Effects of light intensity on soluble microbial products produced by Synechocystis sp. PCC 6803 and associated heterotrophic communities

The success of algal and cyanobacterial cultivation hinges on the robustness of the processes, and a key to this success is managing the microbial community co-existing with the algae or cyanobacteria. In this work, we grew the cyanobacterium Synechocystis sp. PCC6803 in a flat-plate photobioreactor operated as a sequencing batch reactor using two light conditions: continuous incident photosynthetically active radiation of 112 or 364 μmol m−2 s−1 (32 or 90 μmol m−2 s−1 average internal light intensity, respectively). As expected, the higher light condition supported more Synechocystis biomass, but the proportion of photosynthetically produced electrons released as soluble microbial products (SMP; cellular components released during growth and decay) also increased – ~42% compared to 20% at the lower light condition – which corresponded to an increase in heterotrophic biomass that grew by oxidizing the SMP. After the increase in incident light intensity, the co-existing community of heterotrophic bacteria showed a large taxonomical transition, with Sphingobacteriales declining and Comamonadaceae increasing. Over the course of 7 days, however, the microbial community structure stabilized, gradually moving towards the initial composition. We suggest that changes in the microbial community resulted from changes in SMP composition from Synechocystis at the different light intensities.