3. Results and discussion
Anthropogenic activities have led to metal pollution in the water bodies as it threatened some of the aquatic Fatimah Hashim et al. / Procedia Environmental Sciences 30 ( 2015 ) 15 – 20 17 organism and this includes the free-living amoebae. Decreasing population of amoebae due to the presence of heavy metal will leads to imbalance of the natural ecosystem as amoebae act as predator for bacteria. The presence of heavy metal in the aquatic environments makes them a suitable toxicant to be evaluated for their effects towards amoebae. To date, there are only a few reports on cytotoxic and genotoxic effects of heavy metal on amoebae like cadmium, mercury, lead and zinc on these free-living amoebae [7]. However, there are no study focusing the effect of Mn on Acanthamoeba. Therefore, it is important to evaluate the effect of Mn towards Acanthamoeba as it would indicate the presence of exceed level of Mn in aquatic environment. 3.1 Determination of Mn Fifty percent inhibition concentration (IC50) on Acanthamoeba sp. In this study, Acanthamoeba sp. were exposed to various concentrations of Mn i.e 0, 30, 60, 90, 120, 150 and 180 ppm for 24 hours at 30qC in a 96-well plate to determine the cytotoxicity of the metal. The MTT assay was carried out to assess the viability of Acanthamoeba sp. Fig.1 shows the graph of viability of cell against different concentrations of Mn. Based on the graph, the IC50 value obtained was 24 ppm and Acanthamoeba sp. showed the reduction in viability upon exposure to increasing in Mn concentration. Fig 1. Percentage of viability of Acanthamoeba sp. cells assessed by MTT assay exposure to Mn for 24 hours. IC50 value obtained was 24 ppm IC50 value was obtained by using MTT assay with a slight modification on its protocol. After the cells were treated with Mn at varying concentrations for 24 hours at 30o C in a 96-well plate, the medium in the plate was discarded. The plate was washed with PBS solution 3 times before loading the MTT solution. This is to ensure that the glucose content in the medium was completely removed. Presence of glucose in the medium will reduce the efficiency of MTT reaction. MTT specific activity are reduced by cells that are extensively metabolized D-glucose [11]. Cytotoxicity of Mn on Acanthamoeba sp. was determined based on the percentage of viable Acanthamoeba sp. after exposure to Mn for 24 hours at 30o C. In general, the viability of Acanthamoeba sp. decreased with increasing concentrations of Mn (Fig. 1). This shows that the growth of the cells was affected once exposed to Mn. The concentration of Mn that inhibit 50% of the cell population was 24 ppm. The lower IC50 of Mn shows that it has higher toxicity to Acanthamoeba sp. compared to Zn which showed an IC50 of 39 ppm [7]. This shows that Mn have higher toxicity compares to Zn since the IC50 of Mn was lower than Zn. Based on the periodic table of element, Mn have a less stable electron configuration which is [Ar]4s2 3d5 compares to Zn with electronic configuration of [Ar]4s2 3d10. This is supported by Martin-Gonzalez et al. [12] by stating that toxicity of heavy metals decrease with increased stability of electron configuration. In addition, heavy metals have high attraction to negatively-charged group of protein and thiol group [13]. Any alteration that occurs in these proteins will lead to dysfunction of certain enzymes in which may affect certain metabolic reactions in the cell. This phenomenon may have also occurred in Acanthamoeba sp.