Marine Sediments

Because of their low water solubility and hydrophobicity, PAHs in the marine environment rapidly become associated with organic and inorganic suspended particles (Chiou et al. 1998) and they are subsequently deposited into sediments. As PAH solubility decreases with increasing molecular weight, bioaccumulation of PAHs from sediments by marine organisms is generally greater for the lower molecular weight and more water soluble PAHs (Djomo et al. 1996; Porte and Albaigues 1994).

PAH levels in marine sediments are highly heterogeneous depending on geographical location. For example, PAH concentrations as high as 48000 ng/g were found in Venice lagoon (LaRocca et al. 1996), levels between 5700 and 8500 ng/g were found in the Mediterranean littoral of France-Spain (Baumard et al. 1999) and levels of approximately 800 ng/g were found in Todos Santos Bay, Mexico (Macias-Zamora et al. 2002). Shao et al. (2010) carried out an investigation of the deep-sea sub-surface environment, concretely in sub-surface sediments on the Mid-Atlantic Ridge. They found that the total concentration of PAHs was 445 ng/g dry wt sediment, in the sediment 2.7 m beneath the bottom surface at a water depth of 3962 m on the Mid-Atlantic Ridge. The concentrations of phenanthrene and fluorene were relatively high. In addition, PAH-degrading bacteria were found within the sediments.

In marine sediments, some of the most important factors in PAHs biodegradations are size of inoculum, salinity characteristic of the sediment and pre-exposure of the organisms. There has been growing concern over the mounting concentration of PAHs in the marine environment. One of the most studied areas is the mangrove, important estuarine wetland closely tied to human activities and subject to PAHs contamination (Haritash and Kaushik 2009). Chen et al. (2008) studied the degradation of phenathrene by Sphingomonas sp., a bacterial strain isolated from mangrove sediment. They found that in this case, salinity was the most significant factor in biodegradation, followed by inoculum size. They noted that the phenanthrene biodegradation could be best described by the first order rate model, with a kinetic constant of 0.1185 h-1, under the optimal condition (30°C, 15 % salinity, a carbon/nitrogen ratio of 100 : 1 and an inoculum size of 106 MPN g-1 sediment). They found that the biodegradation was lower with more or less salinity than optimal, and with smaller inoculum size than optimal. Chen et al. (2010) studied the degradation of phenanthrene in mangrove sediments, but with different inocula (Sphingomonas sp., a mixture of Sphingomonas sp. and Mycobacterium sp., and without inoculum), different salinities, different sediment types, and presence of other PAHs. They observed that optimal phenanthrene biodegradation could take place in clay loam sediment slurry at low salinity (5-15%) with the inoculation of both Sphingomonas sp. and Mycobacterium sp., and that the presence of other PAHs had little impact in the degradation process.

Tadros and Hughes (1997) investigated the degradation of PAHs by indigenous mixed and pure cultures isolated from coastal sediments. They found two bacterial species degraders of PAHs in their sediments: Pseudomonas sp. and Ochrobac-trum sp. They also found a relationship between bacteria concentration and the degradation of the compounds. The degradations of naphthalene and fluorene were 98.7% and 90.4%, respectively by indigenous mixed microorganisms, both used as sole grown substrate (after 8 days of incubation). Hinga (2003) studied subtidal marine sediments sampled after a fuel oil spill. It was found that the degradation rate of low molecular weight (LMW) polycyclic aromatic hydrocarbons (PAH) was correlated to the sediment total organic carbon (TOC).

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