Nitrosospira bacterium. Coloured transmission electron micrograph (TEM) of a cross-fractured and freeze- etched nitrogen-fixing bacterium (Nitrosospira sp. ). Most of the interior of the bacterium consists of cytoplasm. This is surrounded by the cell envelope. This Gram- negative soil bacterium obtains energy from ammonia and in the process converts it into nitrites. Other bacteria then convert the nitrites into nitrates, the form of nitrogen used by plants. Magnification: x53, 000 at 6x7cm size. x172,000 at 8x10"~PORTRAIT"