Phormidium ambiguum and Leptolyngbya ohadii Exopolysaccharides under Low Water Availability

Abstract

Cyanobacteria can cope with various environmental stressors, due to the excretion of exopolysaccharides (EPS). However, little is known about how the composition of these polymers may change according to water availability. This work aimed at characterizing the EPS of Phormidium ambiguum (Oscillatoriales; Oscillatoriaceae) and Leptolyngbya ohadii (Pseudanabaenales; Leptolyngbyaceae), when grown as biocrusts and biofilms, subject to water deprivation. The following EPS fractions were quantified and characterized: soluble (loosely bound, LB) and condensed (tightly bound, TB) for biocrusts, released (RPS), and sheathed in P. ambiguum and glycocalyx (G-EPS) in L. ohadii for biofilms. For both cyanobacteria upon water deprivation, glucose was the main monosaccharide present and the amount of TB-EPS resulted was significantly higher, confirming its importance in these soil-based formations. Different profiles of monosaccharides composing the EPSs were observed, as for example the higher concentration of deoxysugars observed in biocrusts compared to biofilms, demonstrating the plasticity of the cells to modify EPS composition as a response to different stresses. For both cyanobacteria, both in biofilms and biocrusts, water deprivation induced the production of simpler carbohydrates, with an increased dominance index of the composing monosaccharides. The results obtained are useful in understanding how these very relevant cyanobacterial species are sensitively modifying the EPS secreted when subject to water deprivation and could lead to consider them as suitable inoculants in degraded soils.

Keywords: EPS monosaccharidic composition; biocrusts; biofilms; dehydration; soil restoration; water deprivation.

Figure 1

Chlorophyll a content in cyanobacterial biocrusts (values represent the mean of N = 3, error bars represent SD). Different letters represent significant differences (p < 0.05).

Figure 2

EPSs contents in P. ambiguum (A) and L. ohadii (B) biocrusts (values represent the mean of N = 3, error bars represent SD). Different letters represent significant differences (p < 0.05) in each graph.

Figure 3

Monosaccharidic composition of the EPS extracted from biocrusts: (A) LB-EPS from P. ambiguum, (B) LB-EPS from L. ohadii, (C) TB-EPS from P. ambiguum, (D) TB-EPS from L. ohadii. 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). Symbol *, when present, indicates significant differences between the control and watered-deprived period in each monosaccharide. 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.

Figure 4

Chlorophyll a content in cyanobacterial biofilms (values represent the mean of N = 3, error bars represent SD). Different letters represent significant differences (p < 0.05).

Figure 5

EPSs contents in P. ambiguum (A) and L. ohadii (B) biofilms (values represent the mean of N = 3, error bars represent SD). Different letters represent significant differences in each graph (p < 0.05).

Figure 6

Monosaccharidic composition of the EPS extracted from biofilms: (A) RPS from P. ambiguum, (B) RPS from L. ohadii, (C) Sheath EPS, (D) G-EPS. 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). Symbol *, when present, indicates significant differences between the control and the water-deprived period in each monosaccharide. 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.