Microbial Nutrition Redux
Looking back through the BIO230 archives, I found this posting from last February, which gives a concise summary of the major nutritional needs of bacteria. We have greatly simplified this topic in lecture, which has the unfortunate consequence making it seem that microorganisms are simple to grow in the lab. Many organisms are extremely difficult to culture, as it is frequently difficult to reproduce the conditions found in nature. Those who read to the end of this will find a special bonus point opportunity.
In lecture, we stated that we have two primary concerns when discussing microbial nutrition, an energy source and a carbon source, but actually there are three. Microbial growth is also dependent on a source of electrons, which are need for energy generation, and this gets to the heart of our class discussion of why oxygen is also a significant concern in the culturing of microbes. The idea of an energy source for a microorganism is easy to grasp: the cell utilizes a chemical (such as glucose) or light, and via a complex series of reactions generates ATP that is used for cellular work. This creates a problem however, as the energy is generated by stripping energy from electrons in a process called oxidation. High energy electrons are removed from compounds like glucose during oxidation, and temporarily stored in cellular cofactors like NAD+ which concurrently becomes reduced. In the final steps, ATP is generated.
The problem arises at the end of these reactions, as the cell now has a large pool of reduced NAD+ (chemically NADH), and must recover it to start cellular respiration all over again. If the cell had no way to recover it, a process like cellular respiration or glycolysis would quickly stop. So the cell must finally oxidize NADH back to NAD+ to allow the process to continue. In aerobic respiration, NADH is oxidized with molecular oxygen; oxygen takes the low energy electrons (plus hydrogen ions) from NADH to produce water.
This is not a problem unique to the use of oxygen, but is common to all energy generation reactions. Consider the process of glycolysis which begins with glucose and ends with pyruvic acid. Glycolysis generates two ATP via substrate level phosphorylation, but also results in the synthesis of two molecules of NADH. These molecules of NADH must be oxidized back to NAD+ in order for glycolysis to continue. In oxidative respiration this occurs at the end of electron transport, and in fermentation this occurs by the partial reduction of pyruvic acid to another organic compound such as ethanol.
So the diagram in this posting begins to give a taste of the complexity of biological systems, and shows the many ways that microorganisms all accomplish the same basic feat: transferring low energy electrons from a cellular intermediate (NADH) to a reduced compound in the environment. We do it with oxygen, as do many bacteria, but bacteria are very resourceful. Consequently, the variety of electron donors and acceptors is immense in nature.
Bonus opportunity: I will replace the lowest quiz score to date with a 5 for everyone who tells me in the comment thread why my choice of post title above is a clever pun. Please note that if you hit the submit button that the comment will be held up in moderation to prevent spam comments. I will release the comments as the end of the day on Wednesday, September 21st giving everyone a fair chance to pick up the bonus.