Читать книгу Astrobiology - Charles S. Cockell - Страница 102

Cells take on a diversity of morphologies (shapes) depending on the species, their local environment, and their function. Eukaryotic cells in multicellular organisms can vary from the rectangular cells of plants whose dimensions range from 10 to 100 μm to the long thin cells of neurons that transmit nerve impulses through organs such as your brain, which although thin (from about 4 to 100 μm in width) can be up to 1 m long.

The most consistent patterns of morphology are found in the prokaryotes. Prokaryotic cells fall into a number of major classes of shapes (Figure 5.3).

Figure 5.3 The wide variety of prokaryote shapes as seen under a microscope. (a) Staphylococcus (coccoid bacteria forming grape-like clusters). Here stained purple with the Gram stain. Scale bar: 10 μm.

Source: Reproduced with permission of Y. Tambe.

(b) Rod-shaped Escherichia coli. Scale bar: 5 μm.

Source: Reproduced with permission of Josef Reischig.

(c) Filamentous cyanobacteria from the Arctic. Scale bar: 10 μm. (d) Spiral-shaped spirochaete bacteria. Scale bar: 10 μm. (e) The square-shaped bacterium, Haloquadratum walsbyi, here shown stained and observed under fluorescence. Scale bar: 2 μm.

Many microbes have a coccoid (spherical) morphology and are referred to as coccus. An example is Staphylococcus (Figure 5.3a), which is responsible for some infections in humans. In some species, coccoids aggregate to form pairs (a diplococcus) or even collections of cells that look like bunches of grapes, such as observed in Staphylococcus. Many organisms are rod-shaped or bacilli, which are round-ended cylinders. An example of these organisms is bacteria in the genus Bacillus, or the gut bacterium Escherichia coli (Figure 5.3b). To complicate matters, there are even prokaryotes that are something of a mix between a coccus and a rod (coccobacillus) and can be considered as a slightly elongated coccus.

Microorganisms can be found that are filamentous, such as many cyanobacteria (Figure 5.3c), fungi, and some species of soil bacteria such as microorganisms in the phylum Actinobacteria. Filamentous organisms can have branched morphologies, most commonly observed in fungi and cyanobacteria. Yet other organisms have a spiral shape, such as Spirochaetes (Figure 5.3d), which are responsible for some diseases such as Lyme disease (Borrelia burgdorferi). There are organisms that are disc-shaped, star-shaped, and tapered.

Perhaps the most unusual shape is found in a member of the archaea, Haloquadratum, which adopts a square shape (Figure 5.3e). The organisms divide into sheets, looking somewhat like a sheet of postage stamps, achieving a layer of microbes up to 40 μm in length. The reason for this shape may be linked to its growth in briny pools and the resulting balance of osmotic stresses inside and outside the cells.

Some microorganisms can change their shape under stress. When exposed to nutrient stress or low temperatures, certain species become filamentous in growth. It is thought that the filamentous shape enhances the surface area for nutrient acquisition. Filamentous shapes may also improve attachment to surfaces and enhance the formation of layers of organisms or biofilms. It is clear that prokaryote shape is not just a serendipitous and ephemeral evolutionary feature, but that there is a whole diversity of environmental pressures that might influence microbial shapes, which are only just beginning to be understood.