And that's just it. The belief that a facultative anaerobe will always prefer oxygen is exactly entry level. Emphasis on entry level. I suppose when one first starts out one will learn this definition even now, though I really wish this is changed.
We have realized that 'facultative anaerobe' is a poor descriptor that should not be reserved for 'organisms that prefer oxygen by can respire anaerobically', and had to be redefined to match with newer understandings of microbial physiology.
The electron transport chain is not always as straightforward as is often taught in entry level biology. Not all of the 'archetypal' protein complexes are always present, and you also have other membrane-bound proteins (or protein complexes) that can utilize a range of electron donors and acceptors beyond what you learn early on.
A common case is the lack of the archetypal complex III, which is the cytochrome c reductase complex that transfers electrons from an electron-transfer quinol to a c-type cytochrome. This is what is necessary in the conventional model to allow electrons donated from NADH at the very start of the electron transport chain to be transferred to oxygen as the terminal electron acceptor. In the absence of this complex or another protein/protein complex capable of the same function, something else needs to be responsible for reducing c-type cytochromes. Certain D-lactate dehydrogenases come to mind here.
However, this is relatively inefficient, as well, you actually need D-lactate then for there to be aerobic respiration. Like, a lot of it for effective aerobic respiration. At the same time, the organism may be able to utilize electrons from NADH, which is a lot more of a 'common' energy source than D-lactate, for anaerobic respiration - often with fumarate as the terminal electron acceptor, but not always. So, you can have an organism that cannot efficiently generate energy aerobically, but a lot more so anaerobically.
As more and more microbial species are characterized, more is known about the diversity of energy production and it became really important to define their respiratory capacities better. There are also microorganisms that can thrive in both high and low oxygen levels, or those that can respire (effectively) in all oxic conditions. It's not just about what electron acceptors are utilized, but also about the type and diversity of electron donors too.
Hope that helps clear this all up for you.
Lol, take the L. I want the definition changed. Yeah buddy.
Just an FYI the majority of bacteria I work with lack complex III. You know what they all create more energy with? Oxygen. This is because they can produce more ATP/e-. There is no possibly way for an alternative electron acceptor to produce more ATP/e- because physics. Doesnt matter what limitations are found in the system.
Now if a facultative anaerobes cannot access enough oxygen it may resort to alternative electron acceptors which by definition must produce less energy. Inefficiencies in the system do not alter this fact. Or if they lack the ability they must utilize fermentation, which is extremely inefficient, e.g. the facultative anaerobe Saccharomyces cerevisiae.
Now you keep saying preferred, so I'm guessing you'll try to hunt down a weirdo organism that will be reported to 'prefer' an alternative to oxygen. However preferred is ill defined. If the organism utilizes an alternative electron acceptor when it has a choice, it will not change the fact that it can produce more ATP/e- with oxygen. And thus why would it choose the poor source?Usually because It's found in a extreme niche. And anyways when we find organisms that make these strange choices we never redefine the majority, but would create another definition for this grouping. In this instance we would add a modifier e.g. nitrate-preferring facultative anaerobe.
However as this is just a reefing forum, I promise everyone reading you don't need to ever consider this information. It's truly pointless in the discussion of this filter.