LIGHT-SCATTERING BY MICROORGANISMS IN THE OPEN OCEAN

Recent enumeration and identification of marine particles that are less than 2-mu-m in diameter, suggests that they may be the major source of light scattering in the open ocean. The living components of these small particles include viruses, heterotrophic and photoautotrophic bacteria and the smallest eucaryotic cells. In order to examine the relative contribution by these (and other) microorganisms to scattering, we have calculated a budget for both the total scattering and backscattering coefficients (at 550nm) of suspended particles. This budget is determined by calculating the product of the numerical concentration of particles of a given category and the scattering cross-section of that category. Values for this product are then compared to values for the particulate scattering coefficients predicted by the models of GORDON and MOREL (1983) and MOREL (1988). In order to make such a comparison, we have estimated both the total scattering and backscattering cross-sections of various microbial components that include viruses, heterotrophic bacteria, prochlorophytes, cyanobacteria, ultrananoplankton (2-8-mu-m), larger nanoplankton (8-20-mu-m) and microplankton (> 20-mu-m). Such determinations are based upon Mie scattering calculations and measurements of the cell size distribution and the absorption and scattering coefficients of microbial cultures. In addition, we have gathered published information on the numerical concentration of living and detrital marine particles in the size range from 0.03 to 100-mu-m. The results of such a study are summarized as follows. The size distribution of microorganisms in the ocean roughly obeys an inverse 4th power law over three orders of magnitude in cell diameter, from 0.2 to 100-mu-m. Thus, the size distribution of living organisms is similar to that for total particulate matter as determined by electronic particle counters. For representative values of refractive index, it appears that most of the scattering in the sea comes from particles less than 8-mu-m in diameter, and that most of the backscattering comes from particles less than 1-mu-m. Among the microorganisms that are found in this size range, free-living heterotrophic bacteria may be often most important. These microbes account typically for 10 to 50% of the total particulate scattering and 5 to 20% of backscattering in oligotrophic waters that contain less than 0.5 mg chlorophyll per m3. The second most important source of microbial scattering is cyanobacteria (especially in tropical and temperate waters) and ultrananoplankton. Viruses, which may be very abundant, make little contribution because they have extremely small cellular scattering cross-sections. Large microorganisms, which include nanoplankton > 8-mu-m and microplankton and which efficiently scatter light, contribute little because of their low cellular concentrations. While a significant fraction of the total scattering coefficient may come from the combined contributions of viable procaryotic and eucaryotic cells, only a small fraction of the backscattering appears to come from these microbes. Instead, it appears that the major source of particulate backscattering is small (< 0.6-mu-m), numerically abundant detrital particles. A recent description by electronic particle counting (KOIKE, HARA, TERAUCHI and KOGURE, 1990) of small detrital particles in the size range of 0.4 to 1-mu-m satisfies the requirements for such a source of backscattering.