Thermal stability of amino acids in seafloor sediment in aqueous solution at high temperature

As an essential component of living organisms amino acids (AAs) and their distributions are important for quantifying the extent of the biosphere. Calcareous sediments were heated with NaCl solution in an airtight vessel at elevated temperature (100-300 °C) to estimate the thermal stability of the AAs in seafloor hydrothermal systems. The concentration of total hydrolysable amino acids (THAAs) decreased with time and temperature as AAs were lost from the solid to the liquid phase and decomposed. Consequently, THAAs in the liquid phase increased at 100 and 120 °C and during first 24 h at 150 °C and 9 h at 200 °C, and then started to decrease gradually. No AAs could be detected in the liquid phase after 240 h reaction at 250 and 300 °C. The proportions of decomposed AAs at 100, 150, 200 and 300 °C were 23.5%, 62.7%, 90.1% and 99.7%, respectively. The upper temperature limit for the stable presence of AAs probably lay between 150 and 200 °C. Our results demonstrate that AAs cannot be synthesized or survive in hot hydrothermal waters at temperatures higher than 250 °C, unless a solid phase such as clay minerals protects them significantly, and that a temperature of about 150 °C is optimal for the survival of microorganisms in natural submarine hydrothermal systems.