In this thesis, using Tetrahymena and microbial community as studying organisms, we carried out some research by isothermal microcalorimetric method as follows: 1. Power-time curves of six strains of Tetrahymena were determined by microcalorimetry. Power-time curves and metabolic characteristics were significantly different among Tetrahymena strains, which indicated that they could be the noticeable characteristics in distinguishing the different strains of Tetrahymena. Their Euclidean distances and clustering analysis diagram could be obtained by metabolic characteristics. The results obtained by them were consistent with those obtained by first internal transcriptional spacer (ITS1) gene sequences. And heat production in the increasing period (QLog) was maybe the most specific characteristic of Tetrahymena strains. 2. The endogenous metabolic processes of Tetrahymena cells were monitored by microcalorimetry. Their typical power-time curves showed a decreasing trend on the whole. It was found that total heat production (Qt) and maximum heat output (Pm) decreased significantly with the decrease of cell density. However, cell density and the mixture of different cells did not influence significantly the maximum heat output per cell and metabolic decrease rate. 3. Power-time curves and metabolic properties of T. thermophila BF5 exposed to different La3+, Yb3+ and TBT levels were studied by microcalorimetry. Metabolic increase rate (r) decreased significantly while peak time (PT) increased with the enhancement of pollutant levels. Compared with the measurements of some biomarkers, cell counting and protozoa community toxicity test, microcalorimetry was sensible, easy to use, reproducible, and could provide the responses of the whole cells to pollutants. As a result, the microcalorimetric assay of T. thermophila would be useful for the toxicity evaluation of pollutants. 4. Power-time curves and metabolic characteristics of the native and concentrated microbial communities were determined at four sampling stations with different eutrophic status in Lake Donghu. The values of metabolic parameters were higher at the more eutrophic stations. And metabolic parameters were in good agreement with water chemical variables and structural parameters of microbial communities. Therefore, microcalorimetry has a great potential to be used in microbial biomonitoring.