# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 1 Introduction  
In nature, under a wide variety of conditions, organisms have been reported to directly or indirectly mediate the formation of over 60 different mineral types (e.g., carbonates, oxides, silicates, and sulfides) through a process called biomineralization [1]. Carbonates are perhaps the most studied minerals formed by microbes. Microbially induced carbonate precipitation (MICP) can occur as a result of the conventional metabolic pathways, including oxygenic photosynthesis, aerobic respiration, ureolysis, ammonification, nitrate reduction (denitrification), sulfate reduction, iron reduction and methane oxidation [2, 3, 4, 5, 6, 7, 8]. In Table 1, MICP studies exploiting different microbial metabolisms and metabolism-specific biochemical reactions that lead to the CaCO\({}_{3}\) precipitation were given. As seen in the reactions, eachof the metabolisms generates dissolved inorganic carbon and except aerobic respiration all microbial pathways generate alkalinity in the microenvironment of the bacteria. Figure 1 shows that among the metabolic pathways leading to MICP, urea hydrolysis drew significant attention not only in fundamental research describing the MICP mechanism, but it was also the most investigated metabolic pathway to develop new bio-based technologies. The ratio of application-oriented MICP research to fundamental research was considerably low for other metabolic pathways (Figure 1). Therefore, possible advantages of the other metabolic pathways in MICP applications were overlooked. \begin{table}
\begin{tabular}{c c c c} \hline \hline  
In MICP through urea hydrolysis, ureolytic bacteria produces urease enzyme which catalyzes the hydrolysis of urea into carbonate and ammonium, and thus favors an increase in the pH leading to the precipitation of CaCO\({}_{3}\) in the presence of free calcium ions. This approach has been demonstrated in many laboratory studies and several field technical applications, especially in enhanced oil recovery (EOR), heavy metal removal, atmospheric CO\({}_{2}\) sequestration, soil improvement, and construction restoration [9; 10; 11; 12; 13; 14; 15]. Most of the ureolytic bacteria are strictly aerobic organisms live in soil deposits, and they were used as model organisms in a majority of the studies [16]. However, ureolytic bacteria are not ubiquitous in extreme environmental conditions, such as nutrient and O\({}_{2}\) deficiencies, desiccation, high pressures, high pH, and high salt concentrations [13; 17]. For instance, their growth, and the production of urease are inhibited by anoxic conditions. This results in gradually decreasing urease activity and survival rate of the bacteria over time in different MICP applications such as self-healing concrete, soil reinforcement and bi-ogrouting, due to the lack of oxygen in the deeper zones of the concrete crack and sand/soil layers, respectively [18]. Moreover, the utilization of ureolysis for soil reinforcement under the groundwater table and in oil reservoirs (i.e., in an anaerobic environment) becomes unlikely. Furthermore, the side product of ureolysis is ammonia, which is known for its toxicity in aquatic environment [19]. In addition, there are extra costs related to the removal of ammonium in groundwater [8]. Therefore, alternative microbiological processes capable of inducing CaCO\({}_{3}\) precipitation should be tested and considered to determine the most sustainable option for use in different practical applications. [PERSON] et al. [8] evaluated the theoretical feasibility of ureolysis and three alternative MICP processes (i.e., denitrification, aerobic oxidation, and sulfate reduction) for soil reinforcement based on four factors: (i) substrate solubility, (ii) the calcium carbonate precipitation rate, (iii) calcium carbonate yield and (iv) the amount and type of the by-product. Overall, denitrification was determined to be the most suitable MICP method.