The Micro-Cauldron of Life?

May 30th, 2007 Placozoan Posted in News |

THE QUESTION of the origin of the first living organism has fascinated scientists for decades. Currently the RNA world hypothesis, which proposes RNA as the first self-reproducing molecule, is the most promising. One difficulty is that this hypothesis require a high concentration of simple organic molecules, which were probably present in low concentrations in the earliest oceans. Some suggest adsorption onto solid surfaces as one solution to this problem, now a new solution has been proposed.

There are some hypotheses that the earliest life evolved around hot springs in the ocean floor, which now harbor unusual eubacteria and archaea. These hot springs build up mineral mounds around them that are riddled with pores. Baaske and coworkers demonstrate that a simulated hydrothermal pore system under a thermal gradient (from hot spring to cold ocean) can concentrate small molecules including nucleotides up to 10⁸ times, a hundred times higher than needed for intermolecular interaction.

The mechanism of accumulation operates as follows. In a hydrothermal vent a plugged pore system is sandwiched between the hot vent interior and the cooling outside ocean (Fig. 1b). A temperature gradient across the pore drives two entangled processes: (i) molecules are shuttled up and down the cleft by laminar thermal convection and (ii) thermophoresis drives the molecules along the temperature gradient, i.e., perpendicular to the convection flow. Both processes are indicated by white arrows in Fig. 1c. In combination, they lead to a strong vertical accumulation toward the closed bottom of the cleft. This geological setting is analogous to a Clusius-tube or thermogravitational column (21). We simulate the behavior of rather rapidly diffusing single nucleotides. Even with conventional biotechnological or microfluidic laboratory methods, such small molecules are hard to concentrate because of their considerable diffusion. The simulation shows a strong 1,200-fold downward accumulation of single nucleotides for the 5 mm short, bent cleft of Fig. 1b. As we will see later, a concatenation of three of these pores leads to a 1,200³ = 1.7 x 10⁹-fold accumulation.

The authors found that larger molecules such as DNA would be even more efficiently concentrated, up to molar levels. One advantage of this hypothesis is that the efficient accumulation of biomolecules eliminates the requirement for a semipermeable membrane in the earliest biomolecule factory.

The described accumulation in semiclosed microscopic pores has several synergistic advantages that pertain to the molecular evolution of early life. The enclosure of pore space by mineral precipitates frees life from the need to build a semipermeable organic membrane in its very first evolutionary steps. Microbiological evidence indicates that membrane synthesis appears to be a rather late development (15, 18). Moreover, active transport across a membrane to accumulate molecules in a cell is well known to be a highly evolved process, requiring complex proteins to form vesicles and to actively pump molecules across the membrane.

A final interesting feature of this hypothesis is the hydrothermal pore experiences thermal cycling. This provides an opportunity for a proto-PCR reaction once a molecule that can catalyze DNA polymerization appears. Such a reaction could rapidly build up large amounts of DNA.

The ability of hydrothermal pore systems to undergo thermal cycling and trap biomolecules without the need for a membrane make them an attractive target for research into the origin of life.

Baaske, P.; Weinert, F. M.; Duhr, S.; Lemke, K. H.; Russell, M. J.; Braun, D. “Extreme accumulation of nuceotides in simulated hydrothermal pore systems.” Proceedings of the National Academy of Sciences, USA 2007, 104, 9346.