Bioprospecting for industrially relevant exopolysaccharide-producing cyanobacteria under Portuguese simulated climate

Abstract

Cyanobacterial exopolysaccharides (EPS) are potential candidates for the production of sustainable biopolymers. Although the bioactive and physicochemical properties of cyanobacterial-based EPS are attractive, their commercial exploitation is limited by the high production costs. Bioprospecting and characterizing novel EPS-producing strains for industrially relevant conditions is key to facilitate their implementation in various biotechnological applications and fields. In the present work, we selected twenty-five Portuguese cyanobacterial strains from a diverse taxonomic range (including some genera studied for the first time) to be grown in diel light and temperature, simulating the Portuguese climate conditions, and evaluated their growth performance and proximal composition of macronutrients. Synechocystis and Cyanobium genera, from marine and freshwater origin, were highlighted as fast-growing (0.1-0.2 g L^-1 day^-1) with distinct biomass composition. Synechocystis sp. LEGE 07367 and Chroococcales cyanobacterium LEGE 19970, showed a production of 0.3 and 0.4 g L^-1 of released polysaccharides (RPS). These were found to be glucan-based polymers with high molecular weight and a low number of monosaccharides than usually reported for cyanobacterial EPS. In addition, the absence of known cyanotoxins in these two RPS producers was also confirmed. This work provides the initial steps for the development of cyanobacterial EPS bioprocesses under the Portuguese climate.

Figure 1

Maximum likelihood phylogenetic tree based on 183 partial 16S rRNA gene sequences of cyanobacteria. Gloeobacter violaceus PCC 7421 and G. violaceus PCC 8105 were used as the outgroup. LEGE-CC strains used in this work are indicated in bold and with a black circle. The different colour segments represent strain placement at the order level following. Bootstrap values over 50% are indicated at the nodes.

Figure 2

Biomass productivity of cyanobacterial strains for the period of 15 days. Biomass productivity is expressed in g L⁻¹ day⁻¹ ± standard error (n = 3).

Figure 3

Protein content in percentage of dry weight (DW) of the cyanobacterial strains. Different letters indicate significant differences between strains according to the Nemenyi post-hoc test. Values are given as means ± standard deviation (n = 3).

Figure 4

Biomass proximate composition of the most productive cyanobacterial strains (n = 3).

Figure 5

Plot of the non-metric multidimensional scaling for the biochemical composition of the seven strains.

Figure 6

PCA of biochemical components for the most productive cyanobacterial strains.

Figure 7

Microphotographs of cyanobacterial EPS after PBR cultivation stained with Alcian Blue formulation clustered by origin. Magnification ranged from 100 to 400× and was adapted to cell size to capture the main profile of EPS around the cells. Letters correspond to: (a) Synechocystis salina LEGE 00038, (b) Synechocystis salina LEGE 00041, (c) Synechocystis salina LEGE 06099, (d) Cyanobium sp. LEGE 06140, (e) Synechocystis sp. LEGE 07367 (f) Cyanobium sp. LEGE 15611 and (g) Chroococcales cyanobacterium LEGE 19970.