Question

Can you think of such situations (and organisms) where $O_2$ is not available?

Answer 1

Part 1: Anaerobic situations
Situations or areas where there is lack of oxygen or complete absence of oxygen are anaerobic situations.

Example: The places on earth where either oxygen is completely absent or present in less amount such as soil, mud, hydrothermal vents deep under the sea, and many more.

Part 2: Anaerobic organisms
Organisms which can live in anaerobic conditions are known as anaerobic organisms or anaerobes.

Example: yeast, bacteria such Clostridium butyricum etc

Part 3: Types of anaerobic organisms
Anaerobic organisms are further divided into two categories. Facultative anaerobes They are not strict anaerobes and are flexible in switching between aerobic and anaerobic respiration depending on the availability of oxygen.
Eg: Staphylococcus spp., Escherichia coli, Salmonella, etc.

Obligate anaerobes They are strict anaerobes which require completely anaerobic conditions for their survival and cannot respire in the presence of oxygen.
Eg: Actinomyces, Bacteroides, Clostridium, etc.