Bacterial Storage Granules
No matter how efficient your factories are, they need raw materials, and in an ever-changing environment, a cell cannot always depend upon a steady supply chain. How can you help your cell cope with occasional shortages? If you said stockpile, you are in good agreement with nature. Both bacteria and archaea use storage granules to stockpile essential nutrients. The substrate is usually polymerized for easier packing, like poly-phosphate or the poly-hydroxybutyrate (a carbon store) in these Cupriavidus necator granules. No matter what they pack, storage granules are generally spherical, and exhibit a range of sizes (⇩). When the environment is rich in one nutrient but poor in another, cells may stop growing, but keep adding to their cellular stores. You can see that effect in this C. necator cell, which has been cultured in a medium with carbon but not enough nitrogen; as a result, it has accumulated very large poly-hydroxybutyrate granules which take up much of the cytoplasmic space.
Storage granules are ubiquitous in bacteria, and you will see them in many of the cells in this book. The most common type is poly-phosphate, which has a characteristic dark appearance in electron microscopy images.