Deep marine oil spills: Oil-particles-dispersants interaction and impact on oil biodegradation

The world still runs on oil and such an oil-based society with a yearly increase in global demand urge to explore new oil sources. As a result, oil exploration and extraction have been moved to the deeper ocean marine environments. Oceans are a vital part of the ecosystem, contain phytoplankton and other crucial microorganisms responsible for CO2 absorption, O2 production, the global carbon cycle, and global climate control. While technologies for oil exploration and extractions in more remote and hard-to-reach locations such as deep marine environments have been developed, technologies for potential oil spill mitigation and clean-up remained under-developed and relied on the knowledge, experiences, and technologies adjusted for surface water oil spills. As a result, mitigation and cleaning up the deep marine oil spills often occur by unadjusted methods and technologies, which can cause unforeseen long-term negative impacts on the oceans and disturbing their performance in ecological and many other ecosystem functions.

One example of a deep marine oil spill was the Deepwater Horizon (DWH) oil spill, which occurred 1500 m below the water surface in the Gulf of Mexico in 2010. The clean-up technology applied for this first very deep marine oil spill was the injection of Corexit, an oil chemical dispersant, at the oil wellhead. Chemical dispersants have been used frequently for mitigating surface water oil spills. The application of dispersants to break down the oil into smaller droplets is based on the formation of smaller oil droplets that more readily dissolve into waters, and the dissolved oil compounds are then biodegraded rapidly by oil-degrading bacteria and other microorganisms. Injection of Corexit at the DWH wellhead as well as to the water surface kept the spilled oil in the water column and reduced the pollution of the shorelines. However, in the aftermath of the DWH and researching the consequences to the marine ecosystem, it was discovered that a significant portion of the oil ended up at the seafloor through unknown or not well understood processes and mechanisms. This lack of understanding was the main driver for setting up a research program called C-IMAGE (Center for Integrated Modeling and Analysis of Gulf Ecosystems) on the fate and effects of such deep-water oil spills, which this PhD research was part of it.

This PhD thesis offers a broad picture and aims at building a step-wise understanding of some critical processes determining the fate and biodegradation of oil in deep marine oil spills and the influence of chemical dispersants in each step. The DWH oil spill was taken as the practical example to refer to in our studies, which are described in six chapters.

Chapter 1 (the introduction) provides a brief overview of the state of the art knowledge of oil spills history, oil composition and compound's behavior in water, weathering processes, clean-up technologies, and compares surface and deep oil spill scenarios. Chapters two to five provide results and discussions of specially designed laboratory experiments to allow a step-wise elucidation of the fate and biodegradation of oil in deep marine spills in the absence and presence of chemical dispersants. In these experiments, the field conditions during and after the DWH spill were mimicked as far as possible, using several sets of batch and microcosms experimental systems. The batch experiments were at the scale of 10–100 milliliters, dedicated to oil compound's dissolution by Corexit, interactions with mineral and organic particles, and impacts on biodegradation of oil compounds. The microcosms experiments were performed in microcosm systems at 10–1000 liters scale dedicated to oil-organic particle-biota interactions mimicking conditions in the water column and in and at the seafloor.

In Chapter 2, biodegradation of oil compounds by two bacterial cultures (n-alkane and aromatic degraders) in the absence and presence of Corexit with various dispersants to oil ratios was studied. Our results show that the presence of Corexit does not in all cases enhances biodegradation of (all or some) oil compounds. When the Corexit was at the highest application concentration, the biodegradation was inhibited, especially when only the n-alkane degrading bacterial culture was present. However, with two bacterial cultures, the inhibition was not effective after ten days. This indicates that the initial inhibition of oil biodegradation can be overcome when different bacteria are present in the environment. We conclude that the observed inhibition is related to the enhanced dissolution of aromatic compounds into the water, inhibiting the n-alkane degrading bacteria.

In Chapter 3, the biodegradation of oil compounds was studied in batch experiments, while the oil interacted with marine snow, clay particles, and Corexit. Marine snow are organic particles produced by algae and plankton in the upper layers of the ocean water column, a natural process essential for the whole functioning of the marine ecosystem. During the DWH oil spill and application of chemical dispersants, excessive production of marine snow was observed in the field. Results of our lab experiments show that the presence of marine snow particles enhances oil biodegradation. On the other hand, the presence of Corexit alone or in combination with organic or mineral particles (clay) hampers oil biodegradation. Clay and Corexit have a synergistic effect in increasing the dissolution of benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds in the water and cause a delay in biodegradation of oil compounds through increasing the dissolution of toxic oil compounds. However, the delay in recovery of the biodegradation was reduced by the presence of marine snow particles that adsorbed toxic BTEX compounds from the water phase.

In Chapter 4, the effect of marine snow on the biodegradation of oil compounds in the sediment layer is described based on the results from microcosm experiments. These results show that the presence of marine snow reduces the depletion of oil n-alkanes by 40% on top sediments due to the preferred biodegradation of marine snow organics. Biodegradation of marine snow reduces the oxygen concentration in the sediment layers, resulting in a lower biodegradation of oil compounds in the sediment.

In Chapter 5, invertebrates were included in the microcosm experiments to study the effects of settled oil-associated marine snow. Benthic invertebrate survival and behavior were investigated and compared to the biodegradation of oil compounds in the presence and absence of invertebrates. Bioturbation activities by invertebrates strongly contribute to the oxygenating and nutrient cycling in the sediment layers and ease the aerobic biodegradation. Sedimentation of oil disturbs the benthic ecosystem by slowing down the bioturbation activity and inhibits the biodegradation of the oil compounds in the sediment. The benthic system itself is affected and has a reduced ability to restore conditions favourable for the biodegradation of oil compounds at the seafloor, presumably leading to long-term impacts on sediment ecosystem functioning through the oil at the seafloor.

In Chapter 6, the results from previous chapters are discussed and placed in a broader context. In short, deep marine injection of chemical dispersants in the DWH oil spill protected the shorelines and their habitats from oil pollution. However, the side effect is that this clean-up response puts the deep parts of the marine ecosystem in danger. The most pronounced effects of such an approach are the intensive oil dissolution in the water column, oil components toxicity and stress impulses to the marine (micro) organisms, excessive formation of marine snow, and scavenging of oil particles from the water column to extensive oil sedimentation towards the seafloor. This new understanding generated in our studies suggests that it may be better to design clean-up strategies that are so-called "nature-based", which means using naturally occurring processes that support the self-healing capacities of the environment. This would also entail no or very limited use of chemical dispersants, and if these are used, it should be done with a full understanding of the effects of these dispersants on the complete deep marine system. In this way, seafloor pollution and long-term consequences on the marine ecosystem will be diminished.

We need to improve our understanding of deep marine systems and their response to oil spills much further. The various environmental conditions and processes are far from completely elucidated, and this is needed for a careful evaluation of scenarios for the specific environmental conditions that may dictate the oil spill behavior. This knowledge is required to further minimize damages from future deep oil spills. Though limiting petroleum oil consumption and transition to a non-fossil economy is a fundamental and final solution for oil pollution, however, inevitably, we will still live for decades in a society that will use fossil oil as an energy and chemical resource. If we transform to a renewable and biobased economy, biofuels and bio-lubricants may gradually replace fossil oils; hence, oily compounds are likely to stay in the world's economy. This pleads for budgets to continue this type of research also when the full attention of the global society is focused on climate mitigation and the energy transition. Such research is highly needed to offer the knowledge base to protect our seas and oceans during the coming transition decades and thereafter.