An approach to freshwater cyanobacterial production and excretion: tentative application of a deterministic model

The rates of photosynthesis, respiration and carbon excretion by the cyanobacteriumOscillatoria rubescens D.C. were estimated at a range of light intensities between 0 and 60 μE m−2 s−1 (μmol photon m−2 s−1) using the14C method. A model of the evolution of cell carbon concentration based on the Hobsonet al. (1976) equations and taking excretion into account is presented. This model predicts that the sum of respiration and excretion rates increases more rapidly with light than the rate of photosynthesis and therefore maximum growth of theO. rubescens strain under study should be obtained at low light intensities, approximately 20 μE m−2 s−1 . Light rapidly increases the excretion rate and so induces a deficit in the carbon balance of the cell. In addition, the simultaneous increase in respiration rate, possibly due to photorespiration, contributes to carbon depletion at high irradiances. Thus, this model explains some of our observations, particularly the fact that growth is saturated at lower light intensities than photosynthesis.