Optical properties of marine bacteria

Optical properties of naturally derived bacterioplankton grown in unenriched seawater are described. The beam attenuation coefficient, absorption coefficient and size distribution of cells suspension of bacteria were measured in order to determine their optical efficiency factors. In addition, the bulk refractive index as well as the angular pattern of light scattering were obtained from Mie theory. The cellular scatering efficiency increases with decreasing light wavelength as A , the backscattering efficiency is almost spectrally neutral, and the absorption efficiency exhibits features associated with respiratory cytochromes. Except for backscattering, the efficiences are significantly lower than those for larger biological inicroparticles. We suggest that baceria are a xrtajor source of light scattering in oligotrophic waters, where their contribution to the scattering coefficient may far exceed 50%. This large contribution is caused by the fact that total geometric cross sectional area for the bacteria compensates for their lower scattering efficiency. The contribution by bacteria to particle absorption, although less certain, appears also to be important in oligotrophic waters, and it may even predominate a non-'phytoplankton component. The effects of changes in size and refractive index on optical efficiencies of bacteria are also discussed in terms of a recent hypothesis concerning the regulation of cellular water content.