False-Positive Error by Microparticles in Solid Phase Bioluminescence Inhibition Test and Its Correction for Predicting Soil Toxicity from Soil Solution Toxicity

In this study, light intensities of Aliivibrio fischeri obtained from soil suspension and its filtrate were compared using field soils contaminated with heavy metals. The soils collected from rice paddy and forest had different soil textures with either high or low silt/clay content. The correlation between soil toxicity and soil solution toxicity for A. fischeri showed a high linearity in sandy soils with the slope of 0.586 (determination coefficient; R-2 = 0.709). Meanwhile, a lower correlation in silty/clay soils with the slope of 0.154 (R-2 = 0.067) was observed. When a solid phase bioluminescence inhibition test is carried out using soil suspension with a high silt/clay content, a large amount of A. fischeri seems to adhere to microparticles and/or microaggregates. It may result in the change of light intensity (i.e., distorted toxicity test result) that could not be corrected by the modified basic solid phase test protocol, and thus the toxicity is likely to be overestimated. Such alteration in the light intensity could be partially overcome using a regression equation accounting for microbial loss by adhesion. With the correction for microbial adhesion, the correlation between soil toxicity and soil solution toxicity in the selected soil samples with high silt/clay contents increased from 0.182 (R-2 = 0.130) to 0.571 (R-2 = 0.602). It is expected that soil toxicity can be evaluated through the prediction of soil solution toxicity after the proper correction of microparticles effects.