Bacteria existed long before animals, plants and fungi. They transformed the biosphere, and still play a major role in many ecosystems. Bacterial cells are (usually) not divided into compartments – so they do not have nuclei, chloroplasts, mitochondria and the like. Most are about 1 µm (1/1000 mm) wide, and pictures of individuals need to be taken with a microscope. Achromatium (upper right) is one of the largest bacteria, measuing about 0.1 mm (100 µm). Most of these pictures are of living organisms (the fossil stromatolite is not). Various optical techniques have been used to add strength to the images.
Many bacteria are simply shaped – almost like a ball or a short sausage. The first two pictures show this nicely. The second picture helps to illustrate how small bacteria are, as there is a very small eukaryotic (cells with nuclei, mitochondria, and other compartments) protist moving, lower right, among the bacteria. The picture of a community and the images of sulfur bacteria show bacteria that form long thin filaments. Bacteria can move, swim or glide. The third picture in the upper row and a pink sulfur bacterium in the third row show flagella. Many bacteria form larger aggregates. Some join end to end, or in one case shown here, adhere side by side to form long filaments. Others are bound together within mucus. In some cases these can become very large to form mats. These mats can become very extensive – extending over kilometers. Sometimes they become lithified (stone-like), forming stromatolites. The arrangement of layers within stromatolites are evident in fossils that extend back over 2 billion years, making them among the earliest recognizable forms of life.
The later rows turn to theme of bacterial metabolism. Bacteria are much less restricted in ways that they capture and use energy and materials. Two images show pink bacteria. As with the green of plants, these bacteria use pink pigments to capture radiant energy. This energy is used to break down sulfides so that they can access usable energy. As a byproduct, sulfur bacteria may deposit small beads of sulfur in the cells. Three other filaments, one forming a spiral, are also sulfur metabolizers, but do not carry out photosynthesis. The remaining images are of iron bacteria – they convert one form of iron compounds to other more rust-like forms, releasing and capturing energy in the process. Many secret a mucus to which the rusty iron attached – as is the case of Gallionella. In nature, bacteria like this can form an iridescent skin on the surface of water, or form orange coats on immersed surfaces.