Toward a definition of a bioreceptivity index for granitic rocks: Perception of the change in appearance of the rock

This study involved investigation of the change in appearance of the surface of granite rocks in relation to the amount of chlorophyll present, with the eventual aim of developing a bioreceptivity index. The study involved: (1) inoculating blocks of different varieties of granite with different amounts of a mixed culture of three cyanobacteria so that each block was covered with a different (and known) amount of chlorophyll a, (2) characterising the change in appearance undergone by each block as change in colour, and (3) deciding on the term to use to describe the change, after asking a substantial number of volunteer observers what term they would use. The thresholds between each interval were calculated by statistical analysis. The range of the visual evaluation of the change was divided into seven categories, ranging from inappreciable to very intense. It was found that the amount of chlorophyll a present before the volunteers perceived a change varied depending on the type of rock; considering all of the granite rocks together, the corresponding amount was 0.04 μg clorophyll a/cm2, which caused a colour change (ΔE*) of 3.17.