Fig. 2.36 Baeocyte formation in Dermocarpella gardneri. (a)–(c). Light micrographs of lateral views showing parallel divisions. (d) Top view showing radial divisions. (e) Scanning electron micrograph showing apical pore through which baeocytes escape. (From Montejano and Leon-Tejera, 2002.)

gliding stage for 36 to 48 hours. Heterocysts begin to develop at this stage in the absence of combined nitrogen. Conversion to vegetative filaments is complete after 96 hours.

Baeocytes (endospores) are formed by some coccoid (spherical) cyanobacteria (Figs. 2.34, 2.36). The protoplasm divides several times in different planes without growth between successive divisions. The resulting baeocytes are smaller than the original cell. Baeocytes are similar to bacterial endospores. In Dermocarpella, the baeocytes are released through an apical pore and enlarge to mature organisms (Fig. 2.36) (Montejano and Leon-Tejera, 2002).

Growth and metabolism

In the cyanobacteria there are three nutritional types: (1) facultative chemoheterotrophs, or those organisms capable of growing in the dark on an organic carbon source and of growing phototrophically in the light (only a portion of the cyanobacteria exhibit this condition); (2) obligate phototrophs, or organisms that can grow only in the light on an inorganic medium (some of

these are actually auxotrophs, requiring a small amount of an organic compound that is not used as a source of carbon, invariably meaning a vitamin); (3) photoheterotrophs, or those cells that are able to use organic compounds as a source of carbon in the light but not in the dark (Stanier, 1973).

Facultative chemoheterotrophs are able to grow in the dark on a very narrow range of substrates, being confined to glucose, fructose, and one or two disaccharides. This range of substrates is so small because the pentose phosphate pathway is the sole energy-yielding dissimilatory pathway. The tricarboxylic acid cycle lacks the enzyme-ketoglutarate (2-oxoglutarate) dehydrogenase and succinyl CoA synthetase, rendering it incomplete, and glycolysis likewise appears to be incomplete. Although the tricarboxylic acid cycle does not provide energy, it does provide carbon skeletons from the portions of it that are functional.

The pentose phosphate pathway does not operate in the light in the Cyanophyceae (although it is highly operational in the dark), being inhibited by ribulose-1,5-diphosphate, a product of light metabolism.

Even though some cyanobacteria grow in the dark, they grow very slowly. It is probable that the rate of chemoheterotrophic growth is always limited by the rate of dark ATP synthesis through oxidation of glucose-6-phosphate in the pentose phosphate pathway, a rate that is evidently not very great.