A biosphere is a closed and self-regulating system made up of living organisms and the environment within which they exist, which can include the land, water, atmosphere, or any combination of these. The life containing and sustaining layer of the Earth or any other cosmic body is its biosphere and the life we speak of incorporates both things seen and unseen by the naked eye. A layer or region of a planet and its atmosphere where little exist other than bacteria is still very much a part of that planet’s biosphere. By that same token a planet where no signs of life other than bacteria are found is still one that has a biosphere.
This is evident in the very fact that it can successfully incubate and provide sustenance for development, growth and reproduction of such organisms, as well as efficiently incorporate the remains of the organism once it has expired. It also utilizes the byproducts of their various bodily functions needed for existence while alive in ways that are beneficial to both the continuation of the species and the needed functions and balance within the environment, so that the process can go on in a repetitive cycle of life, sustenance, sustenance byproducts, reproduction, death, recycling of the dead matter, and life again, indefinitely. All the elements required for survival of the organism and the preservation of other non-living matter from a delicately balanced atmospheric composition to the amount of sunlight and other raw matter to the means by which the organism’s byproducts are converted (often by yet another organism) back to the environment are all inseparable parts of a given biosphere.
When we speak of a biosphere centering entirely on bacterial life, the reader may be reminded of the images of Mars exploration and the search for life or more readily its biochemical indicators as we understand these to be, by sampling and analyzing its atmosphere. There is even on this planet life in unexpected places such as bacteria thriving at extremely high temperatures numbering in the hundreds of degrees Fahrenheit or animals capable of hibernating for a hundred years and surviving the most extreme of conditions (a fascinating example are Tardigrades, the apparent super creature of this planet).
The search for life includes the potential that it had existed at some point of the planet’s history or can exist in the future, and this is determined by both the discovery of fossilized remains of such life including the fossilized remains of bacteria and other simple and single-celled organisms as well as the presence of what are known as the building blocks of life on Earth, most fundamentally carbon and hydrogen, but also oxygen, nitrogen, phosphorus and sulfur among others. The evidence of the organic molecule methane in the atmosphere can be an indicator of biological activity and is one of the ingredients looked for in search of a biosphere on other planets.
References:
http://www.questia.com/library/encyclopedia/biosphere.jsp
http://www.britannica.com/EBchecked/topic/66191/biosphere
http://www.jpl.nasa.gov/news/news.cfm?release=2011-018
http://www.newscientist.com/article/mg16422095.100-indestructible.
http://www.extremescience.com/zoom/index.php/life-in-the-deep-ocean/42-deep-sea-hydrothermal-vents
