The a(alpha)Proteobacteria Nitrogen-fixing bacteria: 。Azospirillum 。Grow in soil,. using nutrients excreted by plants 。Fix nitrogen 。Rhizobium 。Fix nitrogen in the roots of plants Figure 27.5• Nitrogen-fixing bacteria: • Azospirillum • Grow in soil, using nutrients excreted by plants • Fix nitrogen • Rhizobium • Fix nitrogen in the roots of plants The  (alpha) Proteobacteria Figure 27.5