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Hydrocarbons in seawater

enzymatic reactions. Factors, such as temperature, electron acceptors to degrade hydrocarbons. Examples
nutrient availability (nitrogen and phosphorus being include species, such as Aromatoleum sp. (denitrifying)
crucial, limiting nutrients), and the physical state of and Desulfoglaeba sp. (sulfate-reducing), which have
the oil (e.g., dispersion) significantly influence aerobic been studied for their ability to activate alkanes by adding
degradation timing, rates, and success. 36,37,91 The obligate fumarate. In addition, under highly reducing conditions,
hydrocarbonoclastic bacteria, mainly belonging to methanogenic bacteria can degrade hydrocarbons to
the class of Gammaproteobacteria, are considered the produce methane. Although the process is complex and
most important hydrocarbon degraders under aerobic often mediated by microbial consortia, species, such as
conditions. These bacteria can use hydrocarbons as the Smithella sp. and Anaerolinea sp. have been associated
sole source of carbon and energy and are ubiquitous in with anaerobic degradation of hydrocarbons. 91,92
the oceans. Some relevant examples include Alcanivorax Aside from these biodegradative phenomena, a series
(e.g., A. borkumensis, A. dieselolei, A. pacificus), of chemical–physical processes may largely contribute
representing the most studied and important genus of to the degradation of hydrocarbon mixtures in the sea,
obligate hydrocarbonoclastic bacteria, specializing in the especially at the surface level. It has recently been
degradation of n-alkanes (straight-chain hydrocarbons) highlighted that complex mixtures of hydrocarbons
and Cycloclasticus (e.g., C. pugetii), a genus known for in contact with water can undergo photo-oxidation,
its ability to degrade PAHs, which are more complex forming soluble organic compounds, such as aldehydes,
and thermodynamically stable compounds. The genus ketones, and alcohols. 36-40 Most studies have focused on
Pseudomonas represents an additional notable group the physical and chemical alterations of hydrocarbons.
of bacteria capable of aerobic biodegradation; several Characterizing the involved photochemical processes
species, such as P. putida and P. aeruginosa, are known and the molecules produced is complex, mainly due
for their ability to degrade a wide range of petroleum to the difficulty of observing these phenomena in
hydrocarbons and are often employed in remediation natural conditions. The dissolved fraction of aliphatic
and cleanup practices after oil spills. 91 hydrocarbons in seawater is essentially transparent
Anaerobic biodegradation becomes critical in anoxic to sunlight, unlike the aromatic component, which
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marine environments, such as deep sediments, sub- is easily photo-oxidized with the formation of
seafloor oil reservoirs, and oxygen-depleted zones. “sensitizing” substances. These sensitizers, in turn,
While generally slower than aerobic processes, it make the aliphatic hydrocarbons photo-oxidizable.
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plays a vital role in the fate of hydrocarbons in these For instance, anthracene easily absorbs sunlight and is
environments. 91,92 Anaerobic microorganisms utilize photo-oxidized to 9,10-endoperoxide and anthraquinone
alternative electron acceptors in the absence of oxygen, (Figure 4). The same phenomenon has been observed
including nitrate, sulfate, ferric iron, or carbon dioxide. for other PAHs, including benzo(a)pyrene. 38,93 In this
The initial activation of hydrocarbons under anaerobic sense, a mixture of aliphatic and aromatic hydrocarbons
conditions is a key step, as the lack of oxygen prevents exposed to sunlight can lead to the initial production
direct monooxygenase activity. One of the most well- of sensitizers derived from the oxidation of some
described mechanisms for alkane activation is fumarate PAHs, which then facilitates the photo-oxidation of
addition, catalyzed by enzymes, such as alkyl succinate the aliphatic components, with the formation of more
synthase. This reaction involves the anoxic addition soluble organic compounds, including aldehydes,
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of fumarate to the alkane chain, forming 1-methylalkyl- ketones, and alcohols. 36-40,93
succinate. This product then undergoes transformations Specifically, it is known that experimental irradiation
and β-oxidation-like steps, eventually forming short- of alkylbenzenes in the presence of anthraquinone
chain fatty acids, methane (methanogenesis), and determines the formation of 1-phenylalkanone, alcohols,
carbon dioxide. Diverse anaerobic consortia are and benzaldehyde. These same compounds have been
often involved, with specific microbes specializing in isolated from seawater in contact with petroleum
different steps of the degradation pathway. The presence mixtures under natural conditions, thus suggesting a
of certain metal ions, such as iron, can also influence photochemical origin. 38,94 Controlled photo-oxidation of
anaerobic degradation rates by acting as electron the water-soluble fraction of gasoline in the presence of
acceptors or mediating electron transfer. 91,92 Some anthraquinone produced the same reaction products,
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examples of bacterial communities operating under suggesting that the phenomenon does not concern
anoxic conditions are denitrifying and sulfate-reducing only a few experimentally chosen model compounds,
bacteria, respectively, utilizing nitrate or sulfate as final but rather involves all complex and heterogeneous

Volume 22 Issue 6 (2025) 33 doi: 10.36922/AJWEP025290224