Bacteriorhodopsin phototrophy

Classification of the metabolic capabilities of microbes can be challenging. With few exceptions, macroorganisms are either photosynthesizing primary producers (photo-autotrophs) or consumers (organotrophs or more commonly, heterotrophs).

For microbes, the story is more complicated. In addition to phototrophy, microbes can be chemo- or litho-trophs meaning they are able to derive energy from the oxidation of inorganic compounds such as reduced sulfur. If they can use light or chemical energy to fix carbon, then they are considered autotrophs. If the energy they acquire can be used to synthesize ATP but not to fix carbon, they are dependent on external sources of organic carbon making them mixotrophs.

An example of a phototrophic mixotroph is pictured above. These are salt loving haloarchaea in salt production ponds near (in?) San Francisco. The red color is due to the transmembrane protein bacteriorhodopsin. Using this protein some haloarchaea can harness sunlight to pump protons across their cell membrane. This establishes a proton gradient across the membrane. This gradient can be used to generate ATP.

There is a large number of scientific papers on bacteriorhodopsin because of its relative simplicity, it has become a model system for the study of membrane associated ion pumps.

Image from here http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2001/03/13/MN152047.DTL

These salt ponds are near San Francisco. If you want another view of the bay area ponds follow this link, select the satellite map and zoom way in. I tried this for a few of the other places I know these salt production ponds exist but the satellite images did not provide good enough resolution. An example is Bon Aire in the Netherlands Antillies