Thermal stability of amino acids in siliceous ooze under alkaline hydrothermal conditions

Hydrothermal systems have been suggested as being favourable environments for the origin and evolution of life on the primitive Earth. To test this hypothesis, it is necessary to investigate under hydrothermal conditions the behaviour of biomolecules such as amino acids (AAs), a major component of organisms. In contrast to submarine hydrothermal systems, hot springs in the Rift Valley in eastern Africa on the thick continental crust often have a high pH (alkaline conditions) as a result of enrichment via Na2CO3 and volatile gases. We reacted siliceous ooze with an aqueous solution of NaCl and Na2CO3 at elevated temperature (100-300 °C) to evaluate the thermal stability of the AAs under alkaline hydrothermal conditions. The AAs in the sediment in peptide form were released to the liquid phase and decomposed through hydrolysis of peptide bonds. Comparison of the results with those from similar experiments using the same sediment sample under neutral conditions reveals that the rates of decomposition of the AAs are significantly inhibited under alkaline conditions. Moreover, AAs remained in both the solid and liquid phases even after heating at 300 °C for 240 h. Our results indicate that AAs are more thermally stable in alkaline solution, indicating that these hydrothermal conditions are more favourable for the evolution of primitive life on the thick continental crust. They also imply that alkaline hydrothermal systems on other planets might be plausible places for extraterrestrial life.