Hydrocarbons are organic compounds formed exclusively from the elements hydrogen and carbon and are straight-chain, branched, or cyclic molecules. The types of carbon-to-carbon bonds in hydrocarbons range from single to triple. Very low mass hydrocarbons are gases. Liquid and solid hydrocarbons spread out as a floating layer, if less dense than ocean water, or sink below the ocean's surface, if more dense.

Hydrocarbons in the ocean originate from point and nonpoint sources. Point sources include oil spills from vessels, facilities, pipelines, and rivers. In 2007, 10.6 million liters of oil were released near South Korea in a barge accident. In 2005, more than 26.5 million liters were released from petroleum refining facilities in the aftermath of Hurricane Katrina. Nonpoint sources include vents in the ocean floor and marine transport operations. The National Research Council reported in Oil in the Sea III, that 46 percent of oil released in the ocean comes from natural leaks from fissures in the ocean floor. It is a matter of debate whether these hydrocarbons come from biotic or, less likely, abiotic processes.

Methane, or natural gas, is the most volatile of the hydrocarbons. Over 85 percent of methane released from deep ocean vents dissolves in the water column as a result of the high pressure and low temperature of the deep ocean. In shallow waters, methane bubbles rapidly rise to the surface. Methane is also released from landfills, oil production, and ocean oil spills. Methane is a greenhouse gas (GHG) with twenty-five times the global warming potential of carbon dioxide (CO2) (Proskurowski, 2008).

Nonvolatile hydrocarbons forming ocean spills may undergo chemical, biological, or physical alteration. Chemical breakdown of surface hydrocarbons in the ocean occurs as a natural photooxidative process in which hydrocarbons, catalyzed by sunlight, react with oxygen. Because hydrocarbons do not absorb light efficiently, the reaction requires other organic matter to absorb and transfer energy to oxygen, which becomes the highly energetic and reactive species, singlet oxygen. Singlet oxygen reacts with hydrocarbons to produce alcohols, aldehydes, ketones, and carboxylic acids in a series of complex reactions. These oxidized products are more water soluble than the parent hydrocarbon.

Less subtle is in-situ burning of oil spills. When immediate removal is important or other oil removal methods are infeasible, an oil spill can be burned to form the products CO2 and water. However, tremendous amounts of toxic carbonaceous particulate matter are released to the air as a dark cloud of smoke that eventually settles on the ocean surface. Although the combustion product CO2 is a GHG and causes global warming, the smoky particulate matter deflects radiation and cools the atmosphere.

Biological breakdown of hydrocarbons occurs with bacteria, fungi, and some phytoplankton. The two pathways to breakdown hydrocarbons are oxidative phosphorylation, or respiration, and a detoxification mechanism that oxidizes the hydrocarbons to water-soluble oxygen-containing compounds that are easily excreted. Respiration of simple hydrocarbons results in the production of CO2, a GHG. Bacteria that have acclimated to environs containing hydrocarbons are faster to initiate biodegradation. Several physical methods are used to contain and remediate oil spills, including floating boom barriers to surround an oil spill, skimming boats to remove oil, sorbent materials that transfer oil away from water, and chemical dispersants to finely divide and dilute oil spills. The method used depends on the type, amount, and location of spill and also weather conditions.

Some ocean flora and fauna have adapted to live near sea vents of crude oil and natural gas. However, high hydrocarbon concentration in the ocean is a stressor to other ocean flora and fauna (National Research Council, 2003). For example, coral appears bleached after hydrocarbon exposure destroys the pigmented zooxanthellae of coral polyps. Bleached coral, if they survive, evince decreased exoskeleton maintenance, growth, and reproduction. In general, coral bleaching may be due to any number of stressors. Another likely stressor is elevated sea surface temperature associated with climate change. Attributing coral bleaching to specific stressors is complicated.

Although hydrocarbons are present in the marine environment, marine organisms are exposed only to the hydrocarbon that is bioavailable. The amount of hydrocarbon that is bioavailable depends on the partitioning of the hydrocarbon between ocean and tissues such as gills and cell membranes. Hydrocarbons can also be bioavailable when ingested (adsorbed to foods). Bioaccumulation of hydrocarbons in organisms requires desorption of hydrocarbons from foods in the gut and transport of hydrocarbons among cells. Birds and waterfowl are exposed to hydrocarbons that adsorb to feathers, skin, bills, and beaks. Even with sub-lethal exposure, the fitness of waterfowl may be compromised. For example, waterfowl with oily feathers cannot maneuver well enough to obtain sufficient food to survive.

The long-term effects of hydrocarbons in the ocean on populations, communities, and ecosystems are difficult to determine due to the magnitude of the studies and multiple variables. The effect of climate change must be factored into studies. Changes in CO2 concentration affect ocean pH while global warming affects ocean currents and ocean temperature. These factors must be included in studies of marine populations and ecosystems.

References

1. National Research Council. Oil in the Sea III: Inputs, Fates, and Effects. Washington, D.C.: National Academy Press, 2003.

2. Proskurowski, Giora, et al. "Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field." Science 319, no. 5863 (February 1, 2008): 604- 607.

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