A method for the experimental determination of light absorption by aquatic heterotrophic bacteria

Abstract

A method has been set up for the experimental determination of the volume coefficient for light absorption in vivo by aquatic heterotrophic bacteria. The application described here is the absorption measurement of the bacterial fraction that passes through the commonly used GF/F filter and remains unaccounted for. The experimental samples were prepared by successive water filtrations through GF/F and 0.22 μm Millipore membranes. Light-transmission and light-reflection measurements of the filter-retained samples were performed using a dual-beam spectrophotometer equipped with an integrating sphere attachment. Sample absorption was derived from the data by a procedure that corrects for the contamination of the results due to the high degree of light scattering by the bacteria. The bacterial absorption was discriminated from fine detritus absorption by bleaching the bacterial respiratory pigments using a K2S208 solution. The absorption amplification caused by multiple scattering in the filter was corrected for by an expression that was obtained experimentally. A test of the method, including error analysis, was performed on samples collected in both marine and inland waters. The relative contributions to light absorption by heterotrophic bacteria and various types of particulate matter were also measured for a typical situation. Combining the measured volume absorption coefficients with backscattering coefficients computed by Mie theory yields a set of input data to multicomponent optical models that is needed to assess the contribution of these heterotrophic bacteria to the radiative transfer process.