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The pollution of marine sediments by petroleum hydrocarbons is a wide problem affecting the beachfronts of numerous regions of the globe and represents a major concern for the implicit mischievous consequences on ecosystem biodiversity, performing, and overall health. PAHs are considered particularly dangerous to ecosystems, since they’re potentially mutagenic and carcinogenic [1].

In situ and laboratory studies reported that the biodegradation effectiveness of petroleum- defiled sediments can be increased by enhancing biomass and/ or exertion of hydrocarbon-demeaning microorganisms through bio-stimulation as well as bio-augmentation strategies. Understanding the factors impacting microbial metabolism and hydrocarbon declination is pivotal to the design of an optimal bioremediation strategy.

The physical nature of the crude canvas, including available face area and number of carbon tittles composing the hydrocarbon chains, is one of the crucial factors affecting hydrocarbon bioremediation [2]. For case, a single large canvas gyroplane has a lower face area for canvas-demeaning microbes to pierce compared to multitudinous small-sized canvas copters. Also, the chemical nature of the revealed petroleum plays a crucial part in biodegradability. Heavy molecular weight hydrocarbon composites can be more recalcitrant than lighter bones, which are easier for microbes to be metabolized due to their advanced rate of prolixity through the canvas-water interface. In addition, un-branched alkanes can be degraded more fluently than fanned alkanes or multiple- ringed sweet hydrocarbons.

The declination rate of hydrocarbons is also told by the vacuity of nutrients as well as by environmental conditions. Nitrogen and phosphorus have been linked as the two most limiting factors for bacterial-mediated hydrocarbon declination, but indeed sulphur and potassium vacuity can affect bioremediation rates. Crude canvas declination is briskly in warm water as heat promotes the breakdown of the revealed petroleum that becomes more available to canvas-demeaning microbes. Again, sub-zero temperatures beget the arrestment of transport channels of cells and decelerate down cytoplasm inflow processes, hampering or inactivating microbial metabolism and hence their biodegradation eventuality [3]. Also, despite some microbes being cold-tolerant, snap-thaw seasonal cycles between downtime and summer may limit the bioavailability of the revealed petroleum, therefore contributing to reduce microbial biodegradation effectiveness.

Deposition grain size can also affect bioremediation yield by impacting the vacuity of hydrocarbons from microbial attack and the prolixity of nutrients and dissolved feasts demanded to support microbial metabolism. Adsorption mechanisms on the small deposition patches similar as ground/ complexion can reduce microbe – hydrocarbon relations, while the reduced interstitial spaces of ground- complexion- dominated sediments can limit gas and solute exchanges and therefore their bioavailability for microbial exertion. Also, pH, by impacting microbial metabolism, can play a part in affecting bioremediation performance, which generally is effective at pH values around 6-8.

Marine oceanographic systems characterized by reduced oxygen vacuity (e.g., sediments of the oxygen minimum zones, sediments characterized by high organic matter content in largely eutrophic systems, or sub-surface sediments) display lower biodegradation rates of hydrocarbon compared to completely oxygenated systems. Indeed, aerobic conditions favour PAHs declination through oxygenaseintermediated conditioning. Generally, the hydroxylation of an sweet ring via a dioxygenase, a multi-component enzyme conforming of reductase, ferredoxin, and terminal oxygenase subunits, represents the first step in the aerobic bacterial declination of PAHs. Dioxygenase exertion allows the conformation of cis-dihydrodiol, re-aromatized to a diol intermediate via dehydrogenase. Intra diol or redundant diol ring- adhering dioxygenase, through either an ortho-fractionalization or Meta- fractionalization pathway, stick diol intercedes, promoting the conformation of catechols or protocetechuate [4]. Similar interceders are also converted, through β-ketoadipate pathway, to citric acid cycle (CAC) interceders. Gentisate, homogentisate, and homo-protocatechuate represent other metabolic routes, whose genes have been described in metagenomes and trascriptomes belonging to Pseudomonas aeruginosa PAO1, Klesbiella Pneumoniae AWD5, and within a bacteria institute composed by Pseudomonas, Aquabacterium, Chryseobacterium, Sphingobium, Novosphingobium, Dokdonella, Parvibaculum, and Achromobacter. The cytochrome P450- intermediated pathway is a farther metabolic pathway used by bacteria to degrade PAHs, which leads to the product of trans-dihydrodiols.

PAHs breakdown also occurs under anaerobic conditions, e.g., under nitrate/sulfate reducing conditions, with a great body of literature indicating that bioremediation can be effective also in anoxic conditions. In marine anoxic surroundings, the reductions of sulfate, Mn (IV), and Fe (III) represents the primary source for terminal electron- accepting processes [5].

Therefore, the breakdown of hydrocarbons intermediated by microbial metabolism under anaerobic conditions can be successful if the hydrocarbon oxidizers are sulfate, Fe (III), or Mn (IV) reducers. Former studies demonstrated that hydrocarbon declination coupled with sulfate reduction prevails in marine anoxic sediments, since sulfate is generally more available than Fe (III). The bio stimulation of sulfate reducers is allowed to be a suitable strategy for promoting the biodegradation of hydrocarbons in anoxic marine sediments. Indeed though different bacterial strains have been linked as able to degrade a large variety of petroleum pollutants in anoxic marine sediments, strategies and tools suitable to increase microbial growth and biodegradation performance still need to be delved and optimized.

1. Rocchetti L, Beolchini F, Ciani M, Dell'Anno A (2011. Appl Environ Soil Sci.

   

2. Dell’Anno A, Beolchini F, Gabellini M, Rocchetti L, Pusceddu A (2009) . Mar Pollut Bull 58(12): 1808-1814.

      

3. Almeida R, Mucha AP, Teixeira C, Bordalo AA, Almeida CMR (2013) . Biodegradation 24(1): 111-123.

      

4. Guarino C, Spada V, Sciarrillo R (2017) . Chemosphere 170: 10-16.

      

5. Beolchini F, Rocchetti L, Regoli F, Dell’Anno A (2010) . J Hazard Mater 182(1-3): 403-407.