Figure 1.

Primeval ZnS-mediated photosynthesis in sub-aerial, illuminated settings. Right: Precipitation of FeS and ZnS nanoparticles (black and grey spots, respectively) around a primeval, sub-aerial hot spring. Note that ZnS and FeS particles precipitate at different distances from the spring. The picture is based upon data from [102,119,179,181,183]; see the main text for further details. Left: A schematic presentation of reactions within a photosynthesizing ZnS nanoparticle, as combined with an energy diagram; the scheme is based on refs. [145,149,190,278]. Initially the absorption of a UV quantum leads to the separation of electric charges. The electrons migrate in the crystal until they are trapped at the surface; the trapped electrons can reduce a CO₂ molecule either via two one-electron transfers [144] or, possibly, in a concerted two-electron reaction. The electron vacancy (hole) is initially reduced by the S^2- ion of the crystal; the ultimate electron equilibration, as discussed in the main text, requires external electron donors, e.g. H₂S. Note that, for simplicity of presentation, the one-electron and two-electron reactions are not discerned; see the main text for further details.