Potential for Microbially Mediated Natural Attenuation of Diluted Bitumen on the Coast of British Columbia (Canada)

Abstract

Western Canada produces large amounts of bitumen, a heavy, highly weathered crude oil. Douglas Channel and Hecate Strait on the coast of British Columbia are two water bodies that may be impacted by a proposed pipeline and marine shipping route for diluted bitumen (dilbit). This study investigated the potential of microbial communities from these waters to mitigate the impacts of a potential dilbit spill. Microcosm experiments were set up with water samples representing different seasons, years, sampling stations, and dilbit blends. While the alkane fraction of the tested dilbit blends was almost completely degraded after 28 days, the majority of the polycyclic aromatic hydrocarbons (PAHs) remained. The addition of the dispersant Corexit 9500A most often had either no effect or an enhancing effect on dilbit degradation. Dilbit-degrading microbial communities were highly variable between seasons, years, and stations, with dilbit type having little impact on community trajectories. Potential oil-degrading genera showed a clear succession pattern and were for the most part recruited from the “rare biosphere.” At the community level, dispersant appeared to stimulate an accelerated enrichment of genera typically associated with hydrocarbon degradation, even in dilbit-free controls. This suggests that dispersant-induced growth of hydrocarbon degraders (and not only increased bioavailability of oil-associated hydrocarbons) contributes to the degradation-enhancing effect previously reported for Corexit 9500A. IMPORTANCE Western Canada hosts large petroleum deposits, which ultimately enter the market in the form of dilbit. Tanker-based shipping represents the primary means to transport dilbit to international markets. With anticipated increases in production to meet global energy needs, the risk of a dilbit spill is expected to increase. This study investigated the potential of microbial communities naturally present in the waters of a potential dilbit shipping lane to mitigate the effects of a spill. Here we show that microbial degradation of dilbit was mostly limited to n-alkanes, while the overall concentration of polycyclic aromatic hydrocarbons, which represent the most toxic fraction of dilbit, decreased only slightly within the time frame of our experiments. We further investigated the effect of the oil dispersant Corexit 9500A on microbial dilbit degradation. Our results highlight the fact that dispersant-associated growth stimulation, and not only increased bioavailability of hydrocarbons and inhibition of specific genera, contributes to the overall effect of dispersant addition.

Metagenomic and metatranscriptomic responses of natural oil degrading bacteria in the presence of dispersants

“…However, all these studies, including this one, present some level of variation between successional patterns over time which reinforce the hypothesis that early and late microbial community assemblages are influenced by both deterministic and stochastic processes (Dini‐Andreote et al, ). However, when putting these taxonomic profiles in context with their corresponding oil degradation rates (Tremblay et al, ) Schreiber et al ., ), it appears that regardless of which oil degrading microbes are present, oil is being degraded.…”

“…West coast sampling and microcosm experiments are described in a submitted manuscript by our group (Schreiber et al ., ). To provide context for the current article, excerpts from the Schreiber et al .…”

“…There is also a proposal to transport diluted bitumen (a heavy tar mixed with light oils) in a pipeline from northern Alberta to Kitimat and then ship this diluted bitumen down the Douglas Channel to access the Pacific Ocean. To evaluate the environmental effects of the aforementioned proposals, we applied 16S rRNA gene amplicon sequencing (Schreiber et al ., ) and shotgun metagenomic and metatranscriptomic analyses (this study) to evaluate the natural microbial community response following hypothetical crude oil spills or various blends. The biodegradation of physically and chemically dispersed crude oil was monitored by conducting microcosm studies using summer and winter seawater freshly procured from three different locations on each coast.…”

Influence of Climatic Parameters on Changes in the Density and Viscosity of Diluted Bitumen after a Spill

“…However, since no nutrients were added to the sea water for our weathering studies, low rates of biodegradation of the tested oil products were expected based on the results of previous studies [34,35]. Of the climatic parameters assessed it has been reported that temperature affects the rates of biodegradation of oils [36][37][38]. A recent study suggests that the biodegradation of AWB is limited in cold water (ca.…”

Effects of pollution on marine organisms

“…The authors noted a significant shift to smaller oil droplets, contributing this observation to the energetic movement of copepod appendages when swimming and feeding (mechanically separating oil droplets), and to the ingestion/egestion of oil after being mistaken for prey. Schreiber et al (2019) focused on microbial degradation of diluted bitumen under various conditions (i.e. summer,…”