerns before symptoms appear. Structure–function relationships for DOM and other organic matter pools are required to understand the Earth’s past and future. First progress in this context was made in the identification of refractory bacterial cell-wall constituents in DOM45

, and by linking specific compounds in DOM to thermal stabilization (this study). The identification of molecular structures in DOM will remain a major challenge for decades to come. Our understanding of DOM will therefore largely depend on the availability and application of new analytical techniques. The reasons and mechanisms behind the chemical stability of DOM in the marine environment, and potential mechanisms that could release carbon and other elements out of refractory molecules, will continue to be a focus of DOM research. It is conceivable that in the future humans may be technologically capable of exploring DOM stabilization as a geoengineering tool to sequester carbon from active cycles. However, only if robust structure–function relationships are established can the otherwise incalculable risks of such an attempt be predicted.

references1. Pomeroy, L. R. Oceans food web, a changing paradigm. Bioscience

24, 499–504 (1974).2. Moore, B., Edie, E. S., Whitley, E. & Dakin, W. J. The nutrition and metabolism

of marine animals in relationship to (a) dissolved organic matter and (b) particulate organic matter of seawater. Biochem. J. 6, 255–296 (1912).

3. Kirchman, D. L., Suzuki, Y., Garside, C. & Ducklow, H. W. High turnover rates of dissolved organic-carbon during a spring phytoplankton bloom. Nature 352, 612–614 (1991).

4. Cherrier, J., Bauer, J. E. & Druffel, E. R. M. Utilization and turnover of labile dissolved organic matter by bacterial heterotrophs in eastern North Pacific surface waters. Mar. Ecol. Prog. Ser. 139, 267–279 (1996).

5. Bauer, J. E., Williams, P. M. & Druffel, E. R. M. 14C activity of dissolved organic carbon fractions in the central North Pacific and Sargasso Sea. Nature 357, 667–670 (1992).

6. Druffel, E. R. M., Williams, P. M., Bauer, J. E. & Ertel, J. R. Cycling of dissolved and particulate organic matter in the open ocean. J. Geophys. Res. 97, 15639–15659 (1992).

7. Hedges, J. I. Global biogeochemical cycles: Progress and problems. Mar. Chem. 39, 67–93 (1992).

8. Hansell, D. A. & Carlson, C. A. Deep-ocean gradients in the concentration of dissolved organic carbon. Nature 395, 263–266 (1998).

9. Hansell, D. A. & Carlson, C. A. Biogeochemistry of Marine Dissolved Organic Matter (Academic Press, 2002).

10. Ogawa, H., Amagai, Y., Koike, I., Kaiser, K. & Benner, R. Production of refractory dissolved organic matter by bacteria. Science 292, 917–920 (2001).

11. Loh, A. N., Bauer, J. E. & Druffel, E. R. M. Variable ageing and storage of dissolved organic components in the open ocean. Nature 430, 877–881 (2004).

12. Dittmar, T. & Koch, B. P. Thermogenic organic matter dissolved in the abyssal ocean. Mar. Chem. 102, 208–217 (2006).

13. Yamashita, Y. & Tanoue, E. Production of bio-refractory fluorescent dissolved organic matter in the ocean interior. Nature Geosci. 1, 579–582 (2008).

14. IPCC Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) (Cambridge Univ. Press, 2007).

15. Peltier, W. R., Liu, Y. & Crowleay, J. W. Snowball Earth prevention by dissolved organic carbon remineralization. Nature 450, 813–819 (2007).

16. Ogawa, H. & Tanoue, E. Dissolved organic matter in oceanic waters. J. Oceanogr. 59, 129–147 (2003).

17. Dittmar, T. & Kattner G. Recalcitrant dissolved organic matter in the ocean: Major contribution of small amphiphilics. Mar. Chem. 82, 115–123 (2003).

18. Meyers-Schulte, K. J. & Hedges, J. I. Molecular evidence for a terrestrial component of organic matter dissolved in ocean water. Nature 321, 61−63 (1986).

19. Opsahl, S. & Benner, R. Distribution and cycling of terrigenous dissolved organic matter in the ocean. Nature 396, 480–482 (1997).

20. Keys, A., Christensen, E. H. & Krogh, A. The organic metabolism of sea water with special reference to the ultimate food cycle in the sea. Mar. Biol. Assoc. UK 20, 181–196 (1935).

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Figure 3 | The distribution of thermogenic DoM in the southern ocean. Thermogenic DOM is expressed as the concentration of total polycyclic aromatic compounds (nmol carbon per litre). The probable structures of these compounds are shown in Fig. 2. The water samples were obtained on CLIVAR cruise I6S (4 February–17 March 2008) onboard RV Roger Revelle. Thermogenic DOM was quantified using the molecular method and algorithm described in ref. 39.

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21. Kaiser, K. & Benner, R. Biochemical composition and size distribution of organic matter at the Pacific and Atlantic Time-Series Stations. Mar. Chem. doi: 10.1016/j.marchem.2008.12.004 (in the press).

22. Koch, B. P., Ludwichowski, K.-U., Kattner, G., Dittmar, T. & Witt, M. Advanced characterization of marine dissolved organic matter by combining reversed-phase liquid chromatography and FT-ICR-MS. Mar. Chem. 111, 233–241 (2008).

23. Mopper, K., Stubbins, A., Ritchie, J. D., Bialk, H. M. & Hatcher, P. G. Advanced instrumental approaches for characterization of marine dissolved organic matter: Extraction techniques, mass spectrometry, and nuclear magnetic resonance spectroscopy. Chem. Rev. 107, 419–442 (2007).

24. Reemtsma, T., These, A., Springer, A. & Linscheid, M. Differences in the molecular composition of fulvic acid size fractions detected by size-exclusion chromatography-on line Fourier transform ion cyclotron resonance (FTICR-) mass spectrometry. Water Res. 42, 63–72 (2008).

25. Stenson, A. C. Reversed-phase chromatography fractionation tailored to mass spectral characterization of humic substances. Environ. Sci. Technol. 42, 2060–2065 (2008).

26. Seitzinger, S. P. et al. Molecular-level chemical characterization and bioavailability of dissolved organic matter in stream water using electrospray-ionization mass spectrometry. Limnol. Oceanogr. 50, 1–12 (2005).

27. Dittmar, T., Whitehead, K., Minor, L. & Koch, B. P. Tracing terrigenous dissolved organic matter and its photochemical decay in the ocean by using liquid chromatography / mass spectrometry. Mar. Chem. 107, 378–387 (2007).

28. Koch, B. P., Witt, M., Engbrodt, R., Dittmar, T. & Kattner, G. Molecular formulae of marine and terrigenous dissolved organic matter detected by electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry. Geochim. Cosmochim. Acta 69, 3299–3308 (2005).

29. Hertkorn, N. et al. Characterization of a major refractory component of marine dissolved organic matter. Geochim. Cosmochim. Acta 70, 2990–3010 (2006).

30. Stenson, A. C., Marshall, A. G. & Cooper, W. T. Exact masses and chemical formulae of individual Suwannee River fulvic acids from ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectra. Anal. Chem. 75, 1275–1284 (2003).

31. Koch, B. P. & Dittmar, T. From mass to structure: An aromaticity index for high-resolution mass data of natural organic matter. Rapid Commun. Mass Spectrom. 20, 926–932 (2006).

32. Kim, S., Simpson, A. J., Kujawinski, E. B., Freitas, M. A. & Hatcher, P. G. High resolution electrospray ionization mass spectrometry and 2D solution NMR for the analysis of DOM extracted by C-18 solid phase disk. Org. Geochem. 34, 1325–1335 (2003).

33. Ruiz-Morales, Y. & Mullins, O. C. Polycyclic aromatic hydrocarbons of asphaltenes analyzed by molecular orbital calculations with optical spectroscopy. Energ. Fuel. 21, 256–265 (2007).

34. Czimczik, C. I. & Masiello, C. A. Controls on black carbon storage in soils. Global Biogeochem. Cycles 21, GB3005 (2007).

35. Masiello, C. A. & Druffel, E. R. M. Black carbon in deep-sea sediments. Nature 280, 1911–1913 (1998). .

36. Druffel, E. R. M. Comments on the importance of black carbon in the global carbon cycle. Mar. Chem. 92, 197–200 (2004).

37. Mannino, A. & Harvey, H. R. Black carbon in estuarine and coastal ocean dissolved organic matter. Limnol. Oceanogr. 49, 735–740 (2004).

38. Kim, S., Kaplan, L. A., Benner, R. & Hatcher, P. G. Hydrogen-deficient molecules in natural riverine water samples: Evidence for the existence of black carbon in DOM. Mar. Chem. 92, 225–234 (2004).

39. Dittmar, T., Koch, B. P., Hertkorn, N. & Kattner, G. A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater. Limnol. Oceanogr. Methods 6, 230–235 (2008).

40. Dittmar, T. The molecular level determination of black carbon in marine dissolved organic matter. Org. Geochem. 39, 396–407 (2008).

41. CLIVAR Carbon Hydrography Data Office <http://whpo.ucsd.edu/groups?id=i06>.42. Nizzetto, L. et al. PAHs in air and seawater along a North–South Atlantic

transect: Trends, processes and possible sources. Environ. Sci. Technol. 42, 1580–1585 (2008).

43. Broecker, W. A., Sutherland, S. & Peng, T.-H. A possible 20th-century slowdown of Southern Ocean deep water formation. Science 286, 1132–1135 (1999).

44. Mullins, O. C., Sheu, E. Y., Hammami, A. & Marshall, A. G. Asphaltenes, Heavy Oils, and Petroleomics (Springer, 2007).

45. Amon, R. M. W., Fitznar, H. P. & Benner, R. Linkages among the bioreactivity, chemical composition, and diagenetic state of marine dissolved organic matter. Limnol. Oceanogr. 46, 287–297 (2001).

acknowledgementsWe thank the crew and colleagues of RV Roger Revelle for their support at sea, and K. Speer and O. Mullins for beneficial discussions. This work was financially supported by the National Oceanic and Atmospheric Administration (NOAA grant GC 05-099) and the US CLIVAR (Climate Variability and Predictability) programme.

additional informationCorrespondence and requests for materials should be addressed to T.D.

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A heat-induced molecular signature in marine dissolved organic matter