The heterocysts of blue-green algae (Myxophyceae)

Investigation of the links between heterocyst and biohydrogen production by diazotrophic cyanobacterium A. variabilis ATCC 29413

This work investigates the effect of heterocyst toward biohydrogen production by A. variabilis. The heterocyst frequency was artificially promoted by adding an amino acid analog, in this case DL-7-azatryptophan into the growth medium. The frequency of heterocyst differentiation was found to be proportional to the concentration of azatryptophan (0-25 µM) in the medium. Conversely, the growth and nitrogenase activity were gradually suppressed. In addition, there was also a distinct shortening of the cells filaments and detachment of heterocyst from the vegetative cells. Analysis on the hydrogen production performance revealed that both the frequency and distribution of heterocyst in the filaments affected the rate of hydrogen production. The highest hydrogen production rate and yield (41 µmol H2 mg chl a(-1) h(-1) and 97 mL H2 mg chl a(-1), respectively) were achieved by cells previously grown in 15 µM of azatryptophan with 14.5 % of heterocyst frequency. The existence of more isolated heterocyst has been shown to cause a relative loss in nitrogenase activity thus lowering the hydrogen production rate.

Regulation of Expression of Nitrate and Dinitrogen Assimilation by Anabaena Species

Anabaena sp. strain 7120 appeared more responsive to nitrogen control than A. cylindrica. Growth in the presence of nitrate strongly repressed the differentiation of heterocysts and fixation of dinitrogen in Anabaena sp. strain 7120, but only weakly in A. cylindrica. Nitrate assimilation by ammonium-grown cultures was strongly repressed in Anabaena sp. strain 7120, but less so in A. cylindrica. The repressive effect of nitrate on dinitrogen assimilation in Anabaena sp. strain 7120, compared to A. cylindrica , did not correlate with a greater rate of nitrate transport, reduction to ammonium, assimilation into amino acids, or growth. Although both species grew at similar rates with dinitrogen, A. cylindrica grew faster with nitrate, incorporated more 13 NO 3 − into amino acids, and assimilated (transported) nitrate at the same rate as Anabaena sp. strain 7120. Full expression of nitrate assimilation in the two species occurred within 2.5 h (10 to 14% of their generation times) after transfer to nitrate medium. The induction and continued expression of nitrate assimilation was dependent on protein synthesis. The half-saturation constants for nitrate assimilation and for nitrate and ammonium repression of dinitrogen assimilation have ecological significance with respect to nitrogen-dependent growth and competitiveness of the two Anabaena species.

Response of a wild type and a non-nitrogen-fixing mutant of Anabaena doliolum towards different amino acids

The effects of various amino acids on growth and heterocyst differentiation have been studied on wild type and a heterocystous, non-nitrogen-fixing (het+ nif-) mutant of Anabaena doliolum. Glutamine, arginine and asparagine showed maximum stimulation of growth. Serine, proline and alanine elicited slight stimulation of growth of wild type but failed to show any stimulatory effect on mutant strain. Valine, glutamic acid, iso-leucine and leucine at a concentration of as low as 0.1 mM were inhibitory to growth of parent type. Methionine, aspartic acid, threonine, cysteine, and tryptophan did not affect growth at concentrations lower than 0.5 mM. But at 1 mM, these amino acids were inhibitory. In addition to the stimulatory effects of glutamine, arginine and asparagine, the heterocyst frequency was also repressed by these amino acids. Glutamine and arginine at 2 mM completely repressed heterocyst differentiation in the mutant strain; however, other amino acids failed to repress the differentiation of heterocysts. Our results suggest that glutamine and arginine are utilized as nitrogen sources. This is strongly supported from the data of growth and heterocyst differentiation of mutant strain, where at least with glutamine there is good growth without heterocyst formation. Studies with glutamine and arginine on other N2-fixing blue-green algae may reveal the regulation of the heterocyst-nitrogenase sub-system.

The developmental biology of heterocyst and akinete formation in cyanobacteria

We will be concerned with the two major differentiated cell types of filamentous cyanobacteria--the heterocyst and the akinete. The former is generally accepted to be the site of aerobic nitrogen fixation in heterocystous cyanobacteria. The latter is a spore-like cell capable of withstanding certain environmental extremes and of germination. A short general introduction to cyanobacteria and their cell types will be included. The remainder of the review will fall into four main sections. The first will deal with the metabolism of the heterocyst and akinete, with particular reference to nitrogen fixation in the former. The next will be concerned with a description of the metabolic and ultrastructural changes associated with heterocyst and a kinete development. A third section will describe the special arrangement of the heterocyst and akinete (one of the features which makes this group of prokaryotes unique), the relationship between the cell types, and methods of altering this normal regular spatial pattern. The final section will describe in detail present theories of pattern control in cyanobacteria and the mechanisms by which the process of differentiation itself is regulated.

Effect of rifampicin (3-(4-methylpiperazinyl-iminomethyl)rifamycin) on heterocyst differentiation in the blue-green algae Anabaena and Calothrix

Chains of multiple heterocysts form in Anabaena und Calothrix filaments on treatment with rifampicin. The multiple heterocysts form irrespective of whether the rifampicin treatment is given in combined nitrogen-free or supplemented medium. This suggests the possibility of involvement of a species of RNA or of protein as intracellular heterocyst inhibitor and indicates that some latent pattern-determining mechanism may operate in the combined nitrogen medium.

The pattern of sporulation in Anabaena circinalis and comments on the role of heterocysts in sporulation in blue-green algae

Cells between two intercalary heterocysts differentiate at random into spores in A. circinalis. One or more cells, which fail to transform into spores, are present between the two adjacent spores, and these cells disorganize later. A critical C:N ratio regulates sporulation and heterocyst formation. During sporulation the reductive ability of the heterocyst gradually diminishes. It is concluded on the basis of this and other evidence that sporulation is regulated by interactions between heterocysts and vegative cells which are manifested in diverse patterns in different species of blue-green algae.