Abstract

The Deepwater Horizon oil spill led to the release of approximately 4.9 million barrels of oil and gas at a depth of 1544 meters below the surface. Due to the unique location at which the spill occurred, oil was able to spread underwater, across the ocean surface and coastlines. As a result, the widespread distribution of the oil made it difficult for humans to clean up, leaving much of the oil untouched. Therefore, understanding the biological processes that are involved in degrading the hydrocarbons is crucial to better comprehend the ongoing recovery process in the Gulf of Mexico. This review highlights findings related to the bioremediation activities by microbes in oiled areas (underwater, ocean surface, and beaches). In addition, due to the unknown influence of chemical dispersants to the environment, the discovered effects of dispersants on microbes were also incorporated. Overall, studies reveal a rapid and effective response by bioremediation microbes to the spilled oil. This response suggests that microbes may possess the ability to help restore balance in the Gulf of Mexico.

Introduction

On April 20, 2010, 21 years after the horrific Exxon Valdez oil spill, one of the most feared environmental disasters struck again. The oil drilling rig Deepwater Horizon exploded and sank, resulting in the largest marine oil spill in history. In total, an estimated 4.9 million barrels of light crude oil were released deep into the Gulf of Mexico from the Macondo well before the broken wellhead was capped on July 14, 2010 (Camilli et al. 2010).

An analysis of the 4.9 million barrels of oil released reveal that only about 25% of the spilled oil was directly cleaned up in the form of direct recovery from wellhead (17%), burning (5%), and skimming (3%) (The Federal Interagency Solutions Group 2010). 75% of the oil was not directly removed or neutralized by humans, posing a great danger to marine life (The Federal Interagency Solutions Group 2010). Thus, understanding the non-human mechanisms involved in degrading the spilled oil is essential to a better understanding of the recovery process in the Gulf of Mexico.

While the United States has experienced large-scale oil spills before, the Deepwater Horizon oil spill was, in many ways, vastly different from most of the spills preceding it. Whereas most of the preceding oil spills occurred at the ocean surface, this spill occurred at a depth of 1544 meters (Camilli et al. 2010). As a result, the hydrocarbons released into this deep marine environment experienced notably distinct chemical, physical and biological influences (Reddy et al. 2011).

Unlike surface spills, where oil will disperse horizontally across the ocean surface, the oil from the Deepwater Horizon spill was able to travel both vertically and horizontally. Because of this, much of the oil remained below the ocean surface, as it took much time to ascend the 1544 meters needed to reach the surface (Camilli et al. 2010). Consequently, the concentration of subsurface oil posed a great danger to the marine life that originally occupied the water space. Therefore, cleaning up the Deepwater Horizon oil spill required new innovative methods.

In many ways, the natural development of bioremediation (the use of biological agents to remove contaminants from the environment) is characteristic of nearly all chemical spills. While humans will mobilize to contain and clean up an oil spill, biological organisms will naturally contribute to this clean-up effort. Through the use of enzymes and cellular respiration (metabolic processes to obtain energy), certain microorganisms will naturally degrade pollutants such as hydrocarbons by converting them into water and gases (Camilli et al. 2010).

Knowing this natural method of oil degradation, clean-up crews tried to help eliminate the buildup of subsurface oil by applying chemical dispersants near the broken well. Chemically, dispersants break down large collections of oil into small droplets (Baelum et al. 2012). The purpose of the breakdown was to dissipate the oil into smaller droplets so that microbes that degrade hydrocarbons, the main chemicals in oil, could proliferate and aid in the process of bioremediation.

Of course, as with all oil spills, some of the oil released eventually rose to the surface. However, the Deepwater Horizon drilling rig was located around 80 kilometers from the Louisiana coast, making it difficult for clean-up crews to reach the oil slicks formed on the ocean surface (Le Hénaff et al. 2012).  Therefore, most of the oil that reached the surface was dispersed across the Gulf of Mexico toward the northern Gulf coastlines due to the influences of oceanic currents and surface wind-induced drift (Le Hénaff et al. 2012). Consequently, much of the Gulf of Mexico coastline was contaminated with oil (Horel et al. 2012). Despite the aggressive oil removal efforts, an abundance of oil on beaches seeped under rocks and into the sand, making it inaccessible to clean-up crews. Therefore, much of the oil removal again hinged on the activity of bioremediation bacteria, causing marine scientists to conduct studies to analyze their activities.

The wide distribution of hydrocarbons throughout the Gulf of Mexico in water columns, at the ocean surface, and on the coast resulted in a heavy reliance on bioremediation since human efforts were inadequate. However, since the Deepwater Horizon oil spill was unique because of its location, magnitude and distribution of oil, many previous studies on bioremediation were not applicable to this spill. Thus, new research had to be done to understand the hydrocarbon-degrading activities of the microbes in oiled environments. Recent studies of bioremediation in subsurface oil sites, surface oil slicks, and beach environments have all provided new insight into the ongoing biological efforts to remove the released oil. This review seeks to highlight the findings related to the activities of these bioremediation bacteria in the environments that were affected by the Deepwater Horizon oil spill.

Microbial Activity in the Oil Plume

Immediately after the discovery of the spill, scientists noticed the surprisingly low amount of oil visible at the ocean surface when compared to the amount of oil released from the well (Reddy et al. 2011). During exploratory and surveillance cruises carried out for governmental organizations (US Coast Guard, National Oceanic and Atmospheric Administration and the US Environmental Protection Agency), a subsurface layer of oil (later referred to as a plume) was discovered at depths between 1030 and 1300 meters (Camilli et al. 2010). These plumes arose due to the chemical composition of the released hydrocarbons and the low-temperature conditions near the sea floor.

While much of the output from the broken Macondo well was oil, a significant portion of the output was also natural gas (Redmond and Valentine 2011). As the oil and gas were released from the well, they began to rapidly ascend from the sea floor, mixing with cold seawater as they rose (Valentine et al. 2012). As they mixed with the seawater, the oil and gas rapidly cooled, causing the hydrocarbon gases to dissolve (Valentine et al. 2012).  This dissolution of hydrocarbons slowed the ascent of both the oil and the gas, resulting in the formation of plumes deep in the ocean (Valentine et al. 2012). In an analysis of the distribution of methane, the primary component of natural gas, throughout the water columns during the spill, Reddy et al. (2011) discovered that methane was trapped at a depth of 1100 meters. In fact, methane was nearly absent at the surface, suggesting that the released methane bubbles completely dissolved upon reaching 1100 meters below the surface (Camilli et al. 2011). The concentration of hydrocarbons in these plumes, both in the dissolved gas form and in the oil form, provided bioremediation microbes with ample substrate to feed on (Redmond and Valentine 2011).

As mentioned before, bioremediation bacteria degrade pollutants such as hydrocarbons through the use of enzymes as part of metabolic pathways. In short, they utilize the hydrocarbons as an energy source by breaking them down (Lu et al. 2012). For microorganisms to use enzymes to successfully break down pollutants, the genes responsible for creating the enzymes that are used in the metabolic pathways must be active.  Lu et al. (2012) utilized functional gene arrays to analyze gene activity. Significantly more functional genes were detected in the microbes obtained from the oil plume samples than in the ones from unoiled sites (Lu et al. 2012). Furthermore, Lu et al. discovered that an abundance of genes involved in hydrocarbon degradation were active in the microbes from the oil plume sample (Hazen et al. 2010, Lu et al. 2012). These findings therefore suggest bioremediation activity by the microbes in the oil plumes.

Like all living organisms, microbes undergo cellular respiration when they use metabolic pathways to obtain energy from energy sources, such as hydrocarbons. Therefore, the measurement of available oxygen is an effective measurement of microbial activity since oxygen is used in aerobic cellular respiration. In analyzing the water columns containing and not containing oil plumes, Camilli et al. (2010) discovered that there was a slight decrease in oxygen concentration, indicating microbial respiration and the breakdown of hydrocarbons. The percent of dissolved oxygen within the plumes averaged 59% while it averaged 67% outside of the plumes (Hazen et al. 2010). In addition, cell densities and phospholipid fatty acid concentrations within the plumes were also higher within the plumes when compared to outside the plumes (Hazen et al. 2010). These measurements all indicate the presence of bacteria, in particular bioremediation bacteria, within the plumes.

Researchers also discovered that no one specific group of bacteria dominated the plumes; instead, the groups of bacteria that dominated the plumes changed over time (Hazen et al 2010). Soon after the Deepwater Horizon spill, in May, Hazen et al. (2010) discovered that a group of Oceanospirillales were prevalent in plume samples but rare in uncontaminated areas. In addition, 16 other groups of Gammaproteobacteria were more present in plume samples when compared to non-plume samples (Hazen et al. 2010). In June however, the population shifted. No longer were Oceanospirillales dominant; instead, Gammaproteobacteria, Colwellia and Cycloclasticus were the dominant microbes as they together accounted for more than 95% of the sequence data (Valentine et al. 2011). Soon, the leaking well was capped, and by September, the population of microbes once again shifted. The groups that were present during the spill in June had diminished in size and the plumes were being dominated by methylotrophs, a group of microorganisms that can degrade one-carbon compounds such as methane (Kessler et al 2011). Thus, while bioremediation continued to occur, the presence of microbes was shifting.

Microbial Response to Dispersants

In order to combat the large oil plumes, clean-up crews decided to use chemical dispersants at great depths. During the spill, chemical dispersants were used both near the site of the broken Macondo well and at the water surface. Dispersants were added to reduce the size of oil droplets in order to prevent the formation of large slicks that could coat and harm the coastal environment (Kujawinski et al. 2011). Prior to the spill, no large-scale application of dispersants in deep water had ever been attempted and thus there were no data that detailed the fate of the chemical dispersants (Kujawinski et al. 2011). Therefore, Kujawinski et al. (2011) set out to study the fate of the dispersants. Through the use of mass spectrometry and liquid chromatography, they discovered that the fate of the dispersants was similar to that of the oil and gas released from the well. Like the outputs from the well, the dispersants also dissolved into the water during their ascent and collected around 1100 meters below sea level (Kujawinski et al. 2011). This suggests that the plumes were a mixture of released oil and added dispersants. The findings of Camilli et al. (2010) supported this view, as they revealed that the dispersant-to-oil ratio was ~10 times higher in the plumes when compared to the ratio at which the dispersant was applied.  Adcroft et al. (2010) also discovered this interaction between the usage of dispersants and the formation of plumes.

Since the use of dispersants, especially in deep-water settings, was previously untested, the effects of these dispersants on deep-water bioremediation bacteria was largely unknown (Baelum et al. 2012). One uncertainty was the toxicity of the chemical dispersants to bioremediation bacteria. Therefore, in addition to studying the fate of the deep-water chemical dispersants, researchers also focused on understanding how deep-sea bacteria reacted to these dispersants. Baelum et al. (2012), in their in situ study, sought to provide insight into this interaction. Their measurements reveal an increase in microbial cell density in environments enriched with both oil and dispersant(Baelum et al. 2012) . Furthermore, the measurements also point to an increase in Macondo crude oil degradation when the chemical dispersant is present (Baelum et al. 2012). These two results suggest that dispersants actually promote the hydrocarbon-degrading activity of bioremediation bacteria. In an attempt to provide an explanation for this increase in activity, Baelum et al. (2012) noted that the hydrocarbon portion of the dispersants were also degraded, suggesting that it was used as a carbon source for microbial growth, allowing them to proliferate. Overall, Baelum et al. (2012) found a high potential for microbial degradation of oil when a chemical dispersant is present.

Microbial Activity in the Surface Water

While several reports have focused on the microbial biodegradation of hydrocarbons in deep-water oil plumes, researchers have also studied microbial activity in surface oil slicks. One of the largest challenges microbes face at the surface level is the scarcity of phosphate, both inside and outside the oil slicks (Edwards et al. 2011). While this phosphate scarcity may theoretically inhibit the metabolic function of the microbes, Edwards et al. actually reveal that the presence of oil still stimulates an enhanced respiration rate (2011). This result is consistent with other studies of deep-sea bacteria where microbes are also responding to the addition of hydrocarbons by increasing respiration rates (Edwards et al. 2011). In fact, calculations of the total microbial hydrocarbon degradation rate and the total flux of carbon from the Macondo well suggest that the microbes in the surface water possessed the metabolic potential to degrade oil at the same rate that the oil was being delivered (Edwards et al. 2011).

Shoreline Microbial Activity

Due to ocean currents, some of the spilled oil ended up reaching the shoreline. While there was a rapid and aggressive clean-up response to the spill, as of this publication, a significant amount of oil still remains trapped in coastal ecosystems, especially the beach ecosystem. The marine sand in the Gulf contains an abundance of microbial communities and provides an environment that boosts microbial metabolism. Similar to the environment in the sea-surface slicks and deep-water plume, beach environments also contain microorganisms capable of metabolizing carbon.

Both Horel et al. (2011) and Kostka et al. (2011) sought to determine the responses of indigenous microbial communities to hydrocarbons. Testing microbes obtained from oiled beach sand and confirmed as oil-degrading microorganisms, both studies found a rapid biodegradation response to crude oil from these microbes (Horel et al. 2011, Kostka et al. 2011). Horel et al. (2011) showed that the most significant amount of growth of hydrocarbon-degrading microbes occurred during the first seven days of the biodegradation process. Kostka et al. (2011) presented a similar result as they showed that bacteria, from Pensacola Beach, were 2 to 4 orders more abundant in the presence of oil contaminants. Kostka et al. (2011) also revealed, through the usage of RNA-based analysis that analyzes gene expression, that these bacteria in oiled sands were active.  In addition, Kostka et al. (2011) points to Alcanivorax as a rapid-responding bacterium. Like Hazen et al. (2010), Kostka et al. (2011) also points to Gammaproteobacteria as an oil-degrading bacterium. Horel et al. (2011) also reveals that contaminant elimination by biological means can reach as high as 91% with 50% of it coming from microbial mineralization. Thus, bioremediation bacteria are capable of a rapid and effective bioremediation response to oil spills.

Discussion

Certain microorganisms throughout the Gulf of Mexico and the shoreline possess the ability to degrade hydrocarbons, especially ones located in the plumes, surface slicks and on beaches (Edwards et al. 2011, Horel et al. 2012, Lu et al. 2011, Hazen et al. 2010). Most importantly, the hydrocarbon-degrading microorganisms in these studies that responded to the spill were all indigenous microorganisms, suggesting that most of these microbes alone can play a significant role in biodegradation (Lu et al. 2012, Edwards et al. 2011, Horel et al. 2012, Kostka et al. 2011, Hazen et al. 2010, Camilli et al. 2010). However, these microbes did not respond simultaneously. Instead, the population of microbes changed over time (Hazen et al. 2010, Valentine et al. 2011, Kessler et al 2011). The lack of a consistent group of bacteria in the plume suggests that while these various microorganisms all play a role in hydrocarbon degradation, studies have not revealed one specific individual group of microorganisms that is consistently present throughout the entire degradation of hydrocarbon (Redmond and Valentine 2011).

While studies indicate the effectiveness of microbes in degrading hydrocarbons, these results do not demonstrate that microbes are able to completely remediate the damage of the oil spill in the Gulf of Mexico. For one, many of these studies were carried out in artificial environments (Edward et al. 2011). Thus, certain factors in the actual environment may alter the behavior of these microbes. Most importantly, while specific groups of hydrocarbon-degrading bacteria have been identified, further study is needed to understand the specific role of each group in responding to an oil spill.

References