Coloured dissolved organic matter CDOM gilvin yellow substance chemistry and origins

When plant tissue decomposes in the soil or in a water body, most of the organic matter is broken down by microbial actions within days or weeks to, ultimately, carbon dioxide and inorganic forms of nitrogen, sulfur and phosphorus. In the course of the decomposition process there is formed, however, a complex group of compounds loosely referred to as 'humic substances'. It is from these humic substances that the yellow-coloured dissolved organic matter in inland and marine waters is derived. The chemistry of humic substances has been reviewed by Schnitzer (1978).

Degradation and other studies indicate that these substances are polymers consisting of aromatic rings that are joined by long-chain alkyl structures to form a flexible network.1195 Figure 3.4 shows some of the many different compounds produced on oxidative chemical degradation of humic substances. A humic substance sample from the water of the Okefenokee Swamp, Georgia, USA, was found to have1357 an average atomic composition of C74H72O46N0 7.

Humic substances vary in size from freely soluble compounds with molecular weights (relative molecular masses) of a few hundred, to insoluble macromolecular aggregates with molecular weights in the hundreds of thousands and perhaps ranging up to the millions. Soil chemists classify humic substances into three main fractions on the basis of solubility behaviour. The soil is first extracted with dilute alkali. The humic material that does not dissolve is referred to as humin. Of the

COOH OH

COOH

COOH

0 COOH

^OCH3

{a) 3,5-Dihydroxybenzoic (b) 1, 2, 4-Benzene (c) Vanillin (d) Catechol acid tricarboxylic acid

COOH

Fig. 3.4 Structures of some of the products liberated on chemical degradation of humic substances.

alkali-soluble fraction, some is precipitated on acidification: this is called humic acid. The humic material remaining in solution is called fulvic acid. In fact all three fractions are chemically very similar and differ mainly in molecular weight, humic acid molecules being larger than those of fulvic acid. They are yellow to brown in colour (the soluble ones giving rise to yellow-brown solutions), hydrophilic and acidic (due to the presence of carboxyl and phenolic groups). Fulvic acid has a higher content of oxygen-containing groups such as carboxyl and hydroxyl. The insolubility of the humin fraction may be due to its being firmly adsorbed or bonded to mineral particles and/or to its having a very high molecular weight.

While some humic material may be formed by oxidation and polymerization directly from the existing phenolic compounds (particularly lignin) in the decomposing plant tissue, it is also true that some saprophytic fungi excrete large amounts of phenolic substances when grown on carbohydrate, and these phenolic substances can undergo oxidation and polymerization to give humic-like material.292 Thus it seems likely that some of the aromatic subunits of the humic materials originate in the plant and some are generated de novo during microbial breakdown. McKnight et al. (2001) compared the fluorescence properties of fulvic acids isolated from streams and rivers receiving predominantly terrestrial sources of organic material with those from lakes with microbial sources. They found that, with excitation at 370 nm, the ratio of emission intensity at 450 nm to that at 500 nm was ~1.4 for terrestrially derived fulvic acid and ~1.9 for the microbially derived material, and they proposed the use of this index as an additional tool for the identification of the origin of fulvic acids within inland waters. The difference in emission spectrum is attributable to the lower content of aromatic carbon (12-17%) in microbially derived fulvic acids than in plant-derived fulvic acids (25-30%).

The relative distribution of the various lignin-derived aromatic sub-units of dissolved humic substances in river and lake waters reflects the botanical composition of the dominant vegetation in the catchments.

Ertel, Hedges and Perdue (1984) found that two Oregon water bodies had quite different phenolic composition in their dissolved humic acid, and these in each case corresponded closely to the phenolic composition of the lignin in the, respectively, non-woody angiosperm- and woody gymnosperm-dominated catchments from which the waters were derived. In both water bodies the humic fraction yielded (on oxidation) four to six times more lignin phenols, relative to total organic carbon, than did the fulvic acid fraction. Moran and Hodson (1994) found that lignin is the primary source (66% of total) of the dissolved humic substances exported from coastal salt marshes in the southeastern USA to adjacent marine environments. Using lignin phenols as biomarkers they concluded that 11 to 75% of the dissolved humic substances in the waters of the southeastern US continental shelf is from vascular plant-dominated environments, about half this material being contributed by salt marshes and half by river export.

It has been shown in Canadian lakes and rivers that the coloured dissolved organic matter is substantially, but not entirely, excluded from the surface ice when it forms in the winter.93 It seems that only the less complex, lower molecular weight compounds are retained within the ice.

The proportion of aromatic carbon in marine humic and fulvic acids is lower than in the corresponding freshwater substances.853 Marine fulvic acid appears in fact to contain only a very small proportion of aromatic residues and is predominantly aliphatic in nature.541,853 Harvey et al. (1983, 1984) propose that fulvic and humic acids in sea water consist mainly of polymeric compounds formed by the oxidative crosslinking of polyunsaturated lipids derived from the biota. This is unlikely, however, to be the whole story, since total dissolved humic material from the eastern equatorial Pacific Ocean has been shown to contain measurable amounts of lignin-derived phenolic residues.901

It seems likely that most of the dissolved yellow colour in inland waters is due to soluble humic substances leached from the soils in the catchment areas, and thus indirectly from the vegetation. Yellow material of the humic type can also be generated by decomposition of plant matter within the water: this could be of significance in productive water bodies. Much of the soluble humic material in river water is precipitated when it comes into contact with sea water.636,1221 Nevertheless, a fraction of the material remains in solution and most of the dissolved yellow colour in coastal sea water is due to humic substances derived from the land in river discharge. For example, Monahan and Pybus (1978) found that in regions with major river discharge off the west coast of Ireland, the concentration of soluble humic material diminished linearly with increasing salinity: the results indicated that essentially all the humic material in these coastal North Atlantic waters originated in the rivers. Since it is mainly the humic acid fraction that is lost in the estuaries,368,407 the contribution of river inflow to yellow colour in the sea is mainly in the form of fulvic acid. The specific absorption (per unit mass, at 440 nm) of marine fulvic acid is much lower than that of marine humic acid,204 but this is made up for by the usually much higher concentration of fulvic acid, so that the two forms of dissolved humic material make comparable contributions to the absorption of light in the ocean.

Boss et al. (2001) found that on the continental shelf in the Middle Atlantic Bight of the northeastern USA, variability of coloured dissolved organic matter was dominated by storms, these being associated with resuspension of sediments and accompanied by an increase in absorption by dissolved material. They concluded that bottom sediments can act as a source of dissolved organic carbon during sediment resuspension events.

As in fresh waters, some formation of dissolved yellow materials takes place within marine waters. Brown seaweeds, for example, actively excrete phenolic compounds, which, probably as a consequence of oxidation and polymerization, give rise to yellow-brown materials of the humic type within the water.1222 It seems possible that this phenomenon might make a significant contribution to the amount of dissolved yellow material in the water near luxuriant brown algal beds. Hulatt et al. (2009) found that green and red macroalgae also excrete CDOM, but at a rate lower than that of brown algae. For brown, red and green species, exudation increased on exposure to light.

It is not certain to what extent the yellow substance (present at low concentrations but still optically significant) in oceanic waters away from the coastal zone is land-derived humic material or is generated within the sea by decomposition of the phytoplankton. Hjerslev (1979), on the basis of his measurements in the Baltic and North Atlantic, considers it unlikely that significant amounts of yellow substance are formed in the sea and attributes it, even in oceanic areas, to river discharge. Jerlov (1976), however, argues that the presence of yellow substance in the upwelling region west of South America proves its immediate marine origin, as this area is practically devoid of fresh water supply from land drainage. In the East Sound estuary, Washington State, USA, there is a persistent layer, 2 to 4 m thick, of phytoplankton. On the basis of their studies, Twardowski and Donaghay (2001) concluded that there is rapid in situ production of coloured dissolved organic matter associated with phytoplankton primary production in this layer, although it contributed ~10% or less of the total dissolved colour in East Sound. In the Chukchi Sea, Hill (2008) found that CDOM concentration was highest in the early spring, well before riverine input had commenced, and proposed that it originated in CDOM formed within sea ice from ice algae, and released into the Arctic Ocean as the surface ice breaks up. Calculations indicated that the energy absorbed by this CDOM has the potential to account for 48% of the springtime ice melt driven by water column heating.

Bricaud, Morel and Prieur (1981), on the basis of their measurements in a variety of waters, suggest that in the ocean away from regions of river discharge, the concentration of yellow substance is determined by biological activity averaged over a long period. Kopelevich and Burenkov (1977) observed a strong correlation between the concentration of yellow substance and the level of phytoplankton chlorophyll in productive oceanic waters. They proposed that oceanic yellow substance is of two kinds: a component resulting from the recent decomposition of phyto-plankton, and a more stable component of much greater age. This latter 'conservative' component would predominate in oligotrophic waters and might, in agreement with Bricaud et al., reflect average biological activity over a long time.

New evidence on the contribution of the terrestrial biosphere to the humic material in the ocean has come from measurement, in the oxidation products of humic substance from ocean water, of specific phenols derived from lignin, a structural polymer present only in land plants. Meyers-Schulte and Hedges (1986), using humic material from the Amazon River as a standard (entirely terrestrially derived), conclude on the basis of measurement of lignin-specific phenols, that about 10% of the humic material in ocean water from the eastern equatorial Pacific is terrestrially derived. Since this is the part of the world ocean least affected by direct river input, it seems likely that the percentage would be higher in other oceanic regions.

The humic material that ends up in the dissolved state in natural waters occurs in a wide range of molecular weights. Vapour-phase osmometry and small-angle X-ray scattering indicate molecular weights of 600 to 1000 for river fulvic acids and 1500 to 5000 for river humic acids.12,853

On the continental shelf the spatial and seasonal distribution of CDOM is substantially determined by river outflow. For example, along the New Jersey (eastern USA) coast a plume of low-salinity water from the Hudson River, with a content of dissolved (and particulate) colour, periodically flows along the coast trapped in an inshore region of 5 to 15 km width and over 100 km long.647 The Amazon River strongly influences the optical properties of the western tropical North Atlantic Ocean at distances over 1000 km from the river mouth in the high-flow season but has much less effect in the low-flow season.301 The Orinoco River also has a major, seasonally varying, effect on the distribution of dissolved colour in this oceanic region.127 At stations on the continental shelf of the US Middle Atlantic Bight along a track extending from Delaware Bay to the Sargasso Sea the concentration of dissolved colour was observed to vary two- to three-fold between seasons in the period August 1993 to April 1994.297 On the continental shelf of the South Atlantic Bight (southeastern USA) the concentration of dissolved colour in the spring of 1993 at a time of high river discharge was up to ten times as high as that measured in the previous summer at a time of low discharge.980

Photobleaching (see next section) can also have major effects on the concentration of dissolved colour on a seasonal scale. In the Middle Atlantic Bight, following shallow stratification in August, photo-oxidation brought about a 70% fall in the concentration of CDOM.1417,1424 In the North Atlantic Ocean and Caribbean Sea the lowest surface concentrations of CDOM are found in the central subtropical gyres, while the highest are found along the continental shelves and within the subpolar gyre.983

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