FRACTAL DIMENSIONS AND POROSITIES OF ZOOGLOEA-RAMIGERA AND SACCHAROMYCES-CEREVISAE AGGREGATES

The fractal nature of microbial aggregates is a function of the type of microorganism and mixing conditions used to develop aggregates. We determined fractal dimensions from length-projected area (D2) and length-number scaling (D3) relationships. Aggregates of Zoogloea ramigera developed in rotating test tubes were both surface and mass fractals, with fractal dimensions of D2 = 1.69 +/- 0.11 and D3 = 1.79 +/- 0.28 (+/- standard deviation), respectively. When we grew this bacteria in a bench-top fermentor, aggregates maintained their surface fractal characteristics (D2 = 1.78 +/- 0.11) but lost their mass fractal characteristics (D3 = 2.99 +/- 0.36). Yeast aggregates (Saccharomyces cerevisae) grown in rotating test tubes had higher average fractal dimensions than bacterial aggregates grown under physically identical conditions, and were also considered fractal (D2 = 1.92 +/- 0.08 and D3 = 2.66 +/- 0.34). Aggregate porosity can be expressed in terms of a fractal dimension, but average porosities are higher than expected. The porosities of yeast aggregates (0.9250-0.9966) were similar to porosities of bacterial aggregates (0.9422-0.9980) cultured under the same physical conditions, although bacterial aggregates developed in the reactor had higher average porosities (0.9857-0.9980). These results suggest that scaling relationships based on fractal geometry may be more useful than equations derived from Euclidean geometry for quantifying the effects of different fluid mechanical environments on aggregate morphology and characteristics such as density, porosity, and projected surface area.