# _microorganisms_
## 1 Introduction  
Many microorganisms, including cyanobacteria, actinomycetes, and certain fungi, produce malodorous compounds [1]. In water reservoirs, several species of cyanobacteria produce earthy- or musty-smelling compounds as secondary metabolites [2; 3; 4]. The main cyanobacterial odorants affecting the quality of drinking water in reservoirs are 2-methylisoborneol (2-MIB) and geosmin [5; 6]. The odor threshold value for both is \(<\)10 ng L\({}^{-1}\), but their presence is often observed at 5 ng L\({}^{-1}\)[7]. Although neither 2-MIB nor geosmin poses a serious health threat to humans or aquatic animals, the perception of drinking water has raised suspicions among consumers about drinking water safety and led to complaints [8]. Worldwide, reports on cyanobacteria associated with 2-MIB and geosmin in drinking water have gradually increased over the last three decades, with more than 30 reports each [9; 10]. Paldang Lake, fed by the Han River, is the largest (storage capacity of -244 \(\times\) 10\({}^{6}\) tons) drinking water reservoir in South Korea [11]. It is located near the capital city of Seoul and serves as the principal water source for more than 25 million people (50% of the population). In this reservoir, the drinking water monitoring standard for odorant compounds is 20 ng L\({}^{-1}\), and analyses of cyanobacterial numbers and unpleasant malodorous compoundsare conducted once weekly, as mandated by the Korean Ministry of Environment in South Korea in 1998. The first report of a cyanobacterial bloom occurred in 2012 when the concentration of geosmin was high; in the fall of 2018, the occurrence of 2-MIB was also high [12]. Since then, algal bloom events have occurred several times, resulting in an increase in the number of earthy/musty odor complaints in the drinking water from Paldang Lake [13]. These odorant events mainly occurred upstream of the North Han River and Paldang Lake. 2-MIB and geosmin are difficult to remove because they are extremely stable and slowly degrade naturally in a water environment or by boiling. In addition, conventional drinking water treatment methods such as chlorine, ozone, coagulation-sedimentation, and rapid sand filtration are ineffective in removing these substances [4; 14; 15; 16]. Therefore, an accurate early analysis of 2-MIB/geosmin-producing cyanobacteria would contribute to better management of drinking water resources. Cyanobacterial species, including those responsible for odorant production, are typically classified and enumerated using microscopy, and odorant concentrations such as of 2-MIB/geosmin are determined using gas chromatography (GC). However, for monitoring purposes, microscopy-based methods are time-consuming and require specially trained personnel because it is difficult to distinguish between odorant producers and non-producers [17]. While GC can provide accurate quantification of odorants in water, it offers no information about their producers at the source and requires specialized equipment [5]. The discovery of the 2-MIB and geosmin synthase genes _mic_ and _geo_ have enabled the development of a quantitative PCR (qPCR) approach to rapidly detect even low gene copy numbers and enable early on-site detection of the responsible cyanobacteria [8; 18]. This approach requires specific primers that are well-designed based on _mic_ and _geo_ sequence information and are applicable to fielding samples from various geographical locations [9]. There may be regional differences within a single species of cyanobacteria. Therefore, it is necessary to establish a qPCR-based analytical method. In addition, because several cyanobacteria can exist in water resources and produce odorants (such as 2-MIB and geosmin), it is necessary to analyze and characterize odorant-producing species simultaneously. However, field studies on 2-MIB/geosmin-producing cyanobacteria using qPCR are lacking in Asia, including Korea, and the existing applications are aimed at detecting only one odorant producer.