What are photosynthetic bacteria doing in the dark?
Photosynthetic bacteria utilise sunlight as a source of energy. This is why most processes in these microbes - including purple, green, helio and cyanobacteria - such as cell division, take place during the day. However, the cells do not sleep at night; they are also active in the dark phases, albeit less than during the day. They mainly consume the sugar built up during the day. Bacteria also dispose of toxic substances, such as oxygen radicals, at night.
Many cyanobacteria produce cyanophycin at night, a type of nitrogen store. The basic building blocks for this nitrogen-rich biopolymer are the amino acids arginine and aspartate. Cyanophycin is then broken down into amino acids during the day and incorporated into the metabolism. This ensures a continuous supply of nitrogen to the cells.
In addition to cyanophycin, cyanobacteria store glycogen. Similar to starch from plants, this is a polysaccharide made up of glucose. Cyanobacteria store glycogen in granules during the course of the day or under unfavourable environmental conditions. However, unlike most bacteria, they do not break down glycogen at night via glycolysis, but primarily utilise a different metabolic pathway (the oxidative pentose phosphate metabolic pathway). One reason for this is that cyanobacteria, in contrast to heterotrophic bacteria, primarily utilise NADPH as a cofactor for enzymes and this is provided via the pentose phosphate pathway.
A special group of cyanobacteria can fix molecular nitrogen (see also here). Corresponding representatives either use special cells, the heterocysts, or separate this process from photosynthesis. The reason for the separation of these two processes is the oxygen sensitivity of nitrogenase, the enzyme that catalyses nitrogen fixation. Single-cell cyanobacteria therefore fix nitrogen at night. To do this, the cells use the sugar built up by photosynthesis during the day as an energy source and produce cyanophycin. This process is controlled by an internal clock. Cyanobacteria are the only known bacteria with a circadian ("almost daily") rhythm. This enables the cells to coordinate the internal metabolic pathways in a day-night rhythm. For example, the internal clock allows cyanobacteria to adjust to expected changes in light and "activate" their photosynthetic apparatus before sunrise or sunset. Another example of different day-night activity is the movement of cyanobacteria in sediments. On coasts, for example, cyanobacteria migrate downwards in strong sunlight and back to the surface in the dark. This mechanism protects the cells from dehydration and excessive light intensity.
Read more:
https://www.wissenschaft.de/umwelt-natur/mikrobe-im-zwei-schicht-betrieb/
© Text: Jörg Toepel Joerg.toepel[at]ufz.de
Figure: Jörg Toepel, use according to CC4.0
