Congrats to Avishek Dutta for his new paper “Detection of sulfate-reducing bacteria as an indicator for successful mitigation of sulfide production” currently available as an early view in Applied and Environmental Microbiology. This was intended to be the second of two papers on a complex experiment that we participated in with BP Biosciences, but the trials and tribulations of peer review led this to be the first. We’re pretty excited about it.
Here’s the quick background. When microbes run out of oxygen the community turns to alternate electron acceptors through anaerobic respiration. One of these is sulfate, which anaerobic respiration reduces to hydrogen sulfide. In addition to smelling bad hydrogen sulfide is pretty reactive and forms sulfuric acid when dissolved in water. For industrial processes this is a problem. Sulfide can destroy products, inhibit desired reactions, and corrode pipes and equipment. To make matters worse, sulfate reducing bacteria (SRBs: those microbes that are capable of using sulfate as an alternate electron acceptor) can form tough biofilms that are hard to dislodge.
One way of dealing with undesired SRBs is to fight biology with biology and add a more preferential electron acceptor. Oxygen would of course work really well, but it typically isn’t feasible to implement oxygen injection on a really large scale. However, nitrate also works well. If nitrate is abundant nitrate reducing bacteria (NRBs) will outcompete SRBs for resources (e.g., labile carbon). Great! Now here’s the challenge… adding massive quantities of nitrate salts is expensive and likely has it’s own ecologically and environmental consequences. So we’d like to do this judiciously, adding just enough nitrate to the system to offset sulfate reduction. But how to know when you’ve added enough? In a really big system (like an oil field) the sulfide production can be happening very far from any possible sampling site so simply measuring the concentration of hydrogen sulfide doesn’t help much. But we can learn some useful things by monitoring the microbial community in the effluent.
The figure above is a schematic of the formation and decay of the biofilm before, during, and after mitigation. In our study the biofilm was presumed to be sulfidogenic and the mitigation strategy was addition of nitrate salts, but the concept applies equally well to any biofilm and any mitigation strategy. The trick – and this is one of those things that seems painfully obvious after the fact but not before – is that you’re looking for the thing you’re mitigating to appear in the effluent. Although this might seem to suggest increased abundance in the system, it actually represents decay of the biofilm and loss from the system. To take this a step further we used paprica to predict genes in the effluent and then identified anomalies in the abundance of genes involved in sulfate reduction. This anomalies provide specific markers of successful mitigation and a means to a general strategy for monitoring the effectiveness of mitigation.
