Importance of thermodynamics dependent kinetic parameters in nitrate-based souring mitigation studies.

Souring is the unwanted formation of hydrogen sulfide (H2S) by sulfate-reducing microorganisms (SRM) in sewer systems and seawater flooded oil reservoirs. Nitrate treatment (NT) is one of the major methods to alleviate souring: The mechanism of souring remediation by NT is stimulation of nitrate reducing microorganisms (NRM) that depending on the nitrate reduction pathway can outcompete SRM for common electron donors, or oxidize sulfide to sulfate. However, some nitrate reduction pathways may challenge the efficacy of NT. Therefore, a precise understanding of souring rate, nitrate reduction rate and pathways is crucial for efficient souring management. Here, we investigate the necessity of incorporating two thermodynamic dependent kinetic parameters, namely, the growth yield (Y), and FT, a parameter related to the minimum catabolic energy production required by cells to utilize a given catabolic reaction. We first show that depending on physiochemical conditions, Y and FT for SRM change significantly in the range of [0-0.4] mole biomass per mole electron donor and [0.0006-0.5], respectively, suggesting that these parameters should not be considered constant and that it is important to couple souring models with thermodynamic models. Then, we highlight this further by showing an experimental dataset that can be modeled very well by considering variable FT. Next, we show that nitrate based lithotrophic sulfide oxidation to sulfate (lNRM3) is the dominant nitrate reduction pathway. Then, arguing that thermodynamics would suggest that S° consumption should proceed faster than S0 production, we infer that the reason for frequently observed S0 accumulation is its low solubility. Last, we suggest that nitrate based souring treatment will suffer less from S0 accumulation if we (i) act early, (ii) increase temperature and (iii) supplement stoichiometrically sufficient nitrate.

A novel relationship for the maximum specific growth rate of a microbial guild.

One of the major parameters that characterizes the kinetics of microbial processes is the maximum specific growth rate. The maximum specific growth rate for a single microorganism (${\mu _{max}}$) is fairly constant. However, a certain microbial process is typically catalyzed by a group of microorganisms (guild) that have various ${\mu _{max}}$ values. In many occasions, it is not feasible to breakdown a guild into its constituent microorganisms. Therefore, it is a common practice to assume a constant maximum specific growth rate for the guild ($\acute{\mu}_{max}$) and determine its value by fitting experimental data. This assumption is valid for natural environments, where microbial guilds are stabilized and dominated by microorganisms that grow optimally in those environments' conditions. However, a change in an environment's conditions will trigger a community shift by favoring some of the microorganisms. This shift leads to a variable ${\acute{\mu}_{max}}$ as long as substrate availability is significantly higher than substrate affinity constant. In this work, it is illustrated that the assumption of constant ${\acute{\mu}_{max}}$ may underestimate or overestimate microbial growth. To circumvent this, a novel relationship that characterizes changes in ${\acute{\mu}_{max}}$ under abundant nutrient availability is proposed. The proposed relationship is evaluated for various random microbial guilds in batch experiments.