Halobacterium in the context of "Proton pump"

Photosynthesis (/ˌfoʊtəˈsɪnθəsɪs/ FOH-tə-SINTH-ə-sis) is a system of biological processes by which photopigment-bearing autotrophic organisms, such as most plants, algae and cyanobacteria, convert light energy — typically from sunlight — into the chemical energy necessary to fuel their metabolism. The term photosynthesis usually refers to oxygenic photosynthesis, a process that releases oxygen as a byproduct of water splitting. Photosynthetic organisms store the converted chemical energy within the bonds of intracellular organic compounds (complex compounds containing carbon), typically carbohydrates like sugars (mainly glucose, fructose and sucrose), starches, phytoglycogen and cellulose. When needing to use this stored energy, an organism's cells then metabolize the organic compounds through cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.

Some organisms also perform anoxygenic photosynthesis, which does not produce oxygen. Some bacteria (e.g. purple bacteria) uses bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, releasing sulfur instead of oxygen, which was a dominant form of photosynthesis in the euxinic Canfield oceans during the Boring Billion. Archaea such as Halobacterium also perform a type of non-carbon-fixing anoxygenic photosynthesis, where the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and produce a proton (hydron) gradient across the cell membrane, and the subsequent ion movement powers transmembrane proton pumps to directly synthesize adenosine triphosphate (ATP), the "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, as far back as the Paleoarchean, preceding that of cyanobacteria (see Purple Earth hypothesis).

Halobacteriaceae, from Ancient Greek ἅλς (háls), meaning "salt", and "bacterium", is a family in the order Halobacteriales and the domain Archaea. Halobacteriaceae represent a large part of halophilic Archaea, along with members in two other methanogenic families, Methanosarcinaceae and Methanocalculaceae. The family consists of many diverse genera that can survive extreme environmental niches. Most commonly, Halobacteriaceae are found in hypersaline lakes and can even tolerate sites polluted by heavy metals. They include neutrophiles, acidophiles (ex. Halarchaeum acidiphilum), alkaliphiles (ex. Natronobacterium), and there have even been psychrotolerant species discovered (ex. Hrr. lacusprofundi). Some members have been known to live aerobically, as well as anaerobically, and they come in many different morphologies. These diverse morphologies include rods in genus Halobacterium, cocci in Halococcus, flattened discs or cups in Haloferax, and other shapes ranging from flattened triangles in Haloarcula to squares in Haloquadratum, and Natronorubrum. Most species of Halobacteriaceae are best known for their high salt tolerance and red-pink pigmented members (due to bacterioruberin carotenoids), but there are also non-pigmented species and those that require moderate salt conditions. Some species of Halobacteriaceae have been shown to exhibit phosphorus solubilizing activities that contribute to phosphorus cycling in hypersaline environments. Techniques such as 16S rRNA analysis and DNA–DNA hybridization have been major contributors to taxonomic classification in Halobacteriaceae, partly due to the difficulty in culturing halophilic Archaea.

Archaerhodopsin proteins are a family of retinal-containing photoreceptors found in the archaea genera Halobacterium and Halorubrum. Like the homologous bacteriorhodopsin (bR) protein, archaerhodopsins harvest energy from sunlight to pump H ions out of the cell, establishing a proton motive force that is used for ATP synthesis. They have some structural similarities to the mammalian G protein-coupled receptor protein rhodopsin, but are not true homologs.

Archaerhodopsins differ from bR in that the claret membrane, in which they are expressed, includes bacterioruberin, a second chromophore thought to protect against photobleaching. Also, bR lacks the omega loop structure observed at the N-terminus of the structures of several archaerhodopsins.