Fructose supports glycogen accumulation, heterocysts differentiation, N2 fixation and growth of the isolated cyanobiont Anabaena azollae

Dried Nostoc commune exhibits nitrogen-fixing activity using glucose under dark conditions after rehydration

Nostoc commune is an edible cyanobacterium that produces a massive gelatinous polysaccharide matrix around the filamentous cells. The polysaccharides, more than 70% of which comprise glucose, are essential for resistance to environmental stresses. In the present study, we collected naturally growing N. commune colonies, dried them for preservation, rehydrated them, and then examined their nitrogen-fixing activity using the acetylene reduction method. As expected, the rehydrated N. commune performed nitrogen fixation after illumination with white light. Notably, under dark, aerobic conditions, the rehydrated N. commune exhibited nitrogen fixation in the presence of glucose. In contrast, under dark, anaerobic conditions, nitrogen fixation was low. Because the natural habitats of N. commune are aerobic but lack carbohydrates, N. commune cells may exhibit glucose utilization activity constitutively.

Regulation of fructose transport and its effect on fructose toxicity in Anabaena spp

Anabaena variabilis grows heterotrophically using fructose, while the close relative Anabaena sp. strain PCC 7120 does not. Introduction of a cluster of genes encoding a putative ABC transporter, herein named frtRABC, into Anabaena sp. strain PCC 7120 on a replicating plasmid allowed that strain to grow in the dark using fructose, indicating that these genes are necessary and sufficient for heterotrophic growth. FrtR, a putative LacI-like regulatory protein, was essential for heterotrophic growth of both cyanobacterial strains. Transcriptional analysis revealed that the transport system was induced by fructose and that in the absence of FrtR, frtA was very highly expressed, with or without fructose. In the frtR mutant, fructose uptake was immediate, in contrast to that in the wild-type strain, which required about 40 min for induction of transport. In the frtR mutant, high-level expression of the fructose transporter resulted in cells that were extremely sensitive to fructose. Even in the presence of the inducer, fructose, expression of frtA was low in the wild-type strain compared to that in the frtR mutant, indicating that FrtR repressed the transporter genes even in the presence of fructose. FrtR bound to the upstream region of frtA, but binding was not visibly altered by fructose, further supporting the hypothesis that fructose has only a modest effect in relieving repression of frtA by FrtR. A. variabilis grew better with increasing concentrations of fructose up to 50 mM, showing increased cell size and heterocyst frequency. Anabaena sp. strain PCC 7120 did not show any of these changes when it was grown with fructose. Thus, although Anabaena sp. strain PCC 7120 could take up fructose and use it in the dark, fructose did not improve growth in the light.

Stimulation of pigment accumulation in Anabaena azollae strains: effect of light intensity and sugars

The influence of high light intensity on the growth and pigment accumulating ability of Anabaena azollae was investigated. A. azollae responded positively to high light intensity (6 klx) and was further evaluated at higher intensities (10 and 15 klx), in the presence of glucose, sucrose and jaggery +/- DCMU. Significant enhancement in phycobiliproteins and carotenoids was observed in the sugar supplemented cultures at high light intensities. SDS-PAGE profiles of whole cell proteins revealed the presence of unique bands in such treatments. Sucrose supplementation induced a 30-90 % increase in carotenoids, phycocyanin and phycoerythrin content at 10 klx. Molecular analysis of the stimulatory and interactive role of sugars on pigment enhancement at high light intensity may aid in better exploitation of cyanobacteria as a source of pigments.