Which of the following is a low-DO filament that is most likely to be found under low F:M conditions?

The correct answer is Sphaerotilus natans, which is a filamentous bacterium commonly associated with low dissolved oxygen (DO) conditions in wastewater treatment processes. Under low food-to-microorganism (F:M) ratios, which indicate that there is less organic matter available for microbial growth relative to the amount of microorganisms present, Sphaerotilus natans is likely to proliferate.

This organism thrives in environments where organic matter is limited, allowing it to form extensive filamentous structures. It can often be found in conditions where oxygen levels are low, making it particularly relevant in the context of wastewater treatment where effluents might not have sufficient DO for optimal bacterial activity. The presence of Sphaerotilus natans can lead to operational challenges, such as foaming and settling problems, which are critical for wastewater management.

In contrast, the other options involve microorganisms that either do not specifically thrive under low-DO conditions or are associated with different environmental conditions. Factors like their metabolic pathways, substrate preferences, and ecological roles in the wastewater treatment ecosystem differentiate them from Sphaerotilus natans.