Asymmetric division: a major cause of cyanobacteria cell death

Cyanobacteria have a significant role in global primary productivity. Forms Prochlorococcus and Synechococcus alone contribute up to 50% of total fixed carbon in low latitudes, a great ecological significance. An individual cell has four potential fates; division, eaten by predators, viral lysis or cell death. The cause of death will greatly influence the carbon flow within the food web. Cell death will lead to cell lysis and a release of dissolved organic carbon, as does viral lysis, but this is very difficult to measure. Many studies have classified cell death as programmed cell death (PCD) however this study promotes the idea of asymmetric division as the major cause.

Cyanobacteria colonies aim to optimize colony fitness therefore PCD would make evolutionary sense, where there is differentiation between cells, in order for different roles to be completed. However in some colonial forms, cell death and DNA fragmentation happens randomly, making PCD a less clear response. PCD is detected from increased caspase activity during cell death. It happens when the metabolism is forced beyond its limits, however, in Trichodesmium colonies approximately 30% of the colony survives during mass cell death. This proportion of the population is known as a “persister” fraction that is more resistant to environmental stress.

Asymmetric division occurs when daughter cells are morphologically or physiologically different which could lead to differing fitness. In Escherichia coli, one daughter cell has a reduced fitness where the parent cell has deposited all its metabolic waste, such as damaged proteins, resulting in an increased fitness and growth of the other daughter cell. Dead cells can also be regularly produced as a by-product and this reduces the burden of damaged proteins on the colony, thus improving overall fitness.

A large proportion of cyanobacteria occur in the top 50m of the water column where excess light can lead to cell death by protein damage. Cell death increases throughout the day and peaks in the late afternoon. Is it possible that the individuals which die in these situations have already accumulated metabolic damage from their parent cell and are therefore more susceptible to environmental conditions?

Protein damage can be discovered by measuring the protein carbonylation which needs to be tested in inclusion bodies produced asymmetrically by the parent cell and track their distribution. To conclude that asymmetric division is a major cause of cell death, determining the amount of oxidative damage along with the fate of a bacterial cell needs to be addressed. This knowledge will provide a better understanding of the whole food web structure and the carbon within it.

Franklin, D.J. (2014) Explaining the causes of cell death in cyanobacteria: what role for asymmetric division? Journal of Plankton Research. 36: 11-17