Development and Germination of the Azotobacter Cyst

Abstract

The fine structure of Azotobacter vinelandii has been studied by means of electron microscopy of ultrathin sections made of the encysting and germinating cells. The organisms were fixed with KMnO4 and embedded in epoxy resin. On an encystment medium the rod-shaped bacteria begin to assume an almost spherical form and then bark-like exine appears in 11/~ to 2 days. The exine thickens and an electron permeable intine forms between it and the shrinking cell body. In 5 days the intine makes up more than half of the cyst volume and begins to show a definite two-layered structure. Meanwhile the peripheral bodies, which may be extensions of the cell membrane of the vegetative cell, disappear as the encystment progresses. The cell wall and membrane of the vegetative cell remain demonstrable as the confining structure of the shrinking central body of the mature cyst. In this central body lipoidal globules appear together with aggregations of nuclear material. Cyst germination begins with an increase in the size of the central body at the expense of the intine. The nuclear aggregations become more diffuse and the lipoidal globules disappear. The exine may be pushed outward and the bark-like fragments separate as the emerging vegetative cell develops. Invagination of the cell wall and membrane may occur at this stage leading to cell division. Empty exines remain as horseshoe-shaped structures.

Ultrastructure of Akinete Development in a Blue-green Alga, <i>Cylindrospermum</i> sp

“…The cyst of Azotobacter is very similar in appearance to an akinete. The intine con tains a granular and fibrillar material in an electron transparent background (Wyss et al 1961). Bacillus endospores are structurally less comparable to the akinete than cysts.…”

Characterization of the gene coding for GDP-mannose dehydrogenase (algD) from Azotobacter vinelandii

“…In A. vinelandii, alginate has been proposed to play an important structural role in cysts (44); however, most of the isolates produce this exopolysaccharide also during vegetative growth. Alginate production in A. vinelandii is thus a unique biological model, and its study at the molecular level will help to answer different questions of biological importance.…”

“…The encystment process takes approximately 5 days, and the mature cysts are surrounded by two capsule-like layers containing a high proportion of alginate (19,35,44); these dormant cells are able to survive in dry soil for long periods. Other particular features of this bacterium are that under some conditions it has multiple copies of its entire chromosome (24,26,34) and that it is capable of fixing nitrogen under aerobic conditions (21).…”

Characterization of Three New Azotobacter vinelandii Alginate Lyases, One of Which Is Involved in Cyst Germination

“…Resuspension of cysts in a medium containing glucose leads to a germination process in which vegetative cells eventually escape from the cyst coat. It has been proposed that an alginate lyase may be involved in the rupture of the coat (43). AlgL is dispensable for germination (38), while the biological function of AlgE7 is unknown.…”