Physiological heterogeneity of suspended microbial aggregates

Abstract

Suspended microbial aggregates, which are always in dynamic equilibrium with suspended cells and cells attached to surface, are very common structures in natural and engineering environmental systems. To study and design physiologically diverse suspended microbial aggregates the physiological classification of chemotrophic prokaryotes in 12 groups formed by four evolutionary periods (fermenting, anaerobic respiring, microaerophilic and facultative aerobic, aerobic prokaryotes) and three parallel lines (Gram-negative, Gram-positive Eubacteria, and Archaea) could be used. This type of physiological heterogeneity has been studied in microbial granules using fluorescence in situ hybridization, identification of 16S rRNA genes, and conferring the physiological properties from the description of the species. In spherical granules with diameter of 2.4 mm cells of aerobes were spread to the depth 0.55 mm below surface (85% of granule volume), facultative anaerobes dominated between the depths 0.55 mm and 0.85 mm (13% of granule volume), and anaerobes were concentrated at the depths from 0.85 to 1.0 mm (2% of granule volume). Percentages of aerobic, facultative anaerobic, and anaerobic species in granules, identified by 16S rRNA gene sequencing, were 69%, 9%, and 2% of total number of bacterial clones, respectively. Another type of physiological heterogeneity on the cellular level was due to the changes of cell physiological status during cell cycle. This type of heterogeneity has been studied in the populations of Escherichia coli, Bacillus megaterium, Saccharomyces cerevisiae, and Candida tropicalis. A significant proportion of cells from the exponential phase were killed after 10 min treatment with 1% solution of allyl alcohol, which specifically kills cells with high activity of alcohol dehydrogenase (ADH). However, there was no such effect in starved cell population. Percentage of cells with high activity ADH in microbial population can be used to monitor its physiological status. Physiological diversity of ecosystem may be due to mechanical mixing of cells from the different inflows. An example of such system is an ecosystem of aeration tank in municipal wastewater treatment plant. This ecosystem contains a mechanical mixture of dead anaerobic and live aerobic bacteria as well as attached and suspended cell aggregates supplied from anaerobic digester, raw sewage, and settling tank.