CYANOBACTERIA 53

Fig. 2.32 Diagrammatic representation of the cyanobacterial circadian clock.

Fig. 2.33 KaiC protein from Synechococcus elongatus, one of the circadian clock proteins in cyanobacteria. Left: Transmission electron micrograph of negatively stained KaiC protein showing the hexamer formed in an ATPcontaining solution. Right: Model of a KaiC hexamer. (From Mori et al., 2002.)

Asexual reproduction

Asexual reproduction occurs by the formation of hormogonia or baeocytes or fragmentation of colonies (Fig. 2.34).

Hormogonia (or hormogones), which are characteristic of all truly filamentous cyanobacteria, are short pieces of trichome that become detached from the parent filament and move away by gliding, eventually developing into a separate filament. Hormogonia are distinguished from vegetative filaments by their gliding motility, the small size of their cells (Figs. 2.34(a), (b), 2.35), and the absence of heterocysts (Meeks and Elhai, 2002). In some species, hormogonia contain gas vacuoles, which control buoyancy. In some filamentous algae, such as Oscillatoria and Cylindrospermum, the entire filament may break

54 THE PROKARYOTIC ALGAE

Fig. 2.34 (a),(b) Formation of a hormogonium in

Oscillatoria. (c) Baeocyte formation in Chamaesiphon incrustans. (d) Baeocyte formation in Dermocarpa pacifica. (After Smith, 1950.)

Fig. 2.35 Nostoc punctiforme. (a) Vegetative cells.

(b) Hormogonia. The hormogonia lack heterocysts and are smaller than vegetative cells. (From Meeks and Elhai, 2002.)

up (Fig. 2.34(a), (b)), whereas in others the hormogonia are produced at the tips of special branches. In some algae, specialized separation discs or necridia (Fig. 2.34) are involved in the breaking of the hormogone from the parent filament, whereas in others, the filament just fractures.

The one common factor in initiation of hormogonium differentiation appears to be a change in some environmental parameter such as an increase or decrease of a nutrient or a change in the quantity of light.

Hormogonia of many strains display positive phototaxis, which is important in the colonization of illuminated portions of the habitats by these photoautotrophic organisms (Meeks and Elhai, 2002). The acquisition of motility in hormogonia comes at a price. Hormogonia lack heterocysts and are unable to fix nitrogen. Phycobiliprotein synthesis ceases, leading to an attrition in light gathering. Hormogonia cells continue to photosynthesize and assimilate exogenous ammonium, although the rates of CO2 fixation and NH4 incorporation are only 70% and 62%, respectively, of those of vegetative cells. Much of the metabolic output is devoted to the synthesis and secretion of mucilage required for gliding motility. Hormogonia remain in the