Light Scattering and Absorption due to Bacterial Activity in Water

There is a growing class of elementary particle detectors ("Large Water Cerenkov Detectors") which have a body of water (thousands of tons) as a sensitive medium. Particles are detected when they interact with the water and produce Cerenkov light, so detection efficiency relies on the transparency of the water. These detectors are active typically for many years and so biological activity is one of the means by which the transparency of the water may be reduced. We present the results of a measurement of light scattering and absorption from a population of E-coli in water, which is used as a model for bacteria in general. The scattering and absorption can be separated by varying the refractive index of the medium using a solute of high molecular weight. We show that the results can be understood simply in terms of light scattering from small spheres (radius % wavelength) with an effective refractive index n, plus a small amount of absorption, predominantly in the ultraviolet. We compare this scattering with Rayleigh scattering in pure water.