Modeling the C Economy of Anabaena flos-aquae

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Abstract
Steady state cultures of Anabaena flos-aquae were established over a wide range of phosphate-limited growth rates while N was supplied as either NH3, NO3−, or N2 gas. At growth rates greater than 0.03 per hour, rates of gross and net carbon fixation were similar on all N sources. However, at lower growth rates (3− and N2 cultures, gross photosynthesis greatly exceeded net photosynthesis. The increase in photosynthetic O2 evolution with growth rate was greatest when N requirements were met by NO3− and least when met by NH3. These results were combined with previously reported measurements of cellular chemical composition, N assimilation, and acetylene reduction (Layzell, Turpin, Elrifi 1985 Plant Physiol 78: 739-745) to construct empirical models of carbon and energy flow for cultures grown at 30, 60, and 100% of their maximal growth rate on all N sources. The models suggested that over this growth range, 89 to 100% of photodriven electrons were allocated to biomass production in the NH3 cells, whereas only 49 to 74% and 54 to 90% were partitioned to biomass in the NO3−-and N2-grown cells, respectively. The models were used to estimate the relative contribution of active, maintenance, and establishment costs associated with NO3− and N2 assimilation over the entire range of growth rates. The models showed that the relative contribution of the component costs of N assimilation were growth rate dependent. At higher growth rates, the major costs for NO3− assimilation were the active costs, while in N2-fixing cultures the major energetic requirements were those associated with heterocyst establishment and maintenance. It was concluded that compared with NO3− assimilation, N2 fixation was energetically unfavorable due to the costs of heterocyst establishment and maintenance, rather than the active costs of N2 assimilation.