On the nature of interstellar grains

Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, approx. 1040 g. The identification is based on Mie scattering caclculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals 1 μ-1 < lambda-1 < 1.94 μ-1 and 2.5 μ-1 < lambda-1 < 3.0 μ-1. Over the more restricted waveband 4000-5000 Angstroem an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigmnets. The strongest of the diffuse interstellar bands are provisionally assigned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The lambda 2200 Angstroem interstellar absorption feature could be due to 'degraded' cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths lambda < 1800 Angstroem could be due to scattering by virus particles. (orig.)