Final answer:
The term "deeply branching" refers to the position in the evolutionary tree of some of the oldest bacteria, which can live in extreme environments and are important for understanding early forms of life and for industrial applications.
Step-by-step explanation:
The term "deeply branching" refers to b) the position in the evolutionary tree of deeply branching bacteria. These bacteria are thought to be some of the most ancient forms of life, sharing a close relationship with the last universal common ancestor (LUCA). Deeply branching bacteria are characterized by their ability to thrive in extreme environments such as high temperatures, acidic conditions, and underwater near volcanoes and thermal ocean vents, which may resemble the conditions of early Earth.
They play a significant role in our understanding of microbial evolution and have various applications in industry. One example of deeply branching bacteria is Acetothermus paucivorans, which is known as a thermophile and has been identified as the deepest branching bacterium. The significance of deeply branching bacteria spans both basic science, by offering insights into ancient life forms, and industry, where some species are utilized for their unique properties.