Congrats to Avishek Dutta for his first publication in the Bowman Lab: Understanding Microbial Community Dynamics in Up-Flow Bioreactors to Improve Mitigation Strategies for Oil Souring. Avishek did a remarkable job of resurrecting a stagnant dataset and turning it into a compelling story.
What is oil souring anyway? When oil is extracted in a production field the pressure of the field drops over time. To keep the pressure up the oil company (or more accurately, the subsidiary of the subsidiary tasked with such things) pumps in water, which is frequently seawater. When the water comes back out through wells there are two options. The most economical thing is to release it back into the environment, which has obvious negative consequences for environmental health. Alternatively, the same water can be reused by pumping it back into the ground. The downside to this is that recycled well water typically induces the production of hydrogen sulfide by sulfate reducing bacteria. The hydrogen sulfide reacts with the oil (“souring” it) and creates its own set of environmental and occupational hazards.
The oil industry has spent quite a bit of effort trying to figure out how to mitigate the process of oil souring. A leading method is to introduce nitrate salts into the system to boost the growth of nitrate reducing bacteria. The nitrate reducers out compete the sulfate reducers for reduced carbon (such as volatile fatty acids) and induce other processes that further impeded sulfate reduction.
Although the basic concept is pretty simple the details of this competition between nitrate and sulfate reducers in oil field aquifers is not well understood. In this study Avishek leveraged samples from up-flow bioreactors, analogs of the oil field aquifer system, for 16S rRNA gene analysis of the bacterial and archaeal communities. The bioreactors are vessels filled with sand, seawater, and sources of bioavailable carbon (the oil itself is a source of carbon, but requires a specialized microbial community to degrade). Some of the bioreactors also contain oil. Water flows continually through the system and nitrate salts can be added at appropriate time-points. For this experiment the nitrate amendment (the mitigation or M phase) was halted (the rebound sulfidogenesis or SG phase) and then restarted (the rebound control or RC phase).
Lots of interesting things emerged from this relatively small-scale experiment. For one thing the oil and seawater samples are not that different from one another during mitigation. However, when the nitrate addition is stopped those two treatments start to diverge, with different sulfate reducing taxa present in each. This divergence (but not necessarily the microbial community) persists after the treatment ends. But not all microbial taxa responded to the rather extreme perturbation caused by the nitrate addition. Desulfobacula toluolica, for example, which should have been out-competed during mitigation, remained a significant member of the community.
We’re currently analyzing the results of a much larger bioreactor study that we expect will shed some new light on these processes, so stay tuned!
