Sugar synthesis in prebiotic oceans: towards the formation of prebiotic RNA

Abstract

Sugars are essential molecules in the metabolism and replication of all extant life. Ribose, a five-carbon sugar, is the sole sugar component of RNA, which is regarded as the biopolymer that supported both replication and metabolism of the first life based on the RNA world hypothesis. The origin of prebiotic RNAs and their components is one of the most critical problems in the RNA world hypothesis. Thus, ribose has been regarded as a particularly important sugar in the origin of life. Extraterrestrial delivery via meteorites is one of the possible sources of ribose on Hadean Earth. Ribose could also be formed on prebiotic Earth from aldehydes by formose reaction. In previous studies, its formation has been limited under alkaline conditions, but under alkaline conditions, product ribose is quickly consumed in the subsequent reactions, although borate has been proposed to mitigate this consumption. Thus, Hadean environments where ribose was available remain unclear. One potential environment that has not been investigated is the open ocean, which potentially had very low ribose yields. We quantitatively evaluated the effects of temperature, pH, and abundance of Ca and glycolaldehyde on the formation of ribose by a sensitive analysis using gas chromatography-mass spectrometry. Among the different chemical conditions, high pH, high glycolaldehyde concentrations, and high Ca abundance substantially increased the yields of ribose. They also increased the reaction rates of the formose reaction. This means that a neutral solution is not suitable for ribose synthesis. However, even in a neutral solution containing diluted formaldehyde and glycolaldehyde, the formation of ribose was confirmed due to the sensitive analysis of this experiment. The quantitative effects of the chemical conditions shown in this study suggest that ribose continuously formed in a near-neutral Hadean open ocean. Although its yield was low, reactions in the vast amount of oceanic water might have served a substantial amount of ribose on prebiotic Earth. Further quantitative investigations on the consumption rate of ribose in the open ocean and other possible sources, such as carbonaceous chondrites and alkaline borate-rich environments, will allow us to evaluate which was the primary source for the prebiotic RNA sugar.