Physiological state of a microbial community in a biomass recycle reactor

The transition in physiological state was investigated between a carbon-limited chemostat population and microbes growing very slowly in a biomass recycle reactor. The mixed microbial population was metabolizing a mixture of biopolymers and linear alkylbenzene sulfonate, formulated to represent the organic load in graywater. Biomass increased 30-fold during the first 14 days after a shift from chemostat to biomass recycle mode. The ratios of ATP and RNA to cell protein decreased over the first days but then remained constant. The specific rate of CO₂ production by microbes in the reactor decreased 6-fold within 24 h after the shift, and respiratory potentials declined 2–3 fold during the first 7 days. Whereas chemostat cultures used equal proportions of organic carbon substrate for catabolism and anabolism, the proportion of organic substrate oxidized to CO₂ rose from 62 to 82% over the first 8 days in a biomass recycle reactor, and eventually reached 100% as this reactor population exhibited no net growth. Biomass recycle populations removed from the system and subjected to a nutritional shift-up did not immediately initiate exponential growth. The physiological state of cells in the biomass recycle reactor may be distinct from those grown in batch or continuous culture, or from starved cells.