Hydrocarbonoclastic bacteria (also known as hydrocarbon degrading bacteria, oil degrading bacteria or HCB) are a heterogeneous group of prokaryotes which can degrade and utilize hydrocarbon compounds as source of carbon and energy. Despite being present in most of environments around the world, several of these specialized bacteria live in the sea and have been isolated from polluted seawater.
The taxonomic diversity of hydrocarbon-degrading bacteria has not changed dramatically if we consider the higher taxa, many studies have provided information on 25 kinds of hydrocarbon-degrading bacteria and 25 kinds of fungi isolated from marine environments. [1] Bacterial genera such as Gordonia , Brevibacterium , Aeromicrobium , Dietzia , Burkholderia and Mycobacterium isolated from oil have been shown to be potential organisms for hydrocarbon degradation. Cerniglia et al. observed that nine cyanobacteria, five green algae, one red alga, one brown alga and two diatoms could oxidise naphthalene. Temperature is crucial because it influences microbial physiology and diversity; the rate of biodegradation generally decreases as the temperature decreases. [1]
Hydrocarbonoclastic bacteria are diazophilic, i.e. they can live in environments extremely poor in nitrogen compounds, which allows them to distribute themselves throughout the environment. They are extremely useful for environmentally friendly biosanitation; the fastest and most complete degradation occurs under aerobic conditions.
Hydrocarbons occur in marine environments where there are oil spills, which makes us understand that they are nutritionally independent of nitrogen sources, a characteristic due to their ability to fix atmospheric nitrogen. In Lagos in a city in Nigeria, nine bacterial strains Pseudomonas fluorescens , P. aeruginosa , Bacillus subtilis , Bacillus sp., Alcaligenes sp., Acinetobacter lwoffi, Flavobacterium sp., Micrococcus roseus and Corynebacterium sp, isolated from the polluted flow that could degrade crude oil, were detected [ citation needed ]; in north-east India they were also detected. In the Louisiana incident in the Gulf of Mexico, about 100 strains were detected and studied, revealing that the isolates all belong to the phylum Proteobacteria and three classes (Alteromonadales, Rhodospirillales and Enterobacteriales). [2]
These organisms are normally present in very small numbers, which gives them an advantage over hydrocarbons such as carbon and energy, as they grow and multiply rapidly. Alcanivorax-like bacteria have been detected in oil-affected environments around the world, including the US, Germany, the UK, Spain, Italy, Singapore, China, the western Philippines, Japan, the mid-Atlantic ridge near Antarctica, and from deepwater sediments in the eastern Pacific Ocean. [3]
Hydrocarbonoclastic bacteria have two fundamental characteristics: (1) specific membrane-bound dioxygenases [4] and (2) mechanisms for optimizing contact with water-insoluble hydrocarbons. [5] Microbial biodegradation occurs wherever oil contamination occurs. However, biodegradation rates are slow and as a result there are severe toxic effects on marine life in the water and on the coast. [6]
The hydrocarbons contained in petroleum have a different behavior in water depending on their chemical nature. This process is called weathering, those with low molecular weight volatilize when they reach the surface.
The rest is attacked by bacteria that are able to do this. These bacteria do not adhere to the oil and do not have a high hydrophobicity of the cell surface. The next stage of degradation involves microorganisms with high cell surface hydrophobicity, which can adhere to residual high molecular weight hydrocarbons. Adhesion is due to hydrophobic fimbriae, fibrils, lipids and proteins of the outer membrane and some small molecules of the cell surface, such as gramicidin S and prodigiosin. [7]
All petroleum products are derived from crude oil whose major constituents are hydrocarbons, that can be separated into four fractions: saturated, aromatic, resin and asphaltene fractions. The susceptibility of hydrocarbons to microbial degradation can be generally ranked as follows: linear alkanes branched alkanes > small aromatics > cyclic alkanes. [8] [9] Some compounds, such as the high molecular weight polycyclic aromatic hydrocarbons (PAHs), may not be degraded at all, asphaltenes and resins are considered to be recalcitrant to biodegradation. [10]
Alkanes are readily biodegraded aerobically in the sea by several different pathways.
The degradation of medium-length ones by Pseudomonas putida starts from the alkane hydroxylase, this enzyme is made up of three components: the membrane-bound oxygenase component and two soluble components called rubredoxin and rubredoxin reductase. [11]
From the oxidation of the methyl group of n-alkanes by the alkane hydroxylase, n-alkanols are released which are further oxidized by a membrane-bound alcohol dehydrogenase in n-alkanals. The n-alkanals are subsequently transformed into fatty acids and then into acyl CoA, respectively by the aldehyde dehydrogenase and by the acyl-CoA synthetase.
This path leads to the release of secondary alcohols. The n-alkanes are oxidized by monooxygenase to secondary alcohols, then to ketones and finally to fatty acids. [12]
Cycloalkanes are degraded by a co-oxidation mechanism, the process leading to the formation of a cyclic alcohol and a ketone. A monooxygenase introduces an oxygen into the cyclic ketone and the cyclic ring is cleaved. [13]
For aromatic compounds there are different pathways, considering toluene at least five are known, [14] each of these is present in specific bacterial species, Burkholderia sp. strain JS150 is unique in using multiple pathways for toluene metabolism: [15]
Oil components that are trapped in marine sediments tend to persist in anaerobic conditions. Some hydrocarbons can be oxidized under anaerobic conditions when nitrate reduction, sulfate reduction, methane production, Fe3+ reduction or photosynthesis are coupled to hydrocarbon oxidation. [16]
Anaerobic bacterium strain HD-1 grows on CO2 in the presence of H
2 or tetradecane. In the absence of H
2, tetradecane is degraded, and the major metabolic intermediate is 1-dodecene [17]
The biodegradation of hydrocarbons is limited by a number of chemical, physical and biological factors.
At the moment, most studies of the ecology of hydrocarbonoclastic bacteria refer to a wide group of genera found principally in marine environments. Since each of them is characterized by a different metabolism, these organisms work together in order to degrade all types of hydrocarbon compounds in a very efficient way. They also play a fundamental role in the carbon biogeochemical cycle and several studies show that some species can create intricate relationships with different marine organisms. [23]
When a release of oil (or whichever kind of hydrocarbon compound) happens in a specific marine area, a lot of bacterial species begin to colonize it, changing the microbial community already present there. Analyzing the dynamics of those communities has led to the discovery of common patterns that are associated with biodegradation, and those information can be useful for the improving of bioremediation methods. Microbial community in situ shuffles since the quantities of nutrients change as the presence of hydrocarbons increases: this ecological situation is able to select only those organisms which can use hydrocarbons as an energy source and possess all the enzymes to do so. In addition, most oil biodegrading species require specific quantities of phosphorus and nitrogen to carry out their catabolic processes. It is possible to state therefore, that hydrocarbonoclastic bacteria rate is limited by the availability of nitrogen and phosphorus mainly. [24]
Several experiments conducted both in vitro and in situ showed the fundamental role that OHCBs (obligate hydrocarbonoclastic bacteria) play during events like an oil spill. The very first microorganism populations that bloom when hydrocarbons are released are the so-called generalists, which can break (through specific enzymes) the most simple bonds in hydrocarbons (generally they are n-alkane degraders); among them, the most common genus is Alcanivorax (the most important species is Alcanivorax borkumensis ) which can degrade aliphatic hydrocarbon compounds. [25] Subsequently, specialists replace generalists to degrade stronger and more complex bounds; among them one of the most known genera is Cycloclasticus which can, for example, degrade aromatic hydrocarbons such as PAH (polycyclic aromatic hydrocarbons). [26]
Up to now, no hydrocarbonoclastic Archaea species have been found, since it appears that they are too sensitive to the effects of an oil spill, as shown by many studies carried out on beaches and coastal waters. [27] Nevertheless, Archaea species could be used as markers of the ecological status of an environment.
Hydrocarbonoclastic bacteria form just a part of the ecological network during bioremediation and biodegradation processes, which involves many direct and indirect relationships and interactions with other communities and with the surrounding environment too. Such interactions include competition for limiting nutrients, predation by protozoa, lysis by phages and cooperative interactions that can decrease or increase degradation of hydrocarbons. [23]
Nutrients availability, as well as nutrients recycling, are important aspects of biodegradation communities. As said before, the amount of phosphorus and nitrogen can modify the structure of a microbial populations and consequently the composition of the community that is shaped by the presence of certain molecules in the ecosystem.
Predation and interactions with phages also affect development of a hydrocarbonoclastic bacterial community. It is possible that the increase of the turnover of biomass (which can be obtained by stimulating the activity of bacteriophages lysis or protozoa predation) could benefit hydrocarbonoclastic populations by stimulating biological remediation. [28] In fact, the presence of oil in the environment can induce prophages [29] and the subsequent lysis of a huge number of bacteria. [30] At the same time, nutrients recycling caused by phages' lysis can trigger a bloom of those species who are favored by the presence of both nutrients and hydrocarbons (used as energy resource). On the other hand, the presence of protozoa can create the opposite situation (it has a negative effect on biodegradation), by limiting the growth of bacterial populations in the ecosystem. [31] That is why interactions with predators are fundamental in marine environments. Nevertheless, in specific occasions, the presence of predators can boost bacterial degradation, as it happens for benzene [32] or toluene. [33] Moreover, in a similar way to what happens with phages, the activity of predation does create a nutritional loop, because predators can remineralize nutrients, which increases bacterial growth.
Since hydrocarbonoclastic bacteria can oxidize long carbon compounds, their metabolism includes part of the large family of biotic reactions in the biogeochemical carbon cycle. Hydrocarbons, especially alkanes, are produced by myriad organisms as waste, for defense, as structural elements, and as chemoattractants. [34] Therefore, this type of biodegradation represents one of the major sinks of hydrocarbon compounds and one of the source of carbon dioxide in marine environments.
In conclusion, hydrocarbonoclastic bacteria can mobilize hydrocarbons from natural sources and use the oxidized carbon atoms and introduce them into their metabolic central pathways. Those oxidized molecules enter the biotic phase of the carbon cycle and can be assimilated by primary and secondary consumers through predation or can assume them after cells' death.
Hydrocarbon-degrading bacteria have many different applications but has specially importance their role in the field of environmental microbiology. [35]
Marine hydrocarbonoclastic bacteria are powerful tools for bioremediation, as they can degrade and convert contaminant oils because of their catabolic versatility. [36] In that way, using biotechnology is possible accelerate the cleaning up of a contaminated site such as coastal regions and offshore after an oil spills or human activities' pollution, but also it is possible to contain and mitigate their damage. [37] They normally bloom after an oil spill or other pollution, and because as they are very versatile metabolically, they can grow on minimal mediums. One example of this is the nitrogen-fixing and heavy oil-degrading bacterium Azospirillum oleiclasticum, which was isolated from an oil production mixture. [38] But A. oleiclasticum is not the only strain that can grow on oil, a 2013 study discovered that there are at least 125 strains, adapted to grow on minimal medium supplemented with crude oil. The predominant bacterial detected by approaches were the Proteobacteria and the most abundant species were in genera Acinetobacter and Stenotrophomons . [39]
They are also used in biosynthesis because they are an extraordinary archive of enzymes like mono and dioxygenases, oxidases, dehydrogenases and others. Furthermore, as they are adapted to grow in hydrocarbon-rich environments, they often synthesize characteristic compounds like polymeric storage substances of industrial interest and bio-detergents with high emulsifying activity. One example of this is the use of the oleaginous yeast Yarrowia lipolytica. As this yeast has a versatile lipid metabolism, by its combination with specific bacterial genes it can use specific enzymatic pathways to bioconvert different lipids (petroleum, alkane, vegetable oil, fatty acid), fats and oils into industrially valuable lipid-derived compounds like isoprenoid-derived compounds (carotenoids, polyenic carotenoid ester), wax esters (WE), polyhydroxyalkanoates (PHAs) and free hydroxylated fatty acids (HFAs). [40]
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may be similar to that of gasoline or lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers.
Bioremediation broadly refers to any process wherein a biological system, living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluents etc., in natural or artificial settings. The natural ability of organisms to adsorb, accumulate, and degrade common and emerging pollutants has attracted the use of biological resources in treatment of contaminated environment. In comparison to conventional physicochemical treatment methods bioremediation may offer considerable advantages as it aims to be sustainable, eco-friendly, cheap, and scalable. Most bioremediation is inadvertent, involving native organisms. Research on bioremediation is heavily focused on stimulating the process by inoculation of a polluted site with organisms or supplying nutrients to promote the growth. In principle, bioremediation could be used to reduce the impact of byproducts created from anthropogenic activities, such as industrialization and agricultural processes. Bioremediation could prove less expensive and more sustainable than other remediation alternatives.
Biological augmentation is the addition of archaea or bacterial cultures required to speed up the rate of degradation of a contaminant. Organisms that originate from contaminated areas may already be able to break down waste, but perhaps inefficiently and slowly.
Geobacter is a genus of bacteria. Geobacter species are anaerobic respiration bacterial species which have capabilities that make them useful in bioremediation. Geobacter was found to be the first organism with the ability to oxidize organic compounds and metals, including iron, radioactive metals, and petroleum compounds into environmentally benign carbon dioxide while using iron oxide or other available metals as electron acceptors. Geobacter species are also found to be able to respire upon a graphite electrode. They have been found in anaerobic conditions in soils and aquatic sediment.
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO2−
4) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.
Mycoremediation is a form of bioremediation in which fungi-based remediation methods are used to decontaminate the environment. Fungi have been proven to be a cheap, effective and environmentally sound way for removing a wide array of contaminants from damaged environments or wastewater. These contaminants include heavy metals, organic pollutants, textile dyes, leather tanning chemicals and wastewater, petroleum fuels, polycyclic aromatic hydrocarbons, pharmaceuticals and personal care products, pesticides and herbicides in land, fresh water, and marine environments.
Cometabolism is defined as the simultaneous degradation of two compounds, in which the degradation of the second compound depends on the presence of the first compound. This is in contrast to simultaneous catabolism, where each substrate is catabolized concomitantly by different enzymes. Cometabolism occurs when an enzyme produced by an organism to catalyze the degradation of its growth-substrate to derive energy and carbon from it is also capable of degrading additional compounds. The fortuitous degradation of these additional compounds does not support the growth of the bacteria, and some of these compounds can even be toxic in certain concentrations to the bacteria.
In biology, syntrophy, synthrophy, or cross-feeding is the phenomenon of one species feeding on the metabolic products of another species to cope up with the energy limitations by electron transfer. In this type of biological interaction, metabolite transfer happens between two or more metabolically diverse microbial species that live in close proximity to each other. The growth of one partner depends on the nutrients, growth factors, or substrates provided by the other partner. Thus, syntrophism can be considered as an obligatory interdependency and a mutualistic metabolism between two different bacterial species.
Rhodococcus is a genus of aerobic, nonsporulating, nonmotile Gram-positive bacteria closely related to Mycobacterium and Corynebacterium. While a few species are pathogenic, most are benign, and have been found to thrive in a broad range of environments, including soil, water, and eukaryotic cells. Some species have large genomes, including the 9.7 megabasepair genome of Rhodococcus sp. RHA1.
Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota. It contains about 250 genera, which makes it the most genus-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically important groups of bacteria belong to this class. It is composed by all Gram-negative microbes and is the most phylogenetically and physiologically diverse class of Proteobacteria.
Biomining is the technique of extracting metals from ores and other solid materials typically using prokaryotes, fungi or plants. These organisms secrete different organic compounds that chelate metals from the environment and bring it back to the cell where they are typically used to coordinate electrons. It was discovered in the mid 1900s that microorganisms use metals in the cell. Some microbes can use stable metals such as iron, copper, zinc, and gold as well as unstable atoms such as uranium and thorium. Large chemostats of microbes can be grown to leach metals from their media. These vats of culture can then be transformed into many marketable metal compounds. Biomining is an environmentally friendly technique compared to typical mining. Mining releases many pollutants while the only chemicals released from biomining is any metabolites or gasses that the bacteria secrete. The same concept can be used for bioremediation models. Bacteria can be inoculated into environments contaminated with metals, oils, or other toxic compounds. The bacteria can clean the environment by absorbing these toxic compounds to create energy in the cell. Bacteria can mine for metals, clean oil spills, purify gold, and use radioactive elements for energy.
Biosurfactant usually refers to surfactants of microbial origin. Most of the biosurfactants produced by microbes are synthesized extracellularly and many microbes are known to produce biosurfactants in large relative quantities. Some are of commercial interest. As a secondary metabolite of microorganisms, biosurfactants can be processed by the cultivation of biosurfactant producing microorganisms in the stationary phase on many sorts of low-priced substrates like biochar, plant oils, carbohydrates, wastes, etc. High-level production of biosurfactants can be controlled by regulation of environmental factors and growth circumstances.
Microbial biodegradation is the use of bioremediation and biotransformation methods to harness the naturally occurring ability of microbial xenobiotic metabolism to degrade, transform or accumulate environmental pollutants, including hydrocarbons, polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), heterocyclic compounds, pharmaceutical substances, radionuclides and metals.
Lily Young is a distinguished professor of environmental microbiology at Rutgers New Brunswick. She is also a member of the administrative council at Rutgers University. She is the provost of Rutgers New Brunswick. She is a member of the Biotechnology Center for Agriculture and the Environment and has her academic appointment in the Department of Environmental Sciences.
Alcanivorax borkumensis is an alkane-degrading marine bacterium which naturally propagates and becomes predominant in crude-oil-containing seawater when nitrogen and phosphorus nutrients are supplemented.
Petroleum microbiology is a branch of microbiology that deals with the study of microorganisms that can metabolize or alter crude or refined petroleum products. These microorganisms, also called hydrocarbonoclastic microorganisms, can degrade hydrocarbons and, include a wide distribution of bacteria, methanogenic archaea, and some fungi. Not all hydrocarbonoclasic microbes depend on hydrocarbons to survive, but instead may use petroleum products as alternative carbon and energy sources. Interest in this field is growing due to the increasing use of bioremediation of oil spills.
The plastisphere consists of ecosystems that have evolved to live in human-made plastic environments. All plastic accumulated in marine ecosystems serves as a habitat for various types of microorganisms, with the most notable contaminant being microplastics. There are an estimate of about 51 trillion microplastics floating in the oceans. Relating to the plastisphere, over 1,000 different species of microbes are able to inhabit just one of these 5mm pieces of plastic.
Oleispira antarctica is a hydrocarbonoclastic marine bacterium, the type species in its genus. It is psychrophilic, aerobic and Gram-negative, with polar flagellum. Its genome has been sequenced and from this information, it has been recognized as a potentially important organism capable of oil degradation in the deep sea.
Bioremediation of petroleum contaminated environments is a process in which the biological pathways within microorganisms or plants are used to degrade or sequester toxic hydrocarbons, heavy metals, and other volatile organic compounds found within fossil fuels. Oil spills happen frequently at varying degrees along with all aspects of the petroleum supply chain, presenting a complex array of issues for both environmental and public health. While traditional cleanup methods such as chemical or manual containment and removal often result in rapid results, bioremediation is less labor-intensive, expensive, and averts chemical or mechanical damage. The efficiency and effectiveness of bioremediation efforts are based on maintaining ideal conditions, such as pH, RED-OX potential, temperature, moisture, oxygen abundance, nutrient availability, soil composition, and pollutant structure, for the desired organism or biological pathway to facilitate reactions. Three main types of bioremediation used for petroleum spills include microbial remediation, phytoremediation, and mycoremediation. Bioremediation has been implemented in various notable oil spills including the 1989 Exxon Valdez incident where the application of fertilizer on affected shoreline increased rates of biodegradation.
Plastic degradation in marine bacteria describes when certain pelagic bacteria break down polymers and use them as a primary source of carbon for energy. Polymers such as polyethylene(PE), polypropylene (PP), and polyethylene terephthalate (PET) are incredibly useful for their durability and relatively low cost of production, however it is their persistence and difficulty to be properly disposed of that is leading to pollution of the environment and disruption of natural processes. It is estimated that each year there are 9-14 million metric tons of plastic that are entering the ocean due to inefficient solutions for their disposal. The biochemical pathways that allow for certain microbes to break down these polymers into less harmful byproducts has been a topic of study to develop a suitable anti-pollutant.
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