Fed-batch addition of urea for Spirulina platensis cultivation -: Thermodynamics and material and energy balances

Previous results of Spirulina platensis cultures, carried out by supplying intermittently urea as nitrogen source at exponentially increasing feeding rate, have been used to perform a bioenergetic and thermodynamic study on the growth of this cyanobacterium as well as on its photosynthetic efficiency. Simple material, enthalpy and Gibbs energy balances have been applied to the results obtained alternately varying either the total time of nitrogen source feeding (9 < t(T) < 15 days) or the overall added mass of urea per unit reactor volume (250 < m(T) < 750 mg l(-1)). The number of photons involved in the autotrophic growth has been estimated for every tested condition. The progressive increase in this bioenergetic parameter during cultivations demonstrated that the photosynthesis efficiency decreased with increasing biomass concentration. The estimated percentages of the different energy components suggest that less than one-third of the absorbed Gibbs energy was fixed by the system at 25 degreesC to increase its own enthalpic content, while only 6% was recovered as ATP and about two-thirds were released as heat. (C) 2003 Elsevier Inc. All rights reserved.