An Alternative Exploitation of Synechocystis sp. PCC6803: A Cascade Approach for the Recovery of High Added-Value Products

Abstract

Microalgal biomass represents a very interesting biological feedstock to be converted into several high-value products in a biorefinery approach. In this study, the cyanobacterium Synechocystis sp. PCC6803 was used to obtain different classes of molecules: proteins, carotenoids and lipids by using a cascade approach. In particular, the protein extract showed a selective cytotoxicity towards cancer cells, whereas carotenoids were found to be active as antioxidants both in vitro and on a cell-based model. Finally, for the first time, lipids were recovered from Synechocystis biomass as the last class of molecules and were successfully used as an alternative substrate for the production of polyhydroxyalkanoate (PHA) by the native PHA producer Pseudomonas resinovorans. Taken together, our results lead to a significant increase in the valorization of Synechocystis sp. PCC6803 biomass, thus allowing a possible offsetting of the process costs.

Keywords: antioxidants; biorefinery; cyanobacteria; high-value compounds; polyhydroxyalkanoate.

Figure 1

Optical microscopy image collected at 1000-fold magnification of a stably growing population of Synechocystis sp. PCC6803.

Figure 2

Schematic representation of the biorefinery strategy.

Figure 3

Carotenoids extraction from Synechocystis sp. PCC6803. (A) Yields are reported as % with respect to dry weight biomass. Black bar is referred to untreated biomass and grey bar is referred to residual biomass after protein extraction (p > 0.05). (B) Total carotenoids (black bars) and chlorophyll a (grey bars) extracted from raw and residual biomass. Each content is expressed as mg of extract pigment per g of biomass dry weight. * is referred to p < 0.05. The line is used to show which samples have been used to carry out statistical analysis.

Figure 4

Lipidic extract from Synechocystis sp. PCC6803. Yields are reported as % with respect to dry weight biomass. Black bar is referred to untreated biomass and grey bar is referred to residual biomass post proteins and carotenoids extraction.

Figure 5

Effect of the protein extract on cell viability. Dose-response curve of HaCaT cells (black circles) and A431 cells (black squares) incubated for 48 h in the presence of increasing concentration (0.01–0.4 mg mL⁻¹) of protein extract. Cell viability was assessed by the MTT assay and expressed as described in the Materials and Methods section. Values are given as means ± SD (n ≥ 3).

Figure 6

Effect of ethanolic extracts from Synechocystis sp. PCC6803 on the viability of immortalized cells. Dose-response curve of cells incubated for 48 h in the presence of increasing concentration (10–200 μg mL⁻¹) of each extract. Cells were incubated with ethanol extracts from both raw (A) and residual (B) biomass on HaCaT (circles) and BALB/c-3T3 (squares). Cell viability was assessed by the MTT assay and expressed as percentage of the control. Values are given as means ± SD (n ≥ 3).

Figure 7

Antioxidant activity of carotenoids from Synechocystis sp. PCC6803. Intracellular ROS levels were determined by DCFDA assay. Cells were pre-incubated in the presence of 80 μg mL⁻¹ of extract obtained from raw biomass (grey bars) or from residual biomass (white bars) for 2 h, prior to UVA treatment (100 J cm⁻²). Black bars refer to untreated cells in the absence (−) or in the presence (+) of UVA stress. Values are expressed as percentage with respect to control (i.e., untreated) cells. Data are shown as means ± SD (n ≥ 3). * indicates p < 0.05. The line is used to show which samples have been used to carry out statistical analysis.

Figure 8

¹H-NMR spectrum of mcl-PHA.