Alcanivorax borkumensis and Bioremediation: The Actual Perspective of a Vintage Bacteria

A. borkumensis [27] is classified within the order Oceanospirillales of the class of the Gammaproteobacteria. Recent phylotaxogenomic analysis [16S rDNA sequencing, DNA–DNA Hybridization (dDDH), Average Nucleotide Identity (ANI), Average Amino Acid Identity (AAI), Percentage of Conserved Proteins (POCP) and comparative genomic studies [20,28] showed that Alcanivorax species formed three clades named Isoalcanivorax gen. nov. and Alloalcanivorax gen. nov., respectively, along with the emended description of the genus Alcanivorax sensu stricto (Table 1). The peculiar physiological and metabolic characteristics of Alcanivorax group are, absolutely recognized, the basis of the application of environmental recovery techniques based on the use of microorganisms. Bioremediation is considered as one of the most important eco-friendly and cost-effective technologies for marine ecological restoration, which leads to a complete decomposition of complex petroleum hydrocarbons of spilled oil into non-toxic compound [29,30]. Bioremediation, including biostimulation and bioaugmentation, has proven to be an effective method for cleaning up residual oil in a variety of environments [31] and has been proposed as the only viable management option that can be implemented on a large scale in marine environments [29,31].

Table 1:Schematic representation of taxonomic distribution of bacteria related to Alcanivoracacae family.

In marine environment, hydrocarbons biodegradation can be limited by many factors (e.g. nutrients, pH, temperature, oxygen and contaminant presence, Figure 1). Biostimulation involves the modification of the environment to stimulate existing bacteria capable of bioremediation. This can be done by addition of various forms of limiting nutrients and electron acceptors, which are otherwise available in quantities low enough to constrain microbial activity. As indicated [31] as the addition of nutrients, oxygen or other electron donors and acceptors to the coordinated site in order to increase the population or activity of naturally occurring microorganisms available for bioremediation. The primary advantage of biostimulation is that bioremediation will be undertaken by already present native microorganisms that are well-suited to the subsurface environment and are well distributed spatially within the subsurface (Figure 2).

Figure 1:Fate of an oil spill. Oil fate is dependent on many natural processes as the oil begins to reach the surface. These processes include spreading, evaporation, and photo-oxidation on the surface of the water. Emulsification and dispersion take place below the surface, which can assist with microbial degradation of the oil. Oil can also sediment to the sea floor. A, seawater / water column.

Figure 2:Schematic description of bioremediation (biostimulation) process in marine environment. A: Specialist that according operative protocols alter the natural condition (insertion of nutrients, surfactants, enhancement of microbial growth) to favour the development of natural microbial population with biodegradative capability; B: seawater (water column); C: crude oil. 1: stimulation of natural bacteria; 2: adhesion of bacterial inoculated to crude oil and initial proliferation; 3: growth of bacterial and increment of biodegradation process.

Bioaugmentation involves the introduction of microorganisms isolated from the contaminated site, from a historical site or carefully selected and genetically modified to support the remediation of petroleum hydrocarbon contaminated sites based on the assumption and/or confirmation that indigenous organisms within the impacted site cannot biodegrade petroleum hydrocarbon. Successful bioaugmentation treatments depend on the use of inocula consisting of microbial strains or microbial consortia that have been well adapted to the site to be decontaminated. Foreign microorganisms (those in inocula) have been applied successfully but their efficiency depends on ability to compete with indigenous microorganisms, predators and various abiotic factors. Factors affecting proliferation of microorganisms used for bioaugmentation including the chemical structure and concentration of pollutants, the availability of the contaminant to the microorganisms, the size and nature of the microbial population and the physical environment should be taken into consideration when screening for microorganisms to be applied (Figure 3).

Figure 3:Schematic description of bioremediation (bioaugmentation) in marine environment. A: Flask with microbial culture of hydrocarbon degrading bacteria and/or hydrocarbonoclastic bacteria; B: seawater (water column); C: crude oil. 1: inoculum of bacteria cultivated in laboratory; 2: adhesion of bacterial inoculated to crude oil and initial proliferation; 3: growth of bacterial and increment of biodegradation process.

Furthermore, recent studies [32-34] have proven how Alcanivorax bacteria is important polypropylene degraders in mesopelagic environments. Alcanivorax strain isolated from plastic marine debris [34] exhibited rapid growth in the presence of weathered low density PE (LDPE) as the sole carbon source; in the same time bacteria related to Alcanivorax borkumensis were indicated to be enriched most abundantly on liquid polypropylene and on its structurally similar branched alkane, pristane. Conceptually, the evolution of the application of this bacteria (or bacteria belonging to the same family) can pass through a synergy with different chemical-physical-engineering strategies and technologies. In fact, the application of particular tools (e.g. bioreactor, systems with activated carbon filters...) can favour an increase in biodegradative capability, which therefore allow the normal limits imposed by the natural physiology of these bacteria to be overcome. However, the search for new genera and increasingly performing bacteria, through bi-prospecting that aims at the study of atypical environmental matrices and which could be innovative sources of microorganisms must always be a present point.

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