MOLECULAR ANALYSIS OF BACTERIAL COMMUNITIES IN A 3-COMPARTMENT GRANULAR ACTIVATED-SLUDGE SYSTEM INDICATES COMMUNITY-LEVEL CONTROL BY INCOMPATIBLE NITRIFICATION PROCESSES

Bacterial community structure and the predominant nitrifying activitie s and populations in each compartment of a three-compartment activated sludge system were determined, Each compartment was originally inocul ated with the same activated sludge community entrapped in polyethylen e glycol gel granules, and ammonium nitrogen was supplied to the syste m in an inorganic salts solution at a rate of 5.0 g of N liter of gran ular activated sludge(-1) day(-1). After 150 days of operation, the sy stem was found to comprise a series of sequential nitrifying reactions (K, Note, T. Ogasawara, Y, Suwa, and T. Sumino, Water Res. 32:769-773 , 1998), presumably mediated by different bacterial populations. Activ ity data showed that all NH4-N was completely oxidized in compartments one and two (approximately half in each), but no significant nitrite oxidation was observed in these compartments. In contrast, all availab le nitrite was oxidized to nitrate in compartment three. To study the microbial populations and communities in this system, total bacterial DNA isolated from each compartment was analyzed for community structur e based on the G+C contents of the component populations. Compartment one showed dominant populations having 50 and 67% G+C contents. Compar tment two was similar in structure to compartment one, The bacterial c ommunity in compartment three had dominant populations with 62 and 67% G+C contents and retained the 50% G+C content population only at a gr eatly diminished level, The 50% G+C content population from compartmen t one hybridized strongly with amo (ammonia monooxygenase) and hao (hy droxylamine oxidoreductase) gene probes from Nitrosomonas europaea. Ho wever, the 50% G+C content population from compartment two hybridized strongly with the hao probe but only weakly with the amo probe, sugges ting that the predominant ammonia-oxidizing populations in compartment s one and two might be different. Since different activities and popul ations come to dominate in each compartment from an identical inoculum , it appears that the nitrification processes may be somewhat incompat ible, resulting in a series of sequential reactions and different comm unities in this three-compartment system.