# _microorganisms_
## 3 Results and Discussion
### Application of qPCR-Based Monitoring in Drinking Water Reservoirs  
Our real-time qPCR method was used to quantitatively monitor 2-MIB- and geosmin-producing cyanobacteria in the North Han River, which is the primary drinking water reservoir in South Korea. Water samples were collected in 2020 from three upstream sites, Uiam Dam (UA), Cheongpyung Dam (CP), and Sambong-ri (SB), and one downstream site, Paldang Dam (PD), between July and October, corresponding to a period of high-level cyanobacterial growth. In addition to the copy number of odorous-producing genes determined with the qPCR analysis, water samples were analyzed for 2-MIB and geosmin concentrations (GC-MS) and cyanobacterial cell abundance (microscopy). The three parameters, _mic_ copy number, 2-MIB concentration, and cyanobacterial cell abundance, followed similar trends (Figure 3A). The trends in 2-MIB concentration and _mic_ copy number were nearly identical, and the overall change in the 2-MIB concentration followed the change in the _mic_ copy number more closely than the change in cell abundance. A previous study of three reservoir sites showed that trends in _mic_ abundance closely followed the total 2-MIB concentration in many but not all samples [17]. The trends in 2-MIB concentration and cyanobacterial cell abundance were also roughly similar; however, at several sampling times within the UA and PD sites, both the _mic_ copy number and the 2-MIB concentration were low, whereas the cyanobacterial cell abundance was high. In particular, at the PD site, when the 2-MIB concentration and _mic_ copy number were low and the cyanobacterial cell abundance was high, cyanobacterial species that did not produce 2-MIB were dominant (Table S1). Both _mic_ copy number and 2-MIB concentration increased gradually, beginning in June, at all sites, peaked on 27 July, and decreased thereafter. This pattern is consistent with the increased growth of 2-MIB-producing cyanobacteria during the warm season (in South Korea, from July to October) [31]. In a previous study, the 2-MIB concentration was highest in July, as determined over a nearly 3-year period [31]. A study of the time courses of _mic_ copy number and 2-MIB concentration showed that both were highest in July and August [17]. At all four sampling sites, the _mic_ copy numbers and 2-MIB concentrations were the highest in June and July, ranging from 1 \(\times\) 10\({}^{1}\) to 9 \(\times\) 10\({}^{3}\) copies mL\({}^{-1}\) and 2 to 31 ng L\({}^{-1}\), respectively. Gene analysis of _mic_ in aquatic systems in other regions showed that it was detected in most samples, with amounts ranging from 10\({}^{3}\) to 10\({}^{6}\) copies L\({}^{-1}\) and 2-MIB concentrations from 7.2 to 45.3 ng L\({}^{-1}\), indicating higher gene and 2-MIB concentrations in drinking water than those observed in this study [32]. Another study of drinking water showed lower 2-MIB concentrations ranging from 2.39 to 7.32 ng L\({}^{-1}\)[33]. Figure 3B shows the time course of the _geo_ copy number as determined with qPCR, the geosmin concentration measured with GC-MS, and the cyanobacteria cell abundance determined with microscopy. Similar to with 2-MIB, all three parameters followed a similar pattern during the study period, with the _geo_ copy number and geosmin concentration tending to be higher than the cyanobacterial cell abundance. [PERSON] et al. [8] investigated the geosmin concentration and _geo_ copy number in the Miponga Reservoir, South Australia, and found that the two parameters were closely related, with _geo_ copy numbers ranging from 10\({}^{2}\) to 10\({}^{5}\) mL\({}^{-1}\), similarly to our results. Notably, the _geo_ and cell counts were lower at the PD site, and the geosmin concentrations, as well as the total number of cyanobacteria, were higher in a period dominated by _Anabaena circinalis_ (Table S1), which is known to be a major producer of geosmin [8].