Pseudokirchneriella subcapitata is a sickle-shaped freshwater green microalga that is normally found in unicellular form. It is the best known and most frequently used species of ecotoxicological bioindicator because of its high growth rate and sensitivity to toxicants. Metronidazole (MTZ) is a routinely used nitroimidazole antibiotic that has caused environmental issues owing to incorrect use. A toxicity test was performed in order to understand the relationship between the MTZ concentrations and response at a physiological level. The study found a growth percentage of (0, 4.8571, 4.5714, -15.1429, -37.1429 %) accordingly. The changes on the transcriptomic level were tested by performing a RT-qPCR. Using ∆∆Ct method to compare the treated samples with low and high MTZ concentration against the control sample. The study found that Exposure to MTZ at the low and high concentrations gave rise to 1.45 fold upregulated pcna gene expression that was differentially expressed in control R. subcapitata. The high group of samples in the high group were clearly distinguishable from those in the control and low treatment groups.