Composting is gaining attention as a technology for treating solid organic waste in an environmentally friendly and economical manner, and it is also useful for producing high-quality organic fertilizers. This technology has been used since ancient times, even before the existence and role of microorganisms were recognized. As time progressed, it became clear that microorganisms played a central role in composting. Since the latter half of the twentieth century, controlled composting technologies based on microbiological knowledge have been developed. In recent years, large-scale commercial composting plants that apply this knowledge have been operated, contributing to sustainable waste disposal and resource recycling. CompostingCompostingBiohydrogen is generally classified into four major stages: the mesophilic phase, the thermophilic phase, the cooling phase, and the maturation phase. Each stage involves various microbial communities that perform diverse reactions. In particular, the thermophilic phase (the second stage) holds significant importance in the compostingComposting process. During this phase of typical compostingComposting, the internal temperatures of the compost pile reach 60–70 °C or higher, enabling the efficient decomposition of complex biological macromolecules. This significantly reduces waste volume and promotes the extinction of pathogenic microorganisms and weed seeds, making this stage indispensable for producing hygienic and safe fertilizers. ThermophilesThermophiles are responsible for reactions during the thermophilic phase, and a wide variety of these bacteria have been isolated from compost. In the compostingComposting plant we operate, a notable feature is that the temperature at the center of the compost exceeds 80 °C, suggesting the presence of extreme thermophilesThermophiles. We have isolated novel extreme thermophilesThermophiles, Calditerricola satsumensis YMO81T and Calditerricola yamamurae YMO722T, from this compost, and it has been confirmed that these strains can grow at temperatures as high as 80 °C. These bacteria are actively involved in the early stages of the thermophilic phase but have relatively poor toleranceTolerances to lower temperatures. However, they are also present in the final product of the compostingComposting process, and how they survive the nonthermophilic phase of compostingComposting remains unclear. Furthermore, to evaluate the impact of high-temperature processes on the quality of the final product, we conducted experimental compostingComposting using Alternanthera philoxeroides, a highly reproductive aquatic plant. The final compostingComposting products were mixed with soil, and cultivation tests were conducted. The results showed that the aquatic plants did not regenerate and died. These results suggest that aquatic plants, pathogenic microorganisms, and weed seeds are inactivated in high-temperature environments. Therefore, sufficiently high temperature processes are extremely important for safe compostingComposting.