Purple Bacteria: Electron Acceptors and Donors.

Purple bacteria form a heterogeneous group of microorganisms capable of growing under anoxic conditions by anoxygenic photosynthesis. They can be divided into purple non-sulfur bacteria, which are able to grow both phototrophically and in darkness, and purple sulfur bacteria, all of them capable to grow in the light but only a few of them in the dark (Imhoff, 2006). The publication of several complete genome sequences of purple non-sulfur bacteria strains of the Rhodopseudomonas palustris and other species, pointed out the metabolic versatility of this group of bacteria. Indeed, it was found that the genome of Rps. palustris contain all the genes needed for switching from chemotrophy to phototrophy, from organotrophy to lithotrophy, from heterotrophy to autotrophy. Under anoxic light conditions, Rps. palustris can use either the reducing power deriving from organic compounds, such as organic acids or aromatic compounds, or the electrons deriving from inorganic compounds, such as H2, S2O3 2 (Huang et al., 2010), NO2 (Griffin et al., 2007), Fe2þ species and even electrodes of a microbial fuel cell (Bose and Newman, 2011; Bose et al., 2014). When growing chemotrophically in the presence of oxygen, respiration occurs and the reducing power can derive, as well as under anoxic light conditions, from either organic or inorganic compounds. When neither light nor oxygen is present, anaerobic respiration can take place, using NO3 or N2O andNO as final electron acceptors (Klemme, 1980; Larimer et al., 2004). Moreover, purple sulfur bacteria are able to use sulfide as an electron donor, oxidizing it to elemental sulfur or even to sulfate, while only some non-sulfur bacteria can use S2, but with different enzymatic pathways. Due to the remarkable complexity of the metabolism of purple bacteria, this article will mainly focus on the main electron acceptors and donors involved in photosynthesis, in respiration and in extracellular electron transport.