Key elements influencing rates of biodegradation of hydrocarbons in the environment
oil a) composition that defines its chemical and physical properties and b) concentration;
abiotic factors (temperature, salinity, presence of water, etc.);
biotic factors (composition of microbial community).
The above factors are usually interdependent, for example, abiotic factors affect hydrocarbons and concentration, and biotic factors are highly dependent on abiotic factors.
Petroleum hydrocarbons can be divided into four classes: (1) saturates, (2) aromatics, (3) asphaltenes (phenols, fatty acids, ketones, esters, and porphyrins), and (4) resins (pyridines, quinolines, carbazoles, sulfoxides, and amides). In general, hydrocarbons have been ranked in the following order of decreasing susceptibility to biodegradation: n-alkanes > branched alkanes > low molecular weight aromatics > cyclic alkanes, with high molecular weight aromatics and polar compounds being extremely recalcitrant. However under some conditions (composition of microbial community, abiotic factors), the order of biodegradation can vary greatly.
It was shown that recalcitrant hydrocarbons can be oxidized and consequently degraded in the presence of other hydrocarbons, such as n-alkanes, that can be readily consumed by microorganisms. This phenomenon is termed co-oxidation, based on the Oxo-degradable technologies in the market.
Biodegradation also depends on dispersibility and weathering dynamics of the plastic materials. In general, plastic with high wax content are characterized by high viscosities and pour points and tend to have much less bio-assimilation or bioavailability.
The rates of uptake and mineralization of many organic compounds by microbial populations in the aquatic environment are proportional to the concentration of the hydrocarbons, conforming to Michaelis-Menten kinetics. Michaelian kinetics have been demonstrated for low-molecular weight highly soluble oil components such as toluene. The microbial degradation of high-molecular weight hydrocarbons, such as long (>C12) alkanes with solubility less than 0.01 mg/liter, occurs at rates that exceed the rates of their dissolution and are a function of the hydrocarbon surface area available for emulsification or physical attachment by cells, and therefore do not display the dependence on concentration.
Abiotic factors in biodegradation of hydrocarbons
Temperature affecting biodegradable plastic
Rates of plastic biodegradation generally decrease at lower temperatures. This is believed to be a result primarily of decrease in enzymatic activity, however, increased solubility and bioavailability of less soluble hydrophobic substances also play an important role. Higher temperatures enhance the rates of hydrocarbon metabolism with general optimum in the range of 30°C to 40°C, most Landfills operate above these temperatures.
Cold-adapted, psychrophilic and psychrotrophic microorganisms (for example, Rhodococcus sp.) are able to grow at temperatures around 0°C. They are widely distributed in nature because a large part of the Earth’s biosphere is at temperatures below 5°C.
Oxygen affecting biodegradable plastic
The initial steps in the catabolism of aliphatic, cyclic, and aromatic hydrocarbons by bacteria and fungi involve the oxidation of the substrate by oxygenases for which molecular oxygen is required. Conditions of oxygen limitation normally exist in aquatic sediments and soils. Oxygen depletion can occur in the presence of easily utilizable substrates that increase microbial oxygen consumption.
Nitrogen and Phosphorus
Along with carbon, five other elements – hydrogen, nitrogen, oxygen, phosphorus, and sulfur – play a major role in life on Earth. The release of hydrocarbons into environments often produces excess of carbon over nitrogen and phosphorus which quickly become exhausted. It is well established that deprivation of nitrogen and phosphorus inhibits microbial plastic degradation in such ecosystems as estuaries, seawater and marine sediments, freshwater lakes, groundwater, and soils.
Salinity affecting biodegradation
Biodegradation rates of plastic in fresh and marine water are comparable and depend on tolerance/adaptation of resident oil-degrading microorganisms to the specific salinity. Because abundance of microorganisms in highly halophilic conditions is greatly reduced, so is plastic biodegradation. It was shown that rates of hydrocarbon utilization start to decrease noticeably in the salinity range 3.3 to 28.4%. Nonetheless, there are several reports about microorganisms able to oxidize petroleum hydrocarbons even in the presence of 30% w/v NaCl. Among such microorganisms are crude oil-degrading Streptomyces albiaxialis, and an n-alkane (C10-C30)-degrading member of the Halobacterium group.
Pressure affecting plastic biodegradation
The importance of pressure as a variable in the biodegradation of hydrocarbons is most probably confined to the deep-sea environment where temperature factor is also at play. Barophiles (piezophiles) are microorganisms that require high pressure for growth, or grow better at pressures higher than atmospheric pressure. Little is known about ability of deep-sea microorganisms to cope with hydrocarbons. In general, plastic which reaches the deep-ocean environment is degraded very slowly by resident microorganisms and some fractions persist for decades.
Water availability
Water availability or water activity (also water potential) ranges from 0.0 to 0.99. Hydrocarbon biodegradation in terrestrial ecosystems may be limited by the available water. Optimal rates of biodegradation in sludge are observed at 30% to 90% of water saturation. The presence of moisture (unsaturated conditions) in a landfill increases gas production because it encourages bacterial decomposition. Moisture may also promote chemical reactions that produce gases which are captured and used for heating of homes or powering businesses. A typical value of L o in most MSW landfills in the United States is about 25% on wet basis. (wt of water/(wt of water + dry solids), the ASTM D5511-12 is tested with 40% solids.
pH affecting biodegradable materials
In contrast to most aquatic biosystems, soil pH can be highly variable ranging from 2.5 in mine refuse to 11.0 in alkaline deserts. Most heterotrophic bacteria and fungi favor a near neutral pH, with fungi being more tolerant to acidic conditions.
Biotic factors affecting biodegradation of plastic
Hydrocarbons in the environment are biodegraded primarily by bacteria and fungi. Algae and protozoa are important members of the microbial community in both aquatic and terrestrial ecosystems, but the extent of their involvement in hydrocarbon biodegradation is largely unknown and most likely is minor.
There are three mechanisms for adaptation of microbial communities to chemical contaminants: (1) induction and depression of enzymes,(2) genetic changes (mutations, horizontal gene transfer), and (3) selective enrichment.
References:
Margesin R, Schinner F. Biodegradation and bioremediation of hydrocarbons in extreme environments. Appl Microbiol Biotechnol. 2001 Sep;56(5-6):650-63.
Prince RC. Petroleum spill bioremediation in marine environments. Crit Rev Microbiol. 1993;19(4):217-42.
Leahy JG, Colwell RR. Microbial degradation of hydrocarbons in the environment. Microbiol Rev. 1990 Sep;54(3):305-15.
