Atmospheric aerosols as prebiotic chemical reactors

Abstract

Aerosol particles in the atmosphere have recently been found to contain a large number of chemical elements and a high content of organic material. The latter property is explicable by an inverted micelle model. The aerosol sizes with significant atmospheric lifetimes are the same as those of single-celled organisms, and they are predicted by the interplay of aerodynamic drag, surface tension, and gravity. We propose that large populations of such aerosols could have afforded an environment, by means of their ability to concentrate molecules in a wide variety of physical conditions, for key chemical transformations in the prebiotic world. We also suggest that aerosols could have been precursors to life, since it is generally agreed that the common ancestor of terrestrial life was a single-celled organism. The early steps in some of these initial transformations should be accessible to experimental investigation.A erosol particles in the atmosphere have long been known to scatter and absorb sunlight and hence to have a substantial influence on the temperature of the Earth. Recent real-time observations of the chemical composition of individual aerosol particles have shown an unexpectedly and remarkably high content of organic molecules (1, 2). A conceptual model can account for this observation by an inverted micelle structure in which surfactants form a spherical monolayer enclosing an aqueous interior (3, 4). Analogies in size, form, and composition between these aerosols and single-celled organisms such as bacteria lead us to propose that similar atmospheric particles could have been the precursors of living systems on Earth. Large populations of aerosol particles would have provided an environment for the concentration of prebiotic molecular species and for their chemical transformation through exposure to the fluctuating fields of humidity, temperature, and sunlight available in the atmosphere at different altitudes and latitudes. Coagulation and division of the particles could have resulted in an increased diversity of molecular species and an early mechanism for reproduction and replication of successful molecular populations.The most probable sequence of events in the evolution of living organisms on Earth started with either the terrestrial synthesis or the arrival from elsewhere in the universe of simple carbonaceous compounds (5). It was long ago recognized that hot water or steam would have reacted with metal carbides to form hydrocarbons as the Earth condensed from cooling gases (6). The precursors of biopolymers-amino acids, sugars, purines, and pyrimidines-as well as a range of surfactants such as long-chain carboxylic acids can be rationalized as prebiotic constituents by such means (7). Moreover, molecules of this type, including hydrocarbons with chain lengths in excess of C 10 , have been found in the interior of carbonaceous meteorites (8). These building blocks are thought to have undergone polymerization and other reactions, and through a series of events to have formed structures of greater...

Organic Aerosols and the Origin of Life: An Hypothesis

“…Figure 2 demonstrates that after enough time has elapsed, the fatty acid peaks have decreased by large fractions and chlorine peaks (m/z = 35 and 37) have increased by large fractions, demonstrating that the fatty acids are on the exterior of a sea salt core. This observation confirms the hypothesis of Ellison et al, (1999) that such would be the structure of marine aerosols, and which led to the connection to the origin of life (Dobson et al, 2000). In the present biotic atmosphere, these fatty acids come from the decay of lipids (Sicre et al, 1990); we examine the supply of fatty acid monomers in the prebiotic era in Section 3 below.…”

“…The idea itself is now buttressed by observations in the contemporary atmosphere (Murphy et al, 1998;Tervahattu et al, 2002aTervahattu et al, , 2002b, and can be traced back through earlier suggestions by Goldacre (1958), Shah (1970) and Lerman (1986Lerman ( , 1992Lerman ( , 1994Lerman ( , 1996. Dobson et al (2000) gave an equation (Kasten 1968) expressing how the balance between gravitation and aerodynamic drag determines the size of aerosol particles having an appreciable residence time in the atmosphere, and Donaldson et al (2001) showed thermodynamically how the free energetics of the collapse of an exterior film of amphiphilic molecules on an aerosol could lead to fission, a process not possible for a homogeneous uncoated liquid aerosol. The fission must be asymmetric; for a prebiotic Earth atmosphere with one bar surface pressure, the larger daughter particle is bacterially-sized while the smaller one is virally-sized .…”

“…This accumulation would have occurred at the expense of the CH 4 , CO 2 , H 2 O and NH 3 inventories. Dobson et al (2000) pointed out that the very large, continually recycled aerosol populations would have been potentially versatile chemical reactors in the prebiotic atmosphere. A wide range of radiation, temperature and humidity fields would have been sampled by aerosols transported by the general circulation; in the present atmosphere temperature ranges from −90…”

Photochemistry of aqueous pyruvic acid

“…Atmospheric aerosols, however, are known to experience a wide range of pH values throughout their atmospheric lifetime, and can reach acidic pH values at which this chemistry will become dominant (56). These aerosol particles have the added advantage of concentrating the reactant species through selective evaporation of water, resulting in high concentrations of reactant monomers (57)(58)(59)(60)(61). Therefore, although this chemistry is unlikely to be an important contributor to bulk ocean chemistry, it will be important in acidic aqueous atmospheric aerosols.…”

In situ observation of peptide bond formation at the water–air interface

“…The air-water interface (atmospheric aerosols in particular) has previously been proposed to be important in prebiotic chemistry (11)(12)(13)(14) because it provides a unique environment for chemistry through its ability to concentrate and align biochemical precursors and to alter the state of ionization of surface species (15)(16)(17)(18)(19). Contemporary marine aerosols have been found to contain the amino acid precursors necessary for peptide bond chemistry (20), enabling the possibility for their use in such reactions.…”