## 1 Introduction
[15] observed a significant seasonal effect on EBPR in a wastewater treatment plant with poor EBPR efficiency during the summer, when the wastewater temperature was approximately 30 \({}^{\circ}\)C. At water temperature states less than 10 \({}^{\circ}\)C, EBPR was reported to be inhibited [16; 17]. In some other studies [18; 19], better EBPR performances were observed at lower temperature (5 \({}^{\circ}\)C) than at higher temperatures (10 and 15 \({}^{\circ}\)C). Similarly, EBPR efficiency has been reported to be improved at high temperatures (20-37 \({}^{\circ}\)C) in some studies [17; 20], whereas better P removal efficiency was observed at lower temperatures (5-15 \({}^{\circ}\)C) in the other studies [21; 22]. In the study performed by [PERSON] et al. [23], the PAOs were found to be lower-range mesophiles, or perhaps psychrophiles, and they predominated at 20 \({}^{\circ}\)C or possibly lower. The GAOs were somewhat mid-range mesophilic organisms with an optimum temperature between 25.0 \({}^{\circ}\)C and 32.5 \({}^{\circ}\)C. [PERSON] and [PERSON] [24] observed PAOs were dominant at 20 \({}^{\circ}\)C in an anaerobic/aerobic (A/O) sequencing batch reactor (SBR) as a result of their higher anaerobic acetate uptake rates and aerobic biomass yields compared to GAOs. However, at 30 \({}^{\circ}\)C, GAOs were able to outcompete PAOs in the A/O SBR because of their higher anaerobic acetate uptake rates than PAOs. [PERSON] et al. [25] reported that the influence of temperature shock on PAO abundance was more serious than that of GAOs in an enriching process. Following temperature shock from 22 \(\pm\) 1 \({}^{\circ}\)C to 29 \(\pm\) 1\({}^{\circ}\)C and then to 14 \(\pm\) 1 \({}^{\circ}\)C, GAOs were more temperature-adaptive and grew better than PAOs at 14 \(\pm\) 1 \({}^{\circ}\)C.  
There are some studies concerning the toxic effects of metal ions on phosphorus removal. However, how temperature variation influences the tolerance of PAOs and GAOs biomasses to metal ion toxicity is unknown. The long-term temperature effect is usually studied to examine the competition between PAOs and GAOs in activated sludge. On the contrary, studying the short-term temperature effect on a successful EBPR system can make the temperature effect more clear on the exact metabolisms of PAOs based on minimum interference of the GAO population. As we know, increasing temperature could speed up the metabolism of microorganisms. However, it is also possible to enhance the toxic effects of heavy metals on microorganism metabolisms. Temperature variations and toxic effects of heavy metals are possibly encountered simultaneously in wastewater treatment plants and easily lead to the failure of biological process, resulting in an unstable status of EBPR system in practice. The metabolisms of biological phosphorus removal systems are quite a bit more complicated than biological carbon and nitrogen removal systems. The reasons for operational instabilities of EBPR, and how to control the system well, are still unknown. Especially, the paths in microorganism metabolisms that are easily affected by temperature and toxicity factors are not understood yet. Temperature and toxicity level variations might need to be considered together to completely understand their influences on biological phosphate removal mechanisms, especially the influence of the interaction between temperature and metal toxicity. Thus, the main purpose of this study was to study how temperature variation affects PAO tolerance to a toxic environment in the presence of copper ions, which is a very popular metal element used in real life and industries. The effluent standard of copper in wastewater is 3 mgL\({}^{-1}\) in Taiwan, indicating that a copper level higher than 3 mgL\({}^{-1}\) probably occurs in raw wastewater. And, the preliminary study (data not shown) showed low levels of copper ions could affect PAO behavior. Thus, this study chose 0-2 mgL\({}^{-1}\) as the studied levels.