Prokaryotic organisms such as single celled bacteria, are the simplest of all life forms. They contain very little biological material and even their DNA floats freely within a bacterial cell. This is where the name prokaryote comes from as the name implies “before the nucleus”, ie these cells existed before organisms had evolved to organise their DNA within a nucleus. The roles of prokaryotes in the fabrication of our very existence and the dependence the ecosystem has on them is often under appreciated. Very little thought or respect is given to these microscopic organisms who remain elusive to the naked eye but yet have laid the basis for life on this planet. Bacteria are often viewed upon as infectious entities that are bad for us. But bacteria also play a huge role in the maintenance of the biosphere and the promotion of our good health as well as being a causal factor for ill health. As insignificant as they seem their significance to life as we know it is immense.
Veterans of the earth
We homosapiens like to consider ourselves the most dominant species on the planet and to some extent we are, we have definitely laid our footprint down and made our presence universal. We have the ability to adapt to our environment and we have done that efficiently over the span our evolutionary timescale. But we have only been on this planet for a fraction of the time bacteria has and its thanks to their existence we even exist. Bacteria can adapt to their environment too but on a much more efficient scale. Not only can they adapt thriving in anything from extreme icy cold to hot spring temperatures to extreme PH levels in acidic habitats, environments humans could only imagine living in. But when we humans have exhausted our resources or when our creations become tools of our demise, bacteria will continue to undoubtedly reign.
The scientific consensus is that the earth was formed 4.5 billion years ago and based on evidence this is what science agrees on(1). The earliest evidence suggests life first existed as chemoautrophs, basic single celled anaerobic organisms, these cells were the first prokaryotic bacteria and they dominated the planet in an atmosphere absent of oxygen around 3.8 billion years ago(2). But their dominance was short lived as around 3 billion years ago evolution took place where a new type of prokaryotic cell called cyanobacteria started to photosynthesise(3).
These new pioneer species laid the foundations for life on this earth as we know it and it was the start of a chain of ecological succession. This was a revolution in the world of prokaryotes. Using only water, carbon dioxide and energy from the sunlight they were able to release small amounts of oxygen into the atmosphere as a waste product of photosynthesis. Oxygen killed many anaerobic organisms on contact and cyanobacteria became the dominant prokaryote species.
From fossils of bacterial mats, otherwise known as stromatolites, we know that initially the oxygen was bound to dissolved iron ions and other reducing substances that could react with oxygen. These ions precipitated as iron oxide (shown as rust) and prevented the accumulation of free oxygen. It wasn’t until 2.5 billion years ago precipitation exhausted the dissolved iron which led to the oceans and atmosphere becoming saturated with oxygen(4). This was the beginning of what scientists called the great oxidation event (GOE). The increase in oxygen was toxic to many anaerobic bacterial and protist species, as it still is to many forms today. However this also led to adaptation by some cells in which they evolved to harness the oxygen to render it harmless to them by using a similar system to photosynthesis called cellular respiration or aerobic respiration. This produced more energy for the cell thus also allowing for bigger more complex life(5). We owe bacteria our gratitude for providing us with a stable oxygen steady state eco-system.
Photosynthetic prokaryotes are also responsible for the cause of our plant life. Evidence suggests chloroplasts are descendants of cyanobacteria that some time in the past entered into a symbiotic mutualistic relationship with a primitive plant like organism, becoming a part of it through endosymbiosis(6). This gives entry to the perfect relationship between oxygen releasers and oxygen consumers. Humans breathe in oxygen and release carbon dioxide where as these oxygen givers recycled carbon dioxide to make usable oxygen. Now its easier to see the significance of our dependence on them. Without oxygen we would die, without plant life we would die.
Another life giving aspect that bacteria has provided us with is nitrogen. The role of nitrogen in the biosphere is crucial to all living things. Nitrogen is important for the synthesis of proteins, nucleic acids and other fundamental components associated with growth. Nitrogen is all around us, 78% of the air we breathe is in fact nitrogen, however its in the unusable form of N2, the triple bond between the two atoms makes it almost inert. Therefore nitrogen needs to be fixed and converted to a usable form NH4 (ammonium) or NO3 (nitrate) ions. Free-living cyanobacteria were initially responsible for biological nitrogen fixation in soils but now prokaryotic bacteria have also formed a symbiotic relationship with legumes in which the nitrogen-fixing bacteria called Rhizobium, subside in the root nodules of the legumes, fixing nitrogen for them there. Biological nitrogen fixation is performed exclusively by prokaryotes and is yet another positive and vital impact they have on our existence and the biosphere(7).
Its easy to forget that we are covered by mostly harmless bacteria inside and out (roughly 100 trillion of them) and all the cells in our body are out numbered by them. But a type of bacteria that is beneficial to us, one that we have formed a type of mutualistic symbiosis with is the microbial flora in our guts. There is between 500-1000 bacterial species in the human gut flora. These anaerobic prokaryotic bacteria help us breakdown sugars and lipids increasing the bio-availability of nutrients(8). The metabolic substrates they leave behind are also beneficial to us, they give us vitamins such as vitamin B12, K and short chain fatty acids. Our friendly bacteria also play an important role in acting as a barrier to harmful pathogenic organisms(9).
The negative impacts bacteria has on humans we all know too well. Anything from infection to disease is due to bacteria, some of which we know can be fatal. E.coli, Salmonella, Tetanus and Staphylococcus are just a few deadly examples of how Bacteria can impact us negatively by invading our bodies, destroying our cells and breaking down our immune system.
Prokaryotes have great genetic diversity and this has led to them being beneficial to us and our environment as well as being the cause of many human deaths. They truly are the dominant species and will most likely continue to be so for many years beyond our existence. They laid the foundations for us to live and now maintain the structure of that biosphere. They hold the potential to eradicate us and have the capability of thriving in conditions we couldn’t dream of surviving in. In essence they are our friend and foe. Most would love to live without them but we certainly couldn’t.