The kinetics of tempeh wastewater treatment using Arthrospira platensis.

The microalga Arthrospira platensis (Spirulina) was used for tempeh wastewater treatment. Microalga growth and the kinetics of chemical oxygen demand (COD) degradation under different light intensities (2,100 and 4,300 lux), tempeh wastewater concentrations (0, 0.5, 1, 1.5% v/v), and sodium nitrate concentrations (0, 0.75, 1, 2, 2.5 g/L) were studied. Improved cell growth in wastewater indicated that mixotrophic growth was preferred. The addition of sodium nitrate up to 2 g/L increased COD removal. The highest COD removal was 92.2%, which was obtained from cultivation with 1% v/v tempeh wastewater, 2 g/L sodium nitrate, 2,100 lux, and the specific growth rate of 0.33 ± 0.01 day-1. The COD removal followed a pseudo-first-order kinetic model with the kinetic constant of 0.3748 day-1 and the nitrate uptake rate of 0.122 g/L-day. The results can be used to design a pilot-scale tempeh wastewater treatment facility using A. platensis for tertiary treatment. Based on the kinetic model, a 20 m3 reactor can treat tempeh wastewater to reduce the COD from 400 to 100 ppm in 4 days and produces approximately 32.8 kg of dried microalgae.

Degradation and synthesis kinetics of quorum-sensing autoinducer in Pseudomonas aeruginosa cultivation.

The quorum-sensing (las and rhl) systems play critical roles in the pathogenicity of Pseudomonas aeruginosa and its synthesis of the important biosurfactants, rhamnolipids. In this work, P. aeruginosa PAO1 and its rhlI and rhlR null mutants were used to study the degradation and synthesis kinetics of the rhl system's autoinducer PAI2 (N-butanoyl-homoserine lactone). The two mutants, lacking the ability of synthesizing PAI2 or RhlR protein, produced insignificant amounts of rhamnolipids while having similar growth profiles as the wild-type culture. The regulatory RhlR:PAI2 complex is thus essential to rhamnolipid synthesis. In batch culture of the wild-type PAO1, the autoinducer PAI2 concentration increased along cell growth, especially during the transition from exponential-growth phase to stationary phase, and began to decrease after entering the stationary phase. The decrease in the stationary phase resulted from a faster PAI2 degradation than its synthesis. The degradation kinetics was studied using PAI2-containing supernatants (from centrifuged broth of wild-type culture) with and without the rhlI(-) mutant cells incapable of PAI2 synthesis. Being insignificant in the cell-free systems, PAI2 degradation was found predominantly cell-associated and could be described empirically by the first-order, exponential decay kinetics with the best-fit degradation constant (k(d)) of 0.195 h(-1). When similarly modeled with a first-order kinetics, PAI2 synthesis in stationary-phase wild-type culture was derived to have a synthesis constant (k(s)) of 0.189 h(-1). The PAI2 concentration in batch cultivation of the rhlR(-) mutant also showed an increase-then-decrease profile. However, the maximum PAI2 concentration was about one third of that from the wild-type culture. The constitutive rate of PAI2 synthesis was therefore significantly lower than the rate attainable with active auto-induction by RhlR-PAI2 complex.

Modeling rhl quorum-sensing regulation on rhamnolipid production by Pseudomonas aeruginosa.

The effect of autoinducer PAI2 on rhamnolipid (RL) production by Pseudomonas aeruginosa was evaluated using an rhlI null mutant of PAO1 added with PAI2 at various concentrations. A model has also been developed to describe the production kinetics regulated by the rhl quorum-sensing system in three steps: First, PAI2 combines with RhlR protein. Second, the activated complex RhlR:PAI2 triggers the transcription (and expression) of the rhlAB operon that encodes for rhamnosyltransferase. Finally, the enzyme catalyzes the RL synthesis. The model describes fairly well the experimental results/profiles from three different studies (this and two others reported in the literature). The overall picture predicted by the model is as follows: The induced enzyme synthesis proceeds at the highest rate following PAI2 addition. The rate decreases with time as the autoinducer is degraded. The enzyme concentration nonetheless continues to increase until reaching the plateau at the exhaustion of autoinducer. Higher added PAI2 concentrations thus give not only higher initial enzyme synthesis rates but also longer induced synthesis. As the enzyme concentration increases with time, the RL production rate also increases, resulting in an accelerated rise in RL concentrations initially. The increase in RL concentrations becomes linear at the exhaustion of PAI2. The best-fit model parameters obtained also provided important insights. To complex half of the intracellular RhlR proteins would require 1.61 microM PAI2, about half of the PAI2 concentration obtained in the stationary-phase culture of wild-type PAO1. On the other hand, to activate the rhamnosyltransferase synthesis at half of its maximum rate would require the binding of 39% of RhlR with PAI2. The maximum RL production rate of the culture was found to be 0.042 g/L.h, and the fully induced culture would require at least 1.61 h to synthesize the enzyme to the necessary level for producing RL at half of the maximum rate.