An Overview of Purple Bacteria: Systematics, Physiology, and Habitats

Abstract

Anoxygenic phototrophic purple bacteria are a major group of photosynthetic microorganisms widely distributed in nature, primarily in aquatic habitats. Nearly 50 genera of these organisms are known and some have become prime model systems for the experimental dissection of photosynthesis. Purple sulfur bacteria differ from purple nonsulfur bacteria on both metabolic and phylogenetic grounds, but species of the two major groups often coexist in illuminated anoxic habitats in nature. Purple sulfur bacteria are strong photoautotrophs and capable of limited photoheterotrophy, but they are poorly equipped for metabolism and growth in the dark. By contrast, purple nonsulfur bacteria, nature’s preeminent photoheterotrophs, are capable of photoautotrophy, and possess diverse capacities for dark metabolism and growth. Several purple bacteria inhabit extreme environments, including extremes of temperature, pH, and salinity. Collectively, purple bacteria are important phototrophs because they (1) consume a toxic substance, H2S, and contribute organic matter to anoxic environments by their autotrophic capacities; (2) consume organic compounds, primarily non-fermentable organic compounds, in their roles as photoheterotrophs; and (3) offer scientists in the photosynthesis community a smorgasbord of molecular diversity for the study of photosynthesis.