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Captain's Cards
Those bacteria are driving me crazy! First, the ones who already lived here started getting rowdy. They tore apart the muscle tissue I’m in and separated all the protein strands. Then, some of their relatives from outside are coming in to help them tear up the place. There were so many of them that I was getting kind of claustrophobic. It also smelled pretty bad. All of those bacteria were making the body decay, which doesn’t smell too great. Some of the relatives who called themselves fermentative bacteria started to break apart my fellow proteins. I had just noticed what they were doing when one of them came over and started spitting an enzyme all over me (not a pleasant experience). This enzyme said his name was Protease and he seemed pretty nice. Soon, however, he stuck a water molecule between me and my neighbor. Our amino acid molecules, of course, reacted with the water and the bond between us was broken. Protease then broke the bond on my other side. I felt so alone being in a single molecule again. I was glad to see Protease leave, and wished I could find a way out too.
I couldn’t see any chance of escape because bacteria and other microorganisms were everywhere. Soon another group of bacteria, the acetogens, started to shoot oxygen gas at my serine molecule. Again, we reacted, but this time we formed lots of gases (hydrogen, nitrogen, and carbon dioxide) as well as acetic acid. I had never been part of an acid before, and it was pretty interesting. Lots of the bacteria went away because my acid and the others lowered the pH. Even the acetogens went away because their job was done. After a while, though, the pH started rising again, and some new bacteria showed up. I shouldn’t call them new because they looked ancient. They were called methanogens, and they looked older than my grandma. These guys brought with them hydrogen and carbon dioxide gases, which made my acetic acid molecule react once again. The reaction produced water, carbon dioxide gas, and methane gas, which I became a part of. I felt very free being in a gas molecule. I was also very excited about being in a biogas and doing my part for the carbon cycle.
Bacteria are one-celled microorganisms. They can be classified into two groups, autotrophs and heterotrophs, according to how they obtain energy. Autotrophs (Latin for self/grow) are able to make their own food from light or chemical sources of energy. Heterotrophs (other/grow) obtain their energy from the tissues or body fluids of other organisms. They break down complex organic compounds that they take in from around them in order to make food. Saprobic bacteria are heteroptrophs that live on decaying material, such as a dead body. By decomposing organic material for energy, these microorganisms help recycle nutrients like nitrogen and carbon back into the environment. If it were not for these decomposers, the organic carbon in dead and rotting organisms would remain locked underground, effectively stopping the carbon cycle. The carbon dioxide in air would be quickly depleted, and there would be none left for plants to carry out photosynthesis. Saprobic bacteria are, therefore, one of the most important links in the carbon cycle.
Saprobic bacteria invade every inch of the dead body, and begin eating and digesting its tissues. There are many types of bacteria that live inside the body. These bacteria are the first to begin the process of decomposition after an organism dies. The initial bacteria start physical break-down of muscles. They tear apart the muscle tissue into individual protein strands. More saprobic bacteria soon move in to help break down the organic matter. Because there are so many bacteria in the soil (about 2 billion in 1 gram of fertile garden soil), decomposition occurs more rapidly after a body is buried. Decay that occurs underground or underwater is known as anaerobic decomposition. One specific type of anaerobic decay that is very important to the carbon cycle is biogas production. This process has three steps: hydrolysis, acidification, and methane formation.
Proteins are broken down by an enzyme called protease that is secreted by fermentative bacteria. This enzyme separates proteins (polypeptides) into amino acids (peptides). It accomplishes this depolymerization through a process known as hydrolysis. In hydrolysis, a water molecule is inserted between the two amino acids that are bonded together. This breaks the bond between them by capping the free reactive ends with the H and the OH. The protein,therefore, is broken down from long chains into its individual molecules, amino acids.
In the next step, a new group of bacteria called acetogens take over. These bacteria decompose amino acids into acetic acid, hydrogen gas, nitrogen gas, and carbon dioxide gas. To do this, they need oxygen which they obtain from O2 dissolved in the body’s fluids or bound in the body’s structure. While acetogens are anaerobic bacteria, oxygen is not as poisonous to them as to some other anaerobes. The chemical reaction that occurs when acetogens decompose amino acids is:
Because acetogens produce acid, the pH of the organic matter falls to about 4 or 5. This low pH is toxic to the next group of bacteria, the methanogens. As the acetogens die out, however, the pH rises and methanogens become more predominant.
Methanogens are bacteria that produce methane gas. They are very anaerobic (oxygen is toxic to them) and sensitive to pH. These bacteria are also very biologically primitive. Methanogens have, therefore, been a part of the carbon cycle for a long time. They combine the acetic acid made by acetogens with hydrogen gas, and carbon dioxide to produce methane gas, water, and carbon dioxide, according to the following equation:
Each year, between 531 and 792 million tons (482 - 718 million metric tons) of methane are released into the atmosphere by methanogens. These incredibly busy bacteria, therefore, recycle a great deal of carbon, making a very significant contribution to the earth’s carbon cycle.