Legumes, a type of plant, require rhizobia, a type of soil bacteria, to survive since these organisms fix nitrogen during photosynthesis. rhizobia use the legumes for food. what would most likely happen to the legume population if rhizobia suddenly became extinct?

The answer is: the legumes will extinct, too.

It is known that plants cannot directly use atmospheric nitrogen. But, some legumes have a symbiotic relationship with bacteria that live in their root system. These bacteria are called Rhizobia and have the ability of nitrogen-fixation. In the root nodules, they use atmospheric nitrogen to convert it into ammonia, and later to ammonium, which can be used by plants. When legumes die, nitrogen from their remaining is released back to the soil where it is available to the other plants.

So, if Rhizobia suddenly became extinct, the symbiotic relationship between will be interrupted. The legumes will not be able to use atmospheric nitrogen without the help of Rhizobia, and eventually, they will extinct, too.