Cyanobacteria are a worldwide group of photosynthetic prokaryotes that can cause nuisance blooms in eutrophic waters. It is generally accepted that their resting cells, akinetes, play an important role in the dispersal, recruitment, initiation of blooms and survival under unfavourable conditions, therefore information on the germination, distribution and abundance of akinetes in natural sediments is essential for understanding the ecology and bloom dynamics of N2-fixing cyanobacteria. The present study describes the effect of irradiance on the germination and subsequent growth of N2-fixing filamentous cyanobacteria developed from natural akinete stock in sediment of Lake Balaton (Hungary) with varying phosphorous supply. The research focuses on the invasive Cylindrospermopsis raciborskii and Aphanizomenon flos-aquae the most abundant species of this lake. In the experiments, the germination of ten filamentous N2-fixing cyanobacteria species was observed. The species assemblages of the germinated cyanobacteria populations showed strong light and phosphorus dependence. Anabaena and Anabaenopsis species became dominant in phosphorous-rich conditions, while in phosphorus-deficient environments Aphanizomenon species and C. raciborskii dominated. Among the germinated filaments we have detected Anabaenopsis cunningtonii and Anabaena compacta, which have not been observed in Lake Balaton previously. Our results suggest that among the filamentous heterocytic cyanobacteria of this shallow lake the invasive C. raciborskii was the best competitor when phosphorus supply and irradiance were low.

Summary 3

I. introduction 4

II. the cyanobacteria 7

III. the heterocyst 9

1. Function and metabolism 9

2. Heterocyst structure 12

(a) Overview 12

(b) The polysaccharide (homogeneous) layer 12

(c) The glycolipid (laminated) layer 12

(d) The septum and microplasmodesmata 12

3. Nitrogen regulation and heterocyst development 12

4. Heterocyst development 13

(a) The proheterocyst 13

(b) Proteolysis associated with heterocyst development 14

(c) RNA polymerase sigma factors 14

(d) Developmental regulation of heterocyst cell wall and nitrogenase gene expression 14

(e) Genome rearrangements associated with heterocyst development 15

5. Genes essential for heterocyst development 15

(a) hetR 15

(b) Protein phosphorylation and the regulation of hetR activity 16

(c) hetR in nonheterocystous cyanobacteria 16

(d) Other heterocyst-specific genes 16

6. Heterocyst spacing 18

(a) Patterns of heterocyst differentiation 18

(b) Genes involved in heterocyst spacing 18

(c) Disruption of heterocyst pattern 18

7. Filament fragmentation and the regression of developing heterocysts 20

8. The nature of the heterocyst inhibitor 20

9. Cell selection during differentiation and pattern formation 20

(a) Cell division 20

(b) DNA replication and the cell cycle 21

(c) Competition 21

10. Models for heterocyst differentiation and pattern control 21

IV. the akinete 23

1. Properties of akinetes 23

2. Structure, composition and metabolism 24

3. Relationship to heterocysts 24

4. Factors that influence akinete differentiation 24

5. Extracellular signals 25

6. Akinete germination 25

7. Genes involved in akinete differentiation 26

V. conclusion 26

Acknowledgements 27

References 28

Cyanobacteria are an ancient and morphologically diverse group of photosynthetic prokaryotes. They were the first organisms to evolve oxygenic photosynthesis, and so changed the Earth's atmosphere from anoxic to oxic. As a consequence, many nitrogen-fixing bacteria became confined to suitable anoxic environmental niches, because the enzyme nitrogenase is highly sensitive to oxygen. However, in the cyanobacteria a number of strategies evolved that protected nitrogenase from oxygen, including a temporal separation of oxygenic photosynthesis and nitrogen fixation and, in some filamentous strains, the differentiation of a specialized cell, the heterocyst, which provided a suitable microaerobic environment for the functioning of nitrogenase. The evolution of a spore-like cell, the akinete, almost certainly preceded that of the heterocyst and, indeed, the akinete may have been the ancestor of the heterocyst. Cyanobacteria have the capacity to differentiate several additional cell and filament types, but this review will concentrate on the heterocyst and the akinete, emphasizing the differentiation and spacing of these specialized cells.