Exopolysaccharide Features Influence Growth Success in Biocrust-forming Cyanobacteria, Moving From Liquid Culture to Sand Microcosms

Abstract

Land degradation in drylands is a drawback of the combined action of climate change and human activities. New techniques have been developed to induce artificial biocrusts formation as a tool for restoration of degraded drylands, and among them soils inoculation with cyanobacteria adapted to environmental stress. Improvement of soil properties by cyanobacteria inoculation is largely related to their ability to synthesize exopolysaccharides (EPS). However, cyanobacterial EPS features [amount, molecular weight (MW), composition] can change from one species to another or when grown in different conditions. We investigated the differences in growth and polysaccharidic matrix features among three common biocrust-forming cyanobacteria (Nostoc commune, Scytonema javanicum, and Phormidium ambiguum), when grown in liquid media and on sandy soil microcosms under optimal nutrient and water, in controlled laboratory conditions. We extracted and analyzed the released EPS (RPS) and sheath for the liquid cultures, and the more soluble or loosely-bound (LB) and the more condensed or tightly-bound (TB) soil EPS fractions for the sandy soil microcosms. In liquid culture, P. ambiguum showed the greatest growth and EPS release. In contrast, on the sandy soil, S. javanicum showed the highest growth and highest LB-EPS content. N. commune showed no relevant growth after its inoculation of the sandy soil. A difference was observed in terms of MW distribution, showing that the higher MW of the polymers produced by P. ambiguum and S. javanicum compared to the polymers produced by N. commune, could have had a positive effect on growth for the first two organisms when inoculated on the sandy soil. We also observed how both RPS and sheath fractions reflected in the composition of the soil TB-EPS fraction, indicating the role in soil stabilization of both the released and the cell attached EPS. Our results indicate that the features of the polysaccharidic matrix produced by different cyanobacteria can influence their growth success in soil. These results are of great relevance when selecting suitable candidates for large-scale cyanobacteria applications in soil restoration.

Keywords: EPS molecular weight distribution; EPS monosaccharidic composition; cyanobacteria liquid culture; sand inoculation; sandy soil microcosms; semiarid soil.

FIGURE 1

Scheme of the experimental design and variables measured in the (A,B) batch culture and sand microcosm experiments.

FIGURE 2

Dry weight (A) and chlorophyll a concentration (B) during the cultivation period of the three cyanobacteria strains. Different letters indicate significant differences among the strains for each day.

FIGURE 3

Time course of the daily values of the F_v/F_m ratios measured during the cultivation period of the three cyanobacteria strains (mean ± SD, n = 9). Different letters indicate significant differences among the strains for each day.

FIGURE 4

Released (A) and total EPS (B) content during the cultivation period of the three cyanobacteria strains. Different letters indicate significant differences among the strains for each day.

FIGURE 5

Chlorophyll a content over time in the control and inoculated sandy soil microcosms. Different letters indicate significant differences among the control and cyanobacteria-inoculated soils for each sampling day.

FIGURE 6

LB-EPS (A) and TB-EPS (B) contents over time in the control and inoculated sandy soil microcosms. Different letters indicate significant differences among the control and cyanobacteria-inoculated soils for each sampling day.

FIGURE 7

MW distribution of the EPS extracted from liquid culture: (A) RPS, (B) sheath, and from sandy soil microcosms: (C) LB-EPS; (D) TB-EPS, of the cultured strains. Relative proportions (%) of MW classes are represented. No data are reported for N. commune in the sandy soil microcosms (C,D) because of the negligible amounts of EPS extracted (See Figure 5).

FIGURE 8

Monosaccharidic composition of the EPS extracted from liquid culture: (A) RPS, (B) sheath, and from sandy soil microcosms: (C) LB-EPS; (D) TB-EPS, of the cultured strains. Molar percentages (%) of single sugars are represented (expressed as moles of the single monosaccharide divided by the total amount of moles of monosaccharides in the EPS × 100). No data are reported for N. commune in the sandy soil microcosms (C,D) because of the negligible amounts of EPS extracted. Abbreviations: Fuc fucose, Rha rhamnose, GalN galactosamine, Ara arabinose, GlcN glucosamine, Gal galactose, Glc glucose, Man mannose, Xyl xylose, Fru fructose, Rib ribose, GalA galacturonic acid, GlcA glucuronic acid.