Erythromycin mediates co-flocculation between cyanobacterium Synechocystis sp. PCC 6803 and filamentous fungi in liquid cultivation without organic compounds [CE and CEF]

Photoautotrophic cyanobacteria assimilate the greenhouse gas carbon dioxide as their sole carbon source for producing useful bioproducts. However, harvesting the cells from their liquid media is a major bottleneck in the process. Thus, an easy-to-harvest method, such as auto-flocculation, is desirable. Here, we found that cyanobacterium Synechocystis sp. PCC 6803 co-flocculated with a natural fungal contamination in the presence of the antibiotic erythromycin (EM) but not without EM. The fungi in the co-flocculated biomass were isolated and found to consist of five species with the filamentous Purpureocillium lilacinum and Aspergillus protuberus making up 71% of the overall fungal population. The optimal co-cultivation for flocculation was an initial 5 mg (fresh weight) of fungi, an initial cell density of Synechocystis of 0.2 OD730, 10 µM EM, and 14 days of cultivation in 100 mL of BG11 medium with no organic compound. This yielded 248 ± 28 mg/L of the Synechocystis-fungi flocculated biomass from 560 ± 35 mg/L of total biomass, a 44 ± 2 % biomass flocculation efficiency. Furthermore, the EM treated Synechocystis cells in the Synechocystis-fungi flocculate had a normal cell color and morphology, while those in the axenic suspension exhibited strong chlorosis. Thus, the occurrence of the Synechocystis-fungi flocculation was mediated by EM, and the co-flocculation with the fungi helped Synechocystis to alleviate the negative effect of EM. Transcriptomic analysis of suspended and flocculated (with the fungi) Synechocystis cells suggested that the EM-mediated co-flocculation was a result of down-regulation of the minor pilin genes and up-regulation of several genes including the chaperone gene for pilin regulation, the S-layer protein genes, the exopolysaccharide-polymerization gene, and the genes for signaling proteins involved in cell attachment and abiotic-stress responses. The EM treatment may be applied in the co-culture between other cyanobacteria and fungi to mediate cell bio-flocculation. Transcriptomic analysis was performed between the Synechocystis in the suspended Synechocystis cells with EM and the Synechocystis cells in the flocculated (with the fungi) Synechocystis-fungi flocculated biomass. The results suggested that the presence of fungi did not significantly altered the expression levels of pillin genes. Thus the altered gene expression levels of pillin genes were results from the EM treatment. Samples include: No fungi 1: The Synechocystis cultivation under BG-11 with 5 µM of erythromycin addition_replicated 1 No fungi 2: The Synechocystis cultivation under BG-11 with 5 µM of erythromycin addition_replicated 2 No fungi 3: The Synechocystis cultivation under BG-11 with 5 µM of erythromycin addition_replicated 3 with fungi 1: The Synechocystis cultivation under BG-11 with 5 µM of erythromycin and 5 mg of fungi addition_replicated 1 with fungi 2: The Synechocystis cultivation under BG-11 with 5 µM of erythromycin and 5 mg of fungi addition_replicated 2 with fungi 3: The Synechocystis cultivation under BG-11 with 5 µM of erythromycin and 5 mg of fungi addition_replicated 3