Effects of Microbial Life

Most microbes live as colonies in communities of multiple species. Some physically bind together, coating surfaces with resilient, tough biofilms. A biofilm, however, is more than a group of bacteria. The glue-like matrix that binds them together comes from a mix of proteins and long chains of complex sugars called polysaccharides that the bacteria themselves secrete. These bacteria grow on various moist surfaces, even in human bodies. A common example is the plaque that coats our teeth.

Biofilms were instrumental in launching life. The earliest-known fossils you can see with your own eyes are stromatolites, finely layered rocks that record the growth of bacterial mats—biofilms—in shallow marine waters.In 2008, analyses of organic globules (small round particles) preserved in 2.7-billion-year-old stromatolites supported their microbial origin.The stromatolites formed when thin layers of sediment (particles that fall to the ocean bottom) settled and subsequently became trapped among the filaments and mats made of bacterial colonies.While the individual bacteria involved were not preserved, they left behind evidence of their biofilm structures, the dominant form of direct evidence for early life on Earth.In 1956, living stromatolites were discovered in Shark Bay, Australia, making them a rare example of a life-form alive today that was first discovered in the fossil record.

The global effects of microbial life, especially bacteria that conduct photosynthesis, are hard to overstate. Photosynthesis, the process in which sunlight and carbon dioxide (CO2) are converted into chemical energy and oxygen, is commonly associated with plants, but cyanobacteria were performing photosynthesis long before plants came into being. In addition to creating Earth's oxygen-rich atmosphere, cyanobacteria living near the surface of the oceans help regulate atmospheric CO2 for the entire planet. They draw carbon out of the atmosphere through photosynthesis. Should a substantial proportion of these bacterial photosynthesizers suddenly die, atmospheric CO2 levels would rapidly rise. Just such a rise occurred at the end of the last glacial period, too quick to be accounted for by geophysical or geochemical processes. Could this mean that a sudden decrease in microbial photosynthesis helped bring on the postglacial climate in which humanity subsequently rose to global prominence? Perhaps.

Microbes also play key roles in breaking down and extracting elements from rocks and getting them into biological circulation. And consider that animals, including nearly all insects, cannot actually digest plant matter made of cellulose, a very stable and hard-to-break-down molecule. Although cellulose is the most readily available food (and energy) source in the world, animals delegate the difficult task of decomposing it to the microbes living in their digestive tracts.

Cows, as we all know, eat grass. But without the microbes that make the enzymes to break down cellulose, cows would starve no matter how much they ate. Communities of specialized microbes live in the rumen (the first compartment of a cow's four-chambered stomach), where day in and day out, they do the hard work of breaking down cellulose. So cows don't simply eat grass; they chew it up and feed it to their internal microbe community, which breaks it down and offers up nutrients in return. Cows chew partly digested food to survive. They must grind grass into particles fine enough for microbes to digest the cellulose. Cows graze quickly and then regurgitate (bring from the rumen back to the mouth) small amounts of grass to grind into finer pieces, chewing for up to ten hours a day. Microbes in the rumen break cellulose into digestible sugars by releasing a substance called cellulase. Grinding food into fine particles helps increase the surface area on which cellulase acts. The cow's second stomach chamber, the reticulum, acts as a mixing chamber that extends the cellulose breakdown that began in the rumen. However, cows do not directly benefit from the sugars produced by microbes. The microbes consume the sugars themselves and produce waste products like acetate, propionate, and butyrate that the cow then absorbs. The cow also absorbs water from the digested food using its third stomach chamber, the omasum. Cows benefit from their rumen microbes further by consuming them in their fourth and final chamber, the abomasum. These microbes are the cow's major source of protein.