THE CORRELATIONS BETWEEN MERCURY SPECIATION AND DISSOLVED ORGANIC MATTER IN THE SEDIMENT OF THE RED SEA

This study addresses the correlations between Hg an d organic matter in recent sediment; samples were collected from the Gulf of Aqaba, Red Sea coasts (o ligotrophic regions) during 2010. In the present st udy TOC analyzer was used to determine Total Organic Ca rbon (TOC) and Total Nitrogen (TN) concentrations and the total amount of mercury (Hg T) in sediment samples were analyzed by Hydra-C merc ury analyzer. The obtained results indicated that, mercury, TOC a nd TN average concentrations in the Red Sea were 85.42 ng g, 5.10 and 4.45 mg L , respectively. The results show that the Total Org anic Carbon (TOC) in sediment represents the sum of various organic comp ounds, which may play a completely different role i n the distribution and accumulation of Hg. slightly c orrelations between the TOC and the concentration o f Hg in the studied sediment arise mainly from the labil e portion of organic matter released. These compoun ds primarily consist of easily degradable algal-derive d lipids and various pigments, which are petrograph ically described as a soluble Organic Matter (OM). The pre served OM in sediment is commonly entrapped within the cell walls of phytoplankton and also appears as a surface coating on sediment particles. The stron g affinity between Hg and OM is due not only to its c hemical reactivity, but also to the physical charac teristic of these labile compounds, which plays the most imp ortant role in the distribution of Hg in sediment.


INTRODUCTION
Anthropogenic mercury in the aquatic environment has been an increased awareness due to their toxicity for the life so it is important to understand the fate and reactivity with such environment. Paraquetti et al. (2004) showed that the trace metal are reacted and sorbed to the suspended particulate matter. In an aquatic environment mercury occurs in different forms including elemental mercury, Ionic mercury and methyl mercury (Berthon, 1995). Methyl mercury is one of the most toxic forms of mercury species; it tends to bioaccumulate and biomagnify in the fatty tissue of fish more than other forms. Watras and Huckabee, (1994) prove that the most mercury in the most fish species occurs as methyl mercury which is transferred to human via the food chain. Mercury like other metals speciation is affected by reaction with organic and inorganic compounds that present in water.
Dissolved Organic Matter (DOM), which is ubiquitous in aquatic environments, is bind to trace metals which affect their speciation, solubility, mobility and toxicity (Buffle et al., 1988). There is increasing evidence that DOM interacts with mercury as well which is altering its speciation and bioavailability in aquatic environments (Loux, 1998). In coastal waters, more than 60% of the dissolved mercury is associated with organic matters or suspended particles (Fitzgerald and Lyons, 1973). Whereas Dissolved Organic Carbon (DOC) appears to be the most important species which control the bioavailability of mercury due to high binding capacity (Barkay et al., 1997, Cai et al., 1999. The
Strong ionic binding is one of the most important reactions between mercury and reduced sulfur sites in soil and DOM which facilitates the mobility of mercury across water column and sediment (Wallschlager et al., 1996) and enhanced results in increased water column's mercury concentrations and help in increased sequestration of mercury in sediments (Cossa and Gobeil, 2000). The pore water's mercury that adsorbed to sediment particles may affect the biota. Generally, the highest mercury concentrations in sediments (maximum 35 mg g −1 ) were found at 30-40 cm depth, corresponding1 to the period of maximum industrial production (Pereira et al., 1998a). However, particulate mercury escapes when the surface sediments are resuspended and transported by the tide to the rest of the lagoon (Pereira et al., 1998b). The contribution of the dissolved mercury was not evaluated well due to the low concentrations found and reported in several studies (Lucas et al., 1986;Pereira et al., 1998a;1998b, Ramalhosa et al., 2001.
This study presents is the first study dealing with the distributions of reactive mercury in sediments, as well as dissolved organic matter, in Red Sea-Aqaba and contributes basic information required for regional balances of carbon and nitrogen fluxes, budgets and cycles associated with mercury.

Study area:
The study area is located at the northern end of the Gulf of Aqaba (Fig. 1), which is the northward extension of the desert-enclosed Red Sea. The maximum depth of the gulf is 1830 m; it is 180 km long and 5-26 km wide. Oligotrophic conditions prevail in the gulf waters and evaporation (350 cm y −1 ) greatly exceeds precipitation (3 cm y −1 ) (Reiss and Hottinger, 1984). Surface sediment samples were collected by SCUBA divers from the upper layer of the sediment (0-2 cm) using cylindrical plastic tubes for the surface samples and using the cores for the core samples (0-17 cm) , after drying; samples were sieved through 63um sieve and preserved in the freezer until analysis.

Dissolved Organic Matter Extractions
Sediment DOM was extracted by adding 10.0 mL of deionized H 2 O to 1.00 g of sediment l in a 15 mL centrifuge tube. The suspensions were shaken on an orbital shaker for 30 min at room temperature (22-18C). Centrifuged at 900× g for 30 min and filtered through 0.45 mm Acrodisk syringe filters. The extraction period was selected to minimize microbial DOM alteration during extraction (Zhou and Wong, 2000).
The concentration of Total soluble Organic Carbon (TOC) in the extracts was determined using a TOC analyzer (Shimadzu's TOC-V Series/Oldenburg University, Germany). Absorbs at 240 nm are obtained using a diode -array detector All DOM solutions were diluted with DI-H2O to set absorbance at 240-0.10 nm to minimize inner filtration effects. Fluorescence measurements were obtained using a spectrofluorometer with the excitation range set from 240-400 nm and the emission range set from 300-500 nm in 3 nm increments. Instrumental parameters were excitation and emission slits, 5 nm; response time, 8 s; and scan speed, 240 nm min −1 (Zhou and Wong, 2000).

Mercury Determination
The total amount of mercury (Hg T ) in sediment samples was determined by taking exact sample weight (200-300 mg) and the samples were analyzed by Hydra-C mercury analyzer, this machine is capable to analyze solid and liquid samples without pre-treatments. For methyl mercury (MeHg) determination, a 0.5-1.0 g sample was extracted using 15 mL (HCl: CH3OH, 1:1 (v/v) for 2 h. After the extraction step, the mixture was centrifuged and the supernatant was separated from the sediment by decantation. Multi extracts were mixed and the pH was adjusted to 4.5 with the addition of acetate buffer, followed by derivatization with the addition of 1 ml of 1% sodium tetraphenylborate. The derivative, phenyl methyl mercury (MeHg Ph) was extracted with hexane and directly analyzed by GC-MS after concentration to about 1mL; solvent exchange to ethyl acetate also was used for the analysis by HPLCfluorescence technique.
In parallel to this procedure, a derivatization with KBr is now being used for other more samples. For mercury determinations, the US EPA Method 7473 was adopted; aliquots of homogenized samples were weighed (0.1-0.3 g) in pre-cleaned nickel boats before placement on the Hydra C auto sampler. Samples were analyzed based on the principle of thermal decomposition, amalgamation and atomic absorption spectrophotometry detection. The tuna fish (BCR-463) reference material was analyzed to evaluate the accuracy of the analytical method, the result (Hg: 2.75 µg g −1 ) was in excellent agreement with the certified value (Hg: 2.85 µg g −1 ).
Two Certified Reference Materials (CRM's) were used to evaluate the accuracy of the analytical methods; the tuna fish BCR-463 and the estuarine sediment ERM-CC580. The results presented in Table 1 show excellent correlation with the certified values. Recoveries in the range of (95-102) % and (68-80) % was achieved for Hg T and MeHg, respectively.

Total Organic Carbon (TOC)
The TOC values range from 2.05 -7.10 mg L −1 with a mean value of 5.10 mg L −1 and a standard deviation of 1.59 (Fig. 2) for the all sampling locations in a depth interval of 0-14 cm. These values are not linearly related to depth. The values were relatively low in the continental slope sediments and increased within the continental rise (Fig. 3).

Total Nitrogen (TN)
The TN concentration ranges from 4.23-4.54 mg L −1 in the superficial sediments (Fig. 2), with a mean value of 4.45 and standard deviation of 0.15 in the all stations sampled at depths from 0-17 cm. The TN concentration in superficial sediments displays a polynomial pattern with increasing depth (Fig. 4).

C/N Ratio
TOC shows a direct relation with TN in the superficial sediments of the deep Gulf of Aqaba and adjusts to a linear regression with a correlation coefficient that explains of the cases (r2 = 0.89; Fig. 5). The carbon-nitrogen ratio (C/N) for the samples, from depths of 0-15 cm, has a mean value of 1.14 with a standard deviation of 0.34. C/N does slightly change with increasing depth. The lowest C/N ratios are observed (C/N = 0.48) and the highest (C/N = 1.57).

Total Mercury
As shown in Fig. 6, the concentration of total mercury (all forms of mercury, including inorganic and methylmercury) in the top centimeters of Gulf of Aqaba sediments ranges from approximately 28.9-176.60 ng of mercury per gram of dry sediment and ( StD 4.25, 18.0 respectively). While sediment mercury concentration generally correlated with organic matter content, the sharp peak in sediment mercury we observed in the top centimeters does correspond to a high organic matter content. It is instead due to the point source of mercury.
Comparisons between variations of organic matter throughout the sediment cores and the concentrations of Hg were carried out using a leaner regression correlation. The correlation coefficients between the TOC, TN and the concentrations of Hg in the core are shows a significant value ( Fig. 7a and b). The relationship between Hg and organic compounds throughout the Aqaba sediment core is also shown as a regression plot reiterating a poor relationship between Hg, TOC. However, the linear relationship between Hg and TN indicates that the distribution of Hg in the Aqaba sediment samples mainly relates to the labile compounds.  Seiter et al. (2004) demonstrated TOC values for sediments under well oxygenated conditions are <0.5%. The mean values recorded in this study are slightly higher than expected (1.02 ± 0.3%, range 0.41-1.42%,), but lower than those recorded in the continental margins (>1.5% in Seiter et al., 2004;4-16% in Arthur et al., 1998). The TOC values are similar to those recorded in the northern Gulf of Mexico (0.34-1.59% in Goni et al., 1997;1998;Gordon and Goni, 2004;0.37-1.3% in Morse and Beazley, 2008) and those from other basins at similar latitudes, i.e.,: The Argentina Basin (0.26-1.72% in Stevenson and Cheng, 1972) and the Arabian Sea (0.04-1.5% in Grandel et al., 2000). The highest value of this study is in agreement with the values described by Seiter et al. (2004) for tropical regions, including the continental margin of western Africa.

DICUSSION
The TN values span over a narrow range (TN = 0.85-0.91%) which are higher than those expected for the northern sector of the Gulf of Mexico (TN = 0.08-0.17% in Goni et al., 1998;Gordon and Goni, 2004). This is explained by the broadest geographical coverage of this study. The obtained TN values are higher for those from the Argentina Basin (TN = 0.031-0.167% in Stevenson and Cheng, 1972) and than those recorded in the continental margin of the Arabian Sea (0.1-0.4% in Suthhof et al., 2000) and than those recorded in the China Sea (0.02-0.19% in Kao et al., 2003).

AJES
The organic material of photoautotrophic origin mixes with continental organic matter near the coast and is usually transported by turbidity currents to the abyssal plain (Suthhof et al., 2000;Mari et al., 2001;Epping et al., 2002;Gordon and Goni, 2004). Values of Carbon-Nitrogen atomic ratio (C/N) suggests an organic matter input derived from vascular plants (Meyers, 1994), characterized by limiting nitrate concentration (Mari et al., 2001). The mean value of our study (C/N = 1.41 ± 0.34) is lower the Redfield ratio (C/N = 5.7; Gordon and Goni, 2004;Weston and Joye, 2005). The TN of the sediments avoids being remineralized into the water column by adsorption to the mineral clay surface, hence recording smaller C/N ratios (Stevenson and Cheng, 1972;Suthhof et al., 2000) that are explained by selective degradation of the organic compounds and by immobilization of N by microorganisms during early diagenesis (Meyers, 1994). Other factors that can indicate the origin of organic matter in the sediments, but not performed in this study, include the Hydrogen index, microscopic observations and carbon stable isotopic composition (Stein, 1990;1991). The stable isotope composition of particulate organic matter in deep seawater samples from the same stations in the Gulf of .70 ‰ in the abyssal plain and δ13C -24.11 to -22.12 ‰ in the continental slope) indicate a photoautotrophic origin, with depleted values caused by the time of residence in the water column with a contribution of suspended material from the sediment (González-Ocampo, 2005;González-Ocampo et al., 2007).
The surface sediments collected in this study presented mercury concentrations comparable to the values recorded in a previous study (Pereira et al., 1998a). In the present study, the highest mercury contents were found in the area near the industrial discharge and were positively correlated to both the organic carbon and total nitrogen. The organic carbon present sediments may result from industrial effluents and also from the decomposition of extensive algae coverage and plankton inside the gulf, as stated by Monterroso et al. (2003).
The previous studies of Schuster et al. (2008) and Selvendiran et al. (2008a) showed that there is a connection of wetlands and riparian areas to streams during hydrologic events can flush stored organic matter and Hg to downstream ecosystems. Both dissolved and particulate C plays an essential role in the transport of Hg fractions. The result of the this study is in agreement with other studies in the region that report DOC as an important transport mechanism for THg (Driscoll et al., 1995;Selvendiran et al., 2008b;Shanley et al., 2008).
There was a consistent relationship between Hg and DOC across our sites with, which is higher than the studied reported by Grigal (2002) for sites across the Northern Hemisphere. The similar slopes between our study and those reported by Grigal (2002) for the THg DOC relationship are likely a function of strong correlation between FTHg and the export of humic material associated with hydrophobic organic matter (Mierle and Ingram, 1991;Grigal, 2002).

CONCLUSION
Sediments can be a source of dissolved mercury and organic matter because of the activity of microorganisms. As the deposited mercury is buried beneath new layers of sediment, organic matter in the sediment is consumed by certain types, by bacteria living a few millimeters below the sediment water interface, beyond the reach of oxygen. These bacteria get their energy through chemical reactions that involve sulfur and iron and produce toxic methylmercury as a byproduct.

ACKNOWLEGMENT
The researchers wish to thank The Jordan Scientific Research Fund (SRSF) for the financial support (No. S1/04/2008).