The upper and lower limits of bacteria size
Microorganisms refer to tiny organisms that are invisible or invisible to the naked eye. So, how big and small can microorganisms be? When reading this article, you can refer to a previous blog post: What is the largest bacteria found so far?
Large prokaryotic cells
Bacteria and archaea have different cell sizes, ranging from 0.2 μm in diameter to more than 700 μm in diameter. Most of the cultured rod-shaped bacteria have a width of 0.5-4 μm and a length of less than 15 μm. Of course, there are also some very large bacteria, such as Epulopiscium fishelsoni (Sticulus fischeri) with a cell length of more than 600 μm. C. fischeri is phylogenically related to Clostridium. It exists in the intestines of surgeons and contains multiple copies of the genome. The reason is that a single copy of the genome can no longer meet the needs of such a large bacteria for transcription and translation.
The largest known bacterium is Thiomargarita (sulfurophilic bacteria in Namibia), with a diameter of about 750 μm, which is directly visible to the naked eye. So far, people do not understand why these cells are so huge. Archaea has not found a strain comparable to the above two, but it may just have not been discovered.
We generally believe that the size of prokaryotic cells is limited by the ability of cells to transport nutrients. The metabolic rate of cells is inversely proportional to the square of the cell size. For very large cells, nutrient absorption will eventually limit the metabolism and make it compete with smaller cells. At the disadvantage.
Compared with Thiomargarita or Epulopiscium, the average size of typical rod-shaped bacteria (such as E. coli) is about 1-2 μm; in contrast, eukaryotic cells can be as small as 2 μm and many are larger than 600 μm. But generally speaking, small eukaryotic cells are not common.
Small size prokaryotic cells
Since the smaller the cell, the greater the advantage in nature, and the natural selection is competitive, so in theory, the nature should be all extremely small bacteria. However, it is not.
If we calculate the basic components (such as protein, nucleic acid, ribosomes, etc.) that a free-growing cell needs to contain, then the lower limit of the cell diameter is about 0.15-0.2 μm. At present, we have also successfully obtained some small prokaryotic cells. For example, there are about 105-106 prokaryotic cells per milliliter of seawater. These cells are often very small, with a diameter of 0.2-0.4 μm; they are found in the deep layers of the earth to have a diameter of about 0.2 μm bacteria and archaeal flora, these cells are collectively referred to as "ultramicrobial bacteria".
Interestingly, when we explored the genome of ultramicrobial bacteria, we found that their genome lacked many genes. The products or functions of these genes must be provided by other microbial cells or host organisms. Therefore, their successful evolution depends on the minimum requirements of biochemistry and other closely connected partners.
Specific surface area, growth and evolution
For cells, small size has many benefits. Small cells have a large specific surface area. The specific values can assume that cocci and bacilli are standard spherical and cylindrical, and then calculate their area/volume (S/V). The S/V ratio of a cell controls many characteristics of the cell, including growth rate and evolution. Because the growth rate of a cell partly depends on the rate at which it exchanges nutrients and waste with the environment, the larger the S/V ratio of a small cell, the faster the exchange rate of nutrients and waste per unit cell volume. Therefore, small cells that grow freely tend to grow faster than large cells. With the same amount of nutrients, there are more small cells than large cells, which in turn will affect cell evolution.
Cell division is accompanied by the duplication of chromosomes, and the duplication process is accompanied by mutations. Because the more the number of replications, the more the total number of mutations in the cell population, and the greater the possibility of evolution. Because prokaryotic cells are very small and haploid, they grow and evolve faster than large cells. Based on the differences in size and chromosomes between prokaryotic cells and eukaryotic cells, we can understand why bacteria and archaea are more adaptable to the environment than eukaryotic cells.
