The role of heterotrophic bacteria in assessing phosphorus stress to Synechocystis sp. PCC6803

Microalgae biofuel production, a possible source of carbon-neutral energy, requires phosphorus (P), a limited resource. This study investigates the relationship between specific growth rate of the cyanobacterium Synechocystis sp. PCC 6803 and P availability. It has been previously suggested, and we show here, that P-limited growth kinetics are well represented by a quota-type model with a single pool of intracellular P. We also demonstrate that the presence of heterotrophic bacteria plays a large role in understanding these kinetics, because the culture’s intracellular P content depends on the level of heterotrophic bacteria. Using batch-growth experiments containing up to a 0.07 biovolume ratio of heterotrophic bacteria to Synechocystis, we found that Synechocystis could grow with intracellular P content down to approximately 0.5 mg P g⁻¹ dry weight biomass, while heterotrophic bacteria maintained roughly 20 mg P g⁻¹ dry weight. Thus, a small fraction of heterotrophic bacteria in a microalgal culture can dramatically increase the apparent content of P in the biomass, which affects how to assess P-stress to a P-limited culture.