Introduction: Cyanobacteria and other phototrophic microorganism have tremendous potential as a resource for a sustainable green economy. The economic viability of phototrophic microorganisms, however, is still limited by the low growth rates and the low areal productivities obtained in most current cultivation systems. To overcome the productivity barriers of phototrophic cultivation, there has been increasing interest in fast-growing strains.

Goals: We discuss the main factors that determine phototrophic culture productivity. In particular, based on quantitative models, we show that high maximal growth rates are not a sufficient or necessary property for high phototrophic productivity. Rather, the light-limited specific growth rate of a phototrophic microorganism is a product of several factors, including the rate of light absorption, quantum yield and the maximal biomass yield per photon.

Materials and Methods: Our analysis builds upon high quality computational models of cyanobacterial growth developed over the past decade. In particular, we make use of genome-scale reconstructions as well as reduced coarse-grained models of metabolism.

Results: Computational models allow us to disentangle the prerequisites for high phototrophic productivity. Maximal productivity of a phototrophic culture is attained at high light intensities, low growth rates and high cell densities. Under such conditions, dense cultures act as an effective light dilution mechanism and alleviate the detrimental effects of photoinhibition. It is shown that antennae truncation does not result in improved culture productivities. Model results are supported by quantitative growth experiments using a high-density cultivation setup based on a membrane-mediated CO2 supply, high light intensities and thin-layer cell suspensions.

Summary: Our results show that (i) current limits of phototrophic productivity can be overcome, that (ii) under optimal culture conditions (medium and light conditions) current model strains consistently reach high growth rates and high final cell densities, and (iii) a high maximal specific growth rate is not itself a determinant of culture productivity.