____ bacteria carry out the conversion of gaseous nitrogen to a biologically useful form of nitrogen.

My answer is "nitrogen-fixing"
(This is fill in the blank)

Thank you this is part of my Unit test and I always forget this one!

Thanks Damon :)

You are welcome :)

That seems reasonable. See for example:

http://www.britannica.com/EBchecked/topic/967311/nitrogen-fixing-bacteria

Your answer, "nitrogen-fixing," is correct. Nitrogen-fixing bacteria are capable of converting atmospheric nitrogen (N2) into a biologically useful form, such as ammonia (NH3) or nitrate (NO3-). These bacteria play a crucial role in the nitrogen cycle as they provide a source of nitrogen that can be used by other organisms, such as plants, to synthesize proteins and other essential molecules. To arrive at this answer, you can use knowledge about the nitrogen cycle and the process of nitrogen fixation, or refer to specific sources such as textbooks or scientific literature on the topic.